Linear DC Power Supplies - Designing & Building Custom DC Power Supplies

today in the workshop we'll be designing and building linear power supplies we'll see how to use transformers rectifiers filters and regulators to build custom power sources for our projects we'll also construct the linear dc power supply for the workshop we've got the power today so welcome to the workshop [Music] [Applause] well hello and welcome to the workshop and today we are going to be working with linear power supplies now we've worked with power supplies before we took an old atx computer supply and made it into a workbench supply we also built another supply that used the power brick of an old computer but both of those power supplies were switching supplies and today we're going to cover linear power supplies now linear power supplies are no longer as popular as switching power supplies and the main reason is the size and the weight of them another reason is that when you get to large power supplies switching power supplies are less expensive to build although for small supplies a linear supply still has some advantages linear power supplies are no longer used with computers except for specialized medical and instrumentation computers but in the early days of personal computers they were the only type of supply that we used and if you think about boxes like the altair or the mca or the southwest technical product computers that came out in the mid 1970s you might remember them as being a lot like boat anchors in terms of size and weight and that was mostly due to the linear power supply that they used the large transformer in the supply really weighs a lot but linear supplies do have some advantages over switching supplies although they're not as efficient they are lower in noise and that can be a real advantage for instrumentation for audio work and for some radio work and so you'll find that a lot of high-end audio equipment for example uses a linear supply another great thing about a linear supply is that you can design and build your own whereas a switching supply is a much more complex project and you're probably better off buying one than building one so i've kind of divided today's video into two different sections at the beginning we'll go through the usual introduction to the topic and we'll do a number of experiments with linear supplies both fixed and variable voltages and both positive and negative ones but after that in the second part of the video we will build a linear bent supply and i'll show you how i went through selecting the components how i went through building and labeling the chassis and putting everything together so we'll have a true project and i made the project using components that you can easily obtain and if you check out the article that accompanies this video you'll find links to the sources for those components so let's begin by learning a little bit about linear power supplies a dc power supply is a device that converts alternating current or ac into direct current or dc it provides a regulated dc output at a specific voltage dc power supplies also provide isolation from the ac line dc power supplies can provide positive or negative output voltages or in many cases both of them these devices can also serve as battery chargers there are two types of dc power supplies linear and switching in a linear power supply the ac mains or line voltage is fed directly into a power transformer in most cases the output of the power transformer is lower than that of the input voltage but you can design dc power supplies to create higher voltages as well the output of the power transformer is fed into a rectifier circuit a rectifier circuit converts the ac into a rather choppy form of direct current the output of the rectifier is fed into a filter circuit which smooths out the output and produces smooth direct current the filter circuit is then connected to a voltage regulator which provides regulated steady voltage at the desired output level in a switching power supply the ac line or mains voltage is fed directly into a rectifier circuit to be converted into high voltage dc this high voltage dc is then used to power a high frequency oscillator this high frequency oscillator is fed into a high frequency transformer because of the higher frequencies involved the high frequency transformer can be made much smaller and lighter than the power transformer used in a linear power supply the output of the high frequency transformer is fed into another rectifier circuit where it again becomes rather choppy and noisy form of direct current this direct current is fed into a filter circuit which smooths it out and removes some of the high frequency noise it is also fed back to a control circuit which then goes back to the oscillator and this is the method that switching power supplies use to regulate their output voltage linear power supplies are not that efficient they have an efficiency of 60 percent or less switching supplies on the other hand are quite efficient and have an efficiency of 80 percent or more linear power supplies require a large power transformer due to the low frequencies used in line or mains voltage because of the high frequencies they use switching supplies can use a much smaller and lighter high frequency transformer linear power supplies however provide a very low noise direct current output the output of a switching power supply often contains remnants of switching noise which can cause interference in audio circuitry and other sensitive devices a linear supply also has a very fast response in other words if a large amount of current is suddenly required a linear supply can provide it very quickly switching supplies are not as fast to respond as linear supplies although in most situations their performance is adequate because of their large power transformers linear supplies are very large in both size and weight switching supplies can be made very compact and lightweight because of the low efficiency of linear supplies they have a very high heat dissipation switching supplies on the other hand are more efficient and have a lower heat dissipation linear supplies are the most cost effective when the application is a low powered one that requires less than 10 watts of power above 10 watts switching supplies are more cost effective today we'll be working with linear power supplies which means we'll be working with power transformers rectifiers and voltage regulators linear power supplies are often used in high-powered audio equipment radio equipment and in instrumentation and medical equipment where their low noise is an advantage so let's take a look at a few of the components we'll be using in order to construct a linear power supply now here are some components that can be used in the construction of linear power supply now over here i have two different power transformers they're both about equivalent although you can see they are different shapes from one another and this style of transformer is one that is meant to mount on a printed circuit board but it's a low profile type of a power transformer this is a more conventional design that just bolts onto a chassis and has all of its connections on this side over here now over here we've got some rectifiers and i've got three different rectifiers for you these two are what are called bread rectifiers and i'll be explaining that term in a few moments and this is one that's meant to bolt onto a chassis and this one is actually meant to be placed on directly onto a printed circuit board and it also has a hole in it so you can bolt a heat sink onto it because these things can get a bit warm when they run this is a more conventional rectifier diode over here this is just a 1n4007 and it's a common rectifier diode that you can also use to build rectifiers now another section of the supply is going to be the filtering section and i've got a 2200 microfarad electrolytic capacitor over here rated at 63 volts and these are 50 volt 10 microfarad tantalum capacitors which can be used on the output of the power supply tantalums actually do have advantages over electrolytic capacitors but when you need a very large value capacitor an electrolytic is pretty well the practical choice and finally these devices over here are voltage regulators they're three pin voltage regulators they look the same but they're actually three different types of three pin regulators and you can get a variety of these you can get them in positive or negative configurations they are used on the output of the supply to provide a smooth regulated voltage these two are actually fixed ones whereas this one can supply a variable voltage so now that we've taken a look at a few of those components let's see how we actually use them in our linear power supplies a transformer is a simple passive electrical component the purpose of the transformer is to transfer alternating current from its input to its output or outputs the transformer will also isolate the input from the output a step up transformer outputs a higher voltage than its input voltage conversely a step down transformer will output a lower voltage than its input voltage an isolation transformer outputs the same voltage as its input voltage a transformer consists of multiple coils of wire wrapped around the common core and its schematic symbol is shown above when alternating current is applied to the input coil the varying magnetic flux induces an output in the output coil or coils the ratio of the number of turns in the coil between input and output determines the transformer's output for a hypothetical example if the input coil has a thousand turns and the output coil is a hundred turns the output will be one-tenth of the input if you reverse that with the input having a hundred turns and the output having a thousand the output will be ten times that of the input the size or gauge of the wire determines the current capability of the transformer and this is why high current transformers are very large devices a common configuration for transformers is to have a center tap on the output a point where the voltage is half the voltage of the total output voltage transformers can also be tapped on the input to allow multiple input voltages and this is a common configuration for equipment that is meant to run on north american power levels which are around 120 volts and european ones which are usually around 220 volts when selecting a transformer you'll select it by its input and output voltage specifications another rating you need to consider is the current capacity of the transformer which will be measured in amperes or volt amps the transformer i've been illustrating so far is a very common design but another design is the toradol transformer while a bit more expensive these transformers have an advantage in that they don't radiate an excessive magnetic field as a magnetic field is constrained to the center of the core this is very useful for audio equipment and other sensitive equipment which may be sensitive to the emf generated by most common transformers a rectifier is a device that converts ac into dc the most common type of a rectifier is a diode but some older designs use selenium plates and some newer designs are using mosfets there are several different configurations for a rectifier the configuration illustrated here is a half wave rectifier and it consists simply of a diode on the output of the transformer note that the output dc signal is actually a series of pulses on the positive side with the negative side being cut off if the diode was reversed it would be the opposite a very common configuration is the full wave rectifier and it can be wired in a number of different fashions the illustration here is one that is very common and is called a bridge rectifier and consists of four diodes you'll note that in this case the output is a series of positive signals but there are twice as many of them as both sides of the ac waveform are included this is a very efficient method of using a rectifier and is probably the most common you can also build a full wave rectifier using only two diodes in a center tap transformer note that in this case however the input voltage which is the output voltage of the transformer needs to be twice as high as that for the bridge rectifier in order to achieve the same dc output voltage the filter circuit smooths the ripple in the dc output allowing for smooth direct current generally a filter is just a capacitor in some cases an inductor is also used to reduce high frequency noise although this is more common in switching supplies than it is in linear supplies the filter capacitor is placed across the voltage output the energy stored in the capacitor can also be useful for current surges occasional surges of current that exceed the ratings of the power supply in this diagram we see the filter capacitor across the output after a full wave rectifier note that there is also a resistor added here this is generally a high value resistor of one kilo or more and is used simply to discharge the capacitor if the power supply is turned off with no load connected to it a voltage regulator is a circuit that provides a steady output voltage from a fluctuating input voltage these can be built to have a fixed voltage output or a variable voltage output you can get positive or negative voltage regulators you can build voltage regulators with an integrated circuit or you can use discrete components you can also add a power transistor to the integrated circuit to increase its current capability here we see a fixed voltage regulator on a power supply that consists of a bridge rectifier and a filter capacitor note that there's also a capacitor on the output of the regulator this is a lower value capacitor and it's usually standard practice to include one and this diagram shows a variable voltage regulator the ground pin on a variable voltage regulator has been replaced by an adjustment pin and it goes to a resistive voltage divider that allows you to set the output voltage now i want to show you in this demonstration the effect of a half wave rectifier and as you recall a halfway rectifier is essentially just a diode now what i have over here is i have my transformer hooked up to my oscilloscope right now and you can see the output over here and of course the output is a 60 hertz signal you can see down over there the frequency is 60 hertz or very close to 60 hertz anyway and what i'm going to do is i'm going to put a diode in series with all of this so i'm going to take off this lead over here and place a diode on and you can see the effect right now the positive side is being passed by the diode the negative side is not being passed and if i reverse the diode we should have the opposite effect so let's just turn that around and there we go and let's just move the trigger point down so we get it there there we have the negative side so the diode is basically just of course allowing current to only pass in one direction and knocking off the current in the other direction and so that is the output of a half wave rectifier so i have a little demonstration here for you and this is a demonstration of the effect of both a load and a filter capacitor on the output of a bridge rectifier now i've got a transformer over here this is not a very high current transformer at all it's about 34 volts center tapped at a very low current actually and it is feeding into a bridge rectifier and the output of the bridge rectifier is currently on the oscilloscope that you can see over here now i'll just remove the scope probe for a moment just so you can see where the baseline is so that's the zero point over there i'll put the probe back in and you can see of course is a great deal of ripple there's no filter capacitor or output load on this at all now one interesting thing about this and this is the nature of the bridge rectifier of course is that the frequency that i'm measuring here is 120 hertz and not 60 hertz i'm not sure if you can see the writing down there and that of course makes sense for the bridge rectifier it's twice the line frequency that i have i live in canada so my line frequency is 60 hertz as it would be in the united states now i'm going to put a load across it what i've got over here is just a few resistors actually these are three 16 ohm power resistors connected in series for a total of 48 ohms as i said this is a very low uh current device over here and we're getting about 35 volts out of it so this should pull about three quarters of an amp here and i'm just going to type that onto the output and observe the scope when i do that notice how the output is reduced quite a bit when i put a load on to it over here now the real thing i wanted to show you though is the effect of a filter capacitor and i've got this is a 2000 microfarad capacitor it's rated at 63 volts now when you do this you have to be absolutely certain you get the polarity right because connecting a large electrolytic capacitor backwards can have some rather negative and explosive consequences so i'm going to touch that across the output and take a look at the scope right now and as you can see there's basically all of the ripple is gone it's a very flat dc output that i get because of the filter capacitor and of course that's the function of the filter capacitor it charges up and discharges and the discharge time is slower than the next cycle so it keeps everything nice and smooth at the top over here and if i remove it we're back to the same display that we had before so as you can see a filter capacitor and a load can really affect the output of your bridge rectifier so let's recap what we've done so far we've taken a transformer and reduced our ac line voltage to a manageable level then we've taken that manageable level ac voltage and fed it into a rectifier device which produced a rather choppy form of direct current or dc then we stuck a capacitor across the output and lo and behold the dc all smoothed out and is wonderful so essentially we've built a linear power supply however what we've built so far is an unregulated power supply so the output voltage won't necessarily always be the same it can be affected by fluctuations on the input ac voltage and it can also be affected by the load that you place across the power supply's output for most applications we want a fixed voltage and in order to obtain that we need to add one more stage to our power supply and that is a voltage regulator now for high currents voltage regulators often employ things like mosfets and discrete circuitry but for lower currents i.e things under about 3 amperes we can make use of integrated circuits that are specifically designed to be voltage regulators and there are a number of common ones out there right now we're going to experiment with three of them a positive voltage regulator a negative voltage regulator and a variable positive one one that can allow you to output different voltages rather than the fixed voltages that the previous two did so let's start off by looking at how we're going to hook up our positive voltage regulator for a fixed positive voltage regulator i'll be using a 7812 which is a 12 volt positive voltage regulator it's in a to220 package and you can see the pin outs over here now this is one of the 7800 series of positive voltage regulators in this series of voltage regulators the last two digits indicate the output voltage so you can see our 7812 gives 12 volts out a 7805 would give 5 volts a 78 15 15 volts etc etc these voltage regulators are available from a number of different manufacturers and in a number of different packages and some of them can handle current up to one and a half amperes they all feature a short circuit shutdown function and they also feature an overheat shutdown function now in the to220 package illustrated here the tab of the package is connected to ground so if you wish to use a heat sink and you will if you're pulling the current near the maximum level you can directly connect your heat sink to ground into the voltage regulator with no problem now here's a circuit that we're going to be using to test our voltage regulator as you can see we have our ac input going into a power transformer and after that going into a bridge rectifier to create some choppy form of dc voltage there's a filter capacitor that's placed across the circuit and you might notice that i don't have a dropping resistor across the capacitor it's not necessary because replacing the voltage regulator on the output of this the voltage regulator is connected next and it just has connections for input ground and output and across the output i also have an output capacitor now i'm not specifying values for these in my experiments i'm using a thousand microfarads for my filter capacitor and 2.2 microfarads for the output capacitor but you could use different values and the output capacitor doesn't even need to be a polarized one you could use something like a 100 nanofarad capacitor if you want it's just there to stabilize the voltage regulator so let's hook this up onto a solderless breadboard and see what our output is now here's a demonstration of the 7812 positive voltage regulator the 12 volt regulator and if you can see the multimeter you can see it's pretty close to 12 volts 12.03 volts is very close to 12. now what i've done is i've taken that transformer that i showed you earlier my test transformer and here's a bridge rectifier over here and the rest of the circuitry is here on the solderless breadboard so you can see the big filter capacitor in here and the smaller capacitor on the output and the 7812 itself now if you're going to be trying to pull appreciable current out of this and of course i'm hardly pulling any current out of it because i haven't got a load on it right now you will require a heat sink on this device and with this device you can just bolt it to the chassis because the tab is connected directly to ground now there are other versions of this voltage regulator more modern versions that are low voltage drop regulators they dissipate less heat they're more efficient and you could substitute those as well in a modern design if you do that i would suggest that you check carefully on the specifications to make sure that the tab on it is grounded because that's not true of all voltage regulators and you certainly don't want to connect the heat sink directly to that that is grounded if the tab isn't grounded you would have to use an insulator or something like that when you're installing the heat sink but other than that consideration this is a very simple regulator to use it's available of course not just in the 12 volt variety but in all the common varieties and so if you're trying to build a logic supply with 5 volts or if you need a 15 volt supply or something this is still an excellent choice for a fixed negative voltage regulator i'll be using a 7912 which is negative equivalent of the 7812 note the pin outs on this regulator they are not the same as the pin outs on the 7812 and that's very important to note because many people just assume they are and if you try to hook it up the other way it obviously won't work now this is again as part of the series of 7900 negative voltage regulators and like the 7800 series the last two digits of the part number indicate the output voltage it has similar specifications with a current up to 1.5 amperes in some packages short circuit shutdown function and an overheat shutdown function now one very important thing to note is that in the to220 package the tab is connected to the input not the ground and so if you are attaching a heat sink to this you cannot ground your heat sink if you're using your chassis as a heat sink which would be grounded you'll have to put some sort of an insulator such as a mica insulator kit in between the voltage regulator and the heatsink itself to avoid attaching that to ground because if you ground the input you'll be shorting it out and that's not a good thing now the circuit for hooking this up is very similar to the positive regulator once again we have our power transformer going to a bridge rectifier with a filter capacitor across it note a couple of things though first of all the hook up to the regulator is different and the ground side is on the positive side not the negative side because we're looking for a negative output again the same considerations go as it did for the positive regulator with the filter capacitors and you can pretty well use any values you wish once again i used a thousand microfarads for my filter capacitor and the output capacitor i used a 2.2 microfarad capacitor if you're using polarized capacitors on the output and you're definitely using them on the input make certain that you have the polarity correct on them so now let's go to the solderless breadboard and take a look at a demo of the 7912 now i'm using the same arrangement for testing for the negative voltage regulator as i did for the positive one i've got my external transformer and my bridge rectifier and i've got it wired up on the solderless breadboard and the big difference over here of course is that the common side is the positive side and not the negative side and since we're used to working with microcontrollers and things where we always deal with positive voltages that is kind of backwards from the way that we think about it but otherwise the hookup is pretty well identical to that of the 7812 regulator the positive one now with the 7912 of course if i wanted to add a heat sink on to it as i mentioned you can't disconnect the heat sink that is connected to ground here because this tab over here is not connected to ground and if you do that you're going to create a very nasty short circuit so you'd have to use some form of an insulator for the heatsink now as you can see the output voltage is pretty good it's about 12.18 volts and it's staying constant which is actually a pretty important thing as well there are two things about a voltage regulator of course the one of the things is that it needs to deliver the correct voltage and this is a 12 volt regulator so it's pretty close to doing that but the other thing is it needs to stay consistent and if you look at this it's consistently a little bit off of 12 volts and that's a good thing too so again this is a easy regulator to use there are low dropout versions of this one as well if you want something more modern but if you need a negative voltage for your project either an audio project or maybe you're doing something with the old rs232 bus then this is an ideal regulator for you for a variable positive regulator i'm going to be using an lm 317 which again is a classic regulator and is available in a t020 package with the pin out that you see over here this is an adjustable voltage regulator it can output from 1.2 to 37 volts and has a maximum input voltage of 40 volts as with the previous regulator it's available in packages with current capabilities up to 1.5 amperes it has the short-circuit shutdown function and the overheat shutdown function in addition to the voltage regulator you're going to require two resistors to adjust the voltage output level this voltage regulator is available in a t020 package and in this package the tab is connected to the output once again you cannot use a grounded heat sink with this if you're going to use a heat sink that's attacked the ground you'll need to attach an insulator to it otherwise you could use a heat sink that is not attached to ground now here's the circuitry for our variable positive test once again we have the transformer bridge rectifier and filter capacitor note the hookup to the lm317 we also have a voltage divider built with a 220 ohm resistor and a 5k potentiometer and this is how we will adjust the output voltage again i have an output capacitor i've used a tantalum capacitor but you could use a ceramic capacitor as well and my filter capacitor i used a thousand microfarads and for the output i used 2.2 microfarads let's go and hook this onto the solderless breadboard and see the results i've got my lm317 regulator hooked up over here on the sawdless breadboard and you can see in the middle here is my little pot now this is the weak link in the chain this little pot actually because it's just a really cheap little trim pot and so because of its tolerance the voltage is having a little bit of difficulty locking in i was trying for about eight volts i've got 7.98 volts 7.99 volts right now which is pretty good what you need to do when you're doing something like this is to let it stabilize for a little while before you adjust it now if i turn the pot we can see lower in the higher end here i'll go all the way down to the bottom i've got a little over a volt over here and if i go up onto the other end i've got the multimeter by the way set on a 20 volt range it isn't auto ranging so i can go all the way up and then i go past the auto range so i can sweep it through a wide level of voltages and it does seem to work pretty well again the tolerance of the pot and the 220 ohm resistor are the key factors into getting some accuracy out of this now this would be a great regulator to use if you need a substandard voltage let's say you needed 7.4 volts or something to emulate the voltage that you would get out of a lipo battery or something you could set this for that specific voltage so these are great regulators to use when one of the standard ones doesn't fit the bill and you could also use it to build a variable power supply if you're doing that i would suggest using something like a precision 10 turn pot so that you could get more accuracy and stability out of it but even despite its age the lm317 is still a great variable voltage regulator so now that we've covered all of the theory behind linear power supplies it's time for the second half of this video and that's where we actually build a linear power supply project now the power supply i'm going to build has a positive output it is variable from about 2 to a little over 20 volts but it also can be switched into three different fixed voltage outputs and in my case they were 3.3 5 and 12 volts now you can build the exact same power supply i did and i've got links for all of the parts that i used in this power supply in the article that accompanies this video on the dronebotworkshop.com website however you could also use this this sort of as a guideline for building your own supply in fact that's really what i hope you go ahead and do you may already have an existing case or you may have an existing power transformer which is by far the most expensive component in the power supply and you'd like to put those to use so you could just use this as a guideline so i'm going to start off by taking a look at a number of different components some of which i used in my power supply and some of which i didn't but it'll give you a rough idea of the things that are available for building your own linear power supply so i've got some parts out here to build a power supply in fact i've got more parts than what i need because i'm kind of waffling between two different designs based upon two different chassis i've got the two chassis over here this chassis is an interesting one it's a taller chassis and it's consisting of a base and a cover and one thing you'll notice is that there's got these plastic sides to it over here that hold it all together there's also a front plate in the back plate of course and none of these are connected to each other electrically the top the sides and the front and the back so if you're interested in shielding this may not be a perfect chassis for you or you would have to shield it with wires going to each metal piece but with this i could use this transformer and this transformer would fit quite nicely into this chassis over here but it doesn't leave a lot of room for everything else the other chassis that i was looking at is this one over here and this is a longer chassis you can see the base over here here's a top for it and it's got ventilation holes in it and because of its height i cannot put this transformer in it's just a bit too tall so what i am going to do is i am going to use this other transformer and i'm going to put it in the chassis and this will fit but of course this is one that needs to be mounted on a printed circuit board or in my case just a perf board because i don't have time to design a printed circuit board now the advantage of this chassis is it's larger and i'll be able to put more things on it i want to show you some of the things i have for it now for the power entry i found this wonderful thing on amazon and i really would highly suggest using something like this if you're building a power supply it makes things so much safer this has uh this is the power plate at the back it has an input of course for a standard size extension cord or a standard size power cord excuse me it's got a power on off switch wired up and it's an illuminated switch as well and in a beautiful way it also has a fuse holder over here for a small fuse and i've got some other fuses over here because i'm going to replace this with a slow blow fuse because with a power supply there can be assertive current when it first turns on i don't want to pop fuses when i don't need to but this is wonderful because this is all assembled into one piece over here so you don't need to worry about the only exposed part really to the high voltage is this piece over here and otherwise everything comes out the hot neutral and ground so that you can wire it right to your transformer and if you're using this style of a transformer you could probably wire it and solder it directly and put some heat shrink over it and really reduce your exposure to 120 volts or 240 volts depending on what part of the world you're in another thing i'm going to use and you've seen these before i've used them in other power supply designs this is a voltmeter and ammeter and since i'm building a variable supply i think that would be a very handy thing to have to be able to know what voltage i'm at and also of course know what current i'm putting you could also get straight volt meter panels if you're not concerned about the current but i think that's an useful thing to know with a bench power supply now down over here i have a number of components and which ones i use will depend upon which chassis i go with now common to them of course is the voltage rate elator the ld1085 voltage regulator that i'll be using for this design and i've also got these little heat sinks for the voltage regulator and they just snap on to the voltage regulator and it's a lot easier than trying to rely on the chassis especially because the tab of this is connected to the center pin uh you can just have a heat sink free floating and i think these could be good enough to vent the heat off of this even at full power now for the front i'm going to use this multi-turn pot this 10 turn pot and that way i'll be able to more accurately dial in the voltage that i want now if i use the larger case and i'm kind of gravitating toward that i can also put this on and what this is a four position rotary switch and with this what i can do is i can allow it to select voltages i'll connect one end of the rotary switch to the 10 turn pot and the other three will go to these and these are just little trim pots and these are 10 turn pots and so they'd be more accurate than the ones i showed you in the experiment with the lm 317 and they should allow me to select between a number of pre-selected voltages and in my case i'm going to do 3.3 5 and 12 volts but of course you could select different voltages and you don't have to use a four position rotary switch you can get a number of different ones i got this switch up on digikey i believe and i couldn't really find an equivalent one on amazon but that's a canadian amazon i think the american one had one there also were some other rotary switches on amazon but they had more than four positions and i thought three extra voltages was quite sufficient you can also get these of course with less positions if you only want to switch between one fixed voltage and a variable one or you could just ignore it and just use this and have a variable output in your power supply because really the design of the supply is up to you what is it that you actually need from a bench power supply and that's the beauty about building your own stuff and otherwise i've just got the terminals that we're going to be using for the output of the power supply pretty standard stuff for banana plugs and i've got a couple of bridge rectifiers if i use the printed circuit board design i'll use this one otherwise i can bolt this to the chassis if i use the tall chassis over here so uh an example of the various parts that you can choose for your power supply again i'll probably use the smaller transformer i think i mentioned before these are 16 volt center tap transformers and so now let's go and prototype our power supply and then we will actually start building it okay here's my test setup where i'm just prototyping the design for my power supply now i'm powering this off of my low current transformer i'm using the center tap which is giving me about just a little under 18 volts ac i'm feeding the bridge rectifier over here here's the big filter capacitor here's my low dropout voltage regulator and i've got an led on the output at the moment i've got the output hooked up to a multimeter over here so you can see the voltage and the meter on the front here seems to be about 0.1 of a volt off of what the multimeter is reading and that's pretty persistent over the range and i can dial it down over here to about its lower voltage and we'll keep going down can actually dropped quite low and here's two volts and i still have a little bit below that 1.9 1.8 volts and so that's the low end of it and if i turn the potentiometer all the way to the other end and it's a 10 turn pot so it's a bit of a journey but it does allow you to dial in pretty precisely on the voltage that you want and the other end is up near about 20 volts here we go it says 19.5 here 19.76 so you can see this panel meter isn't exactly perfect and at that voltage even this led i have on the output is consuming enough current to register on the meter over there um i'll consume a bit more current let's just drop this down to maybe about 10 volts over here go down a bit more oops there we go here we are let's call it 10 volts and i've got uh my load over here three 16 ohm resistors and series uh power resistors i'm just going to try to put this across the output over here you notice how it drops the voltage like crazy this is a low current power transformer uh you can see that i'm reading current over here 230 milliamps which is close to what the transformer is rated at actually i think it's a 300 milliamp transformer now i'm just going to use this meter over here this is going to measure the input voltage right off the rectifier so the raw dc voltage if you wish and i'll disconnect the meter there i'm getting about 22.2 volts dc going into the regulator and right now 10.2 going out now i really haven't had any load on this and um there's a slight slight bit of heat there that i can detect if i really hold my fingers on it but right now it's pretty low and it should be lower than it is with something like the lm317 because of it being a low dropout regulator while it was bread boarding i used this potentiometer this 10 turn trim pod it's also a precision pot and this is a 1k value and i used it to determine the value of the other resistor so i got the maximum range with my pot over here so i've got a value of 510 ohms in there another value which i wrote on the schematic is 560 ohms you could try but i would actually recommend you throw a trim pot inside there and determine what the best value for yours is what you're looking for one is where you get the 10 turn pot turned all the way to one end and it's the maximum voltage and uh the other end the minimum voltage if you don't have it right when you start turning it up you'll get about halfway or three quarters of the way up and you'll be at the maximum voltage and the rest of it is useless so you get the best range by fine-tuning that resistor but if you don't want to do that try 510 or 560 ohms there's a little tantalum capacitor on my output here and of course the meter is attached so that the common of the circuit over here is not the ground over here because it uses that it's dropping resistor the measure current is on the ground side so you have to remember that when you're wiring it that what is negative here is not ground it's only what's negative on the other side which is confusingly a red wire but that's how it came so anyway i think i got a good design i'll show you the schematic for it right now and then we'll get to building it and putting it into a box now when building a dc power supply with a variable output a good choice for voltage regulator would of course be the lm317 as we've already seen this is a very easy to use device and it has very good performance however i'm going to be using a different voltage regulator the ld1085 this is a pin for pin equivalent of the lm317 but the ld1085 is a low dropout regulator and has improved performance and gives off less heat both devices have similar specifications but as you can see the ld 1085 has superior current handling performance so it will allow us to build a three ampere regulated power supply because the ld1085 has the same pin out as the lm317 our final schematic looks a lot like the test circuit we looked at earlier note that the power transformer connection is probably oversimplified as many power transformers have multiple windings to accommodate both 220 and 120 volts and need to be wired appropriately your power transformer will come with a wiring diagram that you'll need to follow for the bridge rectifier you can use any device that handles up to 5 amperes at about 50 volts you could use a bridge rectifier module or if you wish you could just use four diodes i used a 2000 microfarad 63 volt filter capacitor you can use any value similar to 2000 microfarads and you can use multiple capacitors in parallel as well this makes certain that you choose a voltage rating for the capacitor that's high enough for the 24 volt output that we expect to get from our bridge rectifier i chose a 63 volt one because i really like to over specify my capacitors you'll also notice there's an output capacitor in my case i use the 2.2 microfarad tantalum capacitor but any value up to 10 microfarads would work as well and you could also use an electrolytic capacitor note that both the filter capacitor and the output capacitor are polarized and need to be inserted in the correct direction you certainly don't want to wire these in backwards to control the voltage level i used a 10k 10 turn precision potentiometer but you could use a regular 10k pod as well you'll note that in the voltage divider circuit for the voltage reference the 560 ohm resistor has a notation by it that says it was determined by experimentation on my sawdust breadboard i used a 1k precision trim pot to determine this value in fact in my final design i'm going to keep the trim pot because it will allow me to just fine tune the voltage value you can fine tune this value if you find out that your maximum voltage is not at the maximum length of your potentiometer this allows you to get the most precision from the design now as it is this is a good variable power supply but i want to add two features to it i want to add some fixed and variable voltages to it and i also want to add an output voltage meter in order to do that i need to wire into a couple of points here point a is the voltage control point this is where we send the control voltage to the ic to determine the output voltage point b is the negative side which at this point should not be connected to ground if you plan on using an output meter point c is the unregulated 24 volts dc that we're going to get from our bridge rectifier given that we're giving it 18 volts from the transformer to add our voltage selector we'll be using a four position rotary switch three 10k trim pots and a 10k pot all of the potentiometers i used were precision 10 turn potentiometers the rotary switch is wired to select only one of these potentiometers and use it as the voltage control this is the wiring diagram for the volt and ammeter that i'm using my power supply if you're using a different module you'll want to check its wiring diagram as it may differ from this one in my module the negative side is used to detect the current and this is why we can't ground the negative output of the power supply note that the negative output on this is a red wire and i know that's confusing but that's just how the module is built the smaller red wire is the power supply for the module and we're connecting it to the unregulated 24 volt dc output of our power supply the positive output just flows right through and it is sensed by the yellow wire on the module now you may not want to have the output meter or even the multiple voltage selector in that case the diagram over here is all that you need and you can safely ground the negative side and so now i'm going to go and build my power supply no matter what style of chassis you use you're going to need to cut a couple of irregular shaped holes in order to mount a few things for example the power entry unit that has a switch in the fuse and the jack for the power cord is a very irregular shaped device that's going to need to be placed onto the chassis and the same goes for this meter over here this meter is also a just a rectangle but it's more than just drilling a hole so you're going to need a way to cut into the metal and you could do it by drilling a hole and putting a hacksaw blade in but that's rather crude and a little bit difficult to do i've got a bit of a better solution over here for cutting metal on a chassis like this or like the face plates on this one and it's something called a nibbing tool now i'm not certain if you've seen one of these before i actually had one of these years and years ago and it lasted me for a long time and this one is identical to it and the one that i had before i picked up about 30 years ago at radio shack i think this one i got on amazon it's an inexpensive little tool if you look at the tip of the nibbing tool when you press down the handle there's a little blade over here and it actually just eats into the metal and that's called nibbing and when you're using one of these things a couple of things you might want to note though first of all if you're going to do a lot of nibbing you're going to want to wear some gloves because first of all you're going to rub your hand against this little thing over here and you'll probably get a blister if you do enough of it but another reason for wearing gloves of course is that when you're cutting this type of metal you can sometimes get some sharp edges so you definitely want to protect your hands another thing that you might want to consider is the use of goggles as well because you are going to be chewing out little bits of metal and if they happen to shoot up towards your eye that wouldn't be a very pleasant experience so keep that in mind now i'm just going to do a demo here and i won't put the gloves on because i'm just going to do a very little bit of nibbing i've got a chassis over here and this is just an old uh chassis it actually was used for an ab switch for a printer back in the days when we used db25 connectors and it's a nice box so i've kept it and as you can probably see from the back i've already done a little bit of nibbing on it over here and i'll show you how this works i'll just go against an edge over here now normally if you start off with a blank panel like something like this you would end up drilling a 3 8 inch hole to pass the nibbing tool in but i have an entry for it here so i'll just do that and you can see that i just nibbed something over here and i can keep going if i want to i can go in and nib the next bit over here and i can just basically two my way through in order to make a hole and you can also once you've done a piece down over here for example you could go sideways you could put your tool in this way and then start going down so i could start nibbling in this direction and so you draw the outline of what it is that you want to mount and then you can cut the hole for it and it works in a reasonable size chassis most of the common chassis are a light enough gauge metal to support this thing and it's a really useful tool i think it was about 20 bucks on amazon so it's certainly not an expensive tool and a very useful one if you're building chassis for electronics now before you start cutting and drilling it's a very good idea to determine the optimum placement for the components in your project and you can do that just by taking some of the main components and fidgeting around with them now i'm going to be building the power supply in the low profile case so it's going to require that low profile transformer that is mounted on either a perf border printed circuit board perf board in my case even if you're designing a printed circuit board it's a good idea to take a perf board out of the correct size if you have it and just move your components around just to get an idea of the best physical placement before you start deciding where to lay the traces down now this is sort of the layout i've come out with right now the final product may be a little bit of an adjustment with it but i like it because the transformer is pretty well in the center of the chassis it's not quite centered but it's pretty close and that means that the weight won't be off on one side now over here i've got the power entry module and it's going to go back over here it'll have enough room for the wire to clear remember it's going to be a bit further forward and what i'm going to do with this is i'm going to just cut the ends off of this and solder these wires directly to where they go in this case onto the transformer and to the ground because these have lugs on the other end so i can disconnect them and i'm going to have to do that in order to mount the module anyway but i'm trying to avoid having connectors on the circuit board because i want every connection just to be a good durable solder connection if possible now uh on the rest of the circuit board i'm going to have to put the bridge rectifier some big capacitors and of course the voltage regulator itself i've also got my three little trim pots that i'm going to use to set the fixed voltages and there's a few other components but there's plenty of space there for it now i've decided that i'm going to place the potentiometer to do the variable voltage on this end and then next to it i'm going to place the switch which selects between the fixed voltages and between the variable voltage and over here not quite centered but in centered in the group of components on the front i'll place the voltmeter and ammeter and so that'll go into the panel over here so i'll have to do some measurements to figure out where to cut for that and on the very end i will place the output terminals that i'm going to be using so that i can actually have my output and so everything i think should be nicely balanced for weight it should be nicely functional and so i'm going to start making some measurements and start cutting the chassis now there are a number of design considerations when you're laying out the components for your power supply and i wanted to go over them some of them of course have to do with safety and the other ones have to do with performance now first of all safety in this design i've got one circuit board that's going to handle just about everything and nothing is soldered onto the board right now by the way i'm just trying to place the components now i've got all of my ac stuff all of my high voltage ac coming in over here and on this side of the board i've got all of my dc my lower voltage dc out the voltage regulator and all and this is actually an important thing to do to physically separate both the high voltage stuff and the low voltage stuff and you'll see that on commercial designs as well now another safety consideration in this design and it may just be unique to this design is that i have used brass standoffs to mount everything and i used extra standoffs by the way around the transformer because of the weight of it but over here in this corner i noticed that with my power entry i've got one wire this one specifically that comes rather close to this standoff now it's not touching it in any way but it's pretty close and it made me a bit nervous because of course the standoff is going to the chassis which is grounded and so i replaced that standoff with a nylon one so i've got brass ones everywhere else but a nylon one in this corner just to keep things a bit safer now another thing you want to consider is the thermal thing now this transformer is going to give off some heat it's in the middle of the enclosure right now now over here i've got my low voltage dc stuff and there's a couple of items here specifically the regulator and possibly the bridge rectifier that could give off some heat and i've got them up in the corner here and one reason for putting them at the back is that on my chassis i've got these vents and the vents are more toward the back so i wanted to make sure there was some airflow for those components otherwise you also want to try to keep things neat so in order to do that i've made this little assembly and this is my potentiometer and my rotary switch and they just go off to a dupont connector which will be down in the board here and that's an easy way to connect those otherwise my meter the one that goes at the front over here is going to connect in with the wires it was provided with and they'll just go directly to the board i'll also take a couple of output wires some heavier gauge wires and bring those to some lugs and i can use that for the output over here so this way my board can be a self standing assembly that can be unplugged this end over here will be soldered and these can unplug from the module and so those are some of the design considerations i had for my power supply of course you may have a different transformer a different box a different mounting arrangement and you'll have some unique considerations as well now one important but often overlooked aspect of building a project is how you label the chassis you need of course to label the chassis if you've got some controls or output jacks just so you know what they do but it can also make the project look a lot more professional and can definitely make it more useful now there's a number of different ways you could go about labeling if you're not that concerned about the appearance and just want to do something practical then you can use something like this and this of course is just a label maker it allows you to make various sizes of labels and it lets you choose from different fonts etc i've got a standard white label cartridge inside here right now but you can also get clear label cartridges and so those can look fairly good on the chassis because it'll allow the chassis background to come through you've probably already got something like this around your home or office anyway if not they're fairly inexpensive and they're pretty useful for a lot of things so you probably want to pick one up there are other methods however of making a little more professional type of a label one that i've used for years is this now these are rub on transferrings they basically you place this over the area that you want to place the label and then you just use a pencil or something to rub it on the top and it transfers the labeling onto the bottom and you can make a pretty professional looking label using this stuff now this is getting a little harder to find it used to be very easy to find rub on transferring you went to an art or a drafting supply store and they'd have acres and acres of this stuff mostly under the letra set brand name although it's available under different brands for example this is not letraset brand over here however this has become a little more difficult to find in recent years because the original use for this was for drafts people who were doing this on their blueprints and their drawings and they had to make labels on them now of course they do all of that with computers so there really isn't that much of a need for it so it's a little bit difficult to find i actually found a good source for this is ebay and there's a place out in the netherlands that seems to have quite a bit of it so it's not bad stuff to have around your shop for labeling chassis another method that i just recently tried and i tried it on this project is to use this laser water slide decal paper now as the name would imply this is for a laser printer but you can also get it for inkjet printers as well too but it's a different type of a paper now water slide decal paper is interesting stuff if you've ever used model airplanes or built some other type of a model you may have run into water slide deckle paper it isn't the kind of decal that you just peel off the back and stick instead you soak it in water to remove the back and then you press it down and it's a bit of a technique that will require a bit of experimenting i used it on the project today and i had to do it a couple of times before i got it correct now this laser paper doesn't require anything except the paper itself and to put it through your laser printer and it wasn't the easiest thing to put through my laser printer if you have a printer that can do a straight through path it would work a lot better mine unfortunately isn't but i managed to print a few of them and i managed to destroy a few of them too before i got the technique of putting it on with the some of these for example the ones for inkjet you actually after printing it need to spray on a compound in order to thicken it up a bit before you peel the backing off but you don't need to do that with this laser paper now regardless of whether you're using this or rub on transfers another thing you might want to take a look at is something like this and this is clear transparent um plastic basically that you it's a clear finish that you can spray on to things and i've used this for a number of different things it's great to use on chassis because after a while with wear and tear things like rub on transfers are going to wear off and this stuff can put a coating over them that's transparent and yet protects them i use them for other things too if you notice the big blue pegboard i have behind me with all the tools well that's sprayed with a couple of coats of this as well and it's lasted about six years it's very durable so this is easily picked up at any hardware store i think you can also get it at amazon and i'll show you the results i use the laser water slides deco paper and this was what i ended up with for my my chassis now it's not the most perfect looking thing in the world but it does do do the purpose i mean it gives me the labels for all the different controls i've got the positive and negative output i didn't really need a label for my ammeter and voltmeter because its function was obvious but the voltage selection over here that's a good thing to have labeled and the adjustment voltage is labeled and i use the water slide decal paper now if you are using this stuff you just soak it in water for just a few seconds it doesn't have to be in there for long then place the entire thing down with the backing on it sort of hold one edge of it and start pulling the backing off and go along here and smooth it down and as i said it took me a few tries but i ended up getting it so i've got my chassis all nibbed out and cut out it's nibbed out in the back over here for the power entry module and it's drilled so it's ready to go and it is all labeled as well so i'm now ready to do the final assembly of my power supply i've got my circuit board all wired up and i've got my chassis all prepared it's all labeled and all of the components that go on to the chassis are there right now so let's take a look at the circuit board now i did mine on perf board but of course this would be an excellent candidate for a printed circuit board and as you can see the dominant feature of it is of course the transformer with its input side over here it's output side over here now my transformer needed to be strapped i needed to put the two input coils in parallel because i'm using 120 volts had i been using 220 volts it would have gone in series and i also had to put these coils in series to get the full output voltage this is an 18 volt transformer now you can see on this side of the board i've got all the dc components the rectifier the voltage regulator itself the three parts for adjusting the three voltages the fixed voltages that i have and also another trim pot and this is the one that i'm using for the other resistor in the voltage divider circuit i just decided to use that so that if i needed to fine tune it i could just use the pod over here and here's a connector that goes over to the two controls i have in the front and otherwise i've got two leads to go out and snap on to the meter to the ammeter and these two leads here of course are the output leads themselves this is all the input stuff for the uh power for the ac power and for the ground and speaking of ground i've got something a little bit odd over here i guess because i'm using a perf board not a printed circuit board what i did is i took a lug i took a circular lug and put it down here in the board if you can see that and then on the bottom of it over here that i've had some wires coming in from the lug this is my main grounding point and i wired this up as you can see mostly with this 20 gauge wire this thicker wire over here is 16 gauge wire and this is actually silicon wire i have it left over from my outdoor robot project and thought that i would use it but otherwise i've used 20 gauge wire except down over here if you can see these wires are a little hard to see because i used green on a green background but these are the wires that are just using wire wrapped wire and they're going to the resistor and switch because this is a high resistance section in the low current section so wire wrap wire is by far easier to work with but i use the heavier gauge wire for everything else because of the current set we're going to be carrying and so that's the circuit board itself i'll just put this aside and we'll look at the chassis now here's the chassis and as you can see i've got the front panel of the chassis all done i've got the uh switch over here to select my voltage and i've got my voltage adjustment i've got the ammeter mounted here and i've got the uh the output terminals the positive and negative mounted over here and you can see that inside the chassis i put some plastic tape just down under where the circuit board was just another precaution because the chassis is grounded and if anything got lodged underneath the board and touched one of the 120 volt leads i know it's very very unlikely but you do have to remember it's being worked on in a workshop there could be a little nut or something that would make it through the panel through the ventilation holes into that you've got to think of the worst case scenarios and try to prevent it from happening so you can see also i've got the power entry on the back over here that's all set up as well so this is good to go this one post over here again is uh being done with the nylon post everything else is brass i'm ready to mount my board on connect it up and then i should have a working power supply and so here's our final product our power supply it's all plugged in and ready to go i still have the case off of it but otherwise it's complete all of the front panel controls and the meter are hooked up and so is the 120 volts ac and i do have 120 volts on it now but i haven't flipped it on so let's just do that and as you can see we have a output voltage it's reading 9.9 volts i'm on the adjustable voltage over here so i can set my voltage down quite low let's see what the minimum level is it's about 1.4 volts and if i go all the way to the other end and it's a 10 turn pot so i've got several turns to give it and we're near the end there we are 23.5 volts so that's almost the voltage that we get off of the rectifier and that's because of course we have a low dropout regulator i'm going to set this back down to about 10 volts now and the 10 turn pot makes it very easy to dial in the exact voltage and there we go there's 10 volts now of course i have the selector switch over here so i have 3.3 volts 5 volts and 12 volts and those levels naturally were set by those three trim pots on the circuit board so if you want to use different voltages you can set them accordingly now i want to do one thing here now i've got a resistive load and i'm going to put it onto the power supply right now at 10 volts and it's pulling about 0.7 of an amp here i want you to notice that the 10 volt stayed at 10 volts unlike the experiment that we did on the solderless breadboard where we had a very minimal current transformer on there this transformer can give out a lot of current so the voltage stays steady and that's true when you go to the fixed voltages so well there's 3.3 there's 5 volts and there's 12 volts in all cases the voltage stays steady even when it's under load so it looks like it's a working power supply the only thing remaining to do is to put the cover onto it and then i'll have a new test instrument for my workbench one that i built myself now one very important consideration when building your own linear power supply is that of safety you are after all connecting up to the ac line or mains voltage and depending upon where you are in the world that can range from 110 to 240 volts ac and that voltage can be deadly and so be very careful when you're working with that never work on it when it is live i would strongly recommend using one of those power entry modules like i did they're inexpensive and they have a solution that provides both a fuse and a switch and an enclosed module and it doesn't expose the ac and if you're at all nervous about working with alternating current of that level then simply don't do it use this video as a reference instead now if you want some more information about linear power supplies or if you'd like to get the parts list for the one that i constructed you'll find all of that on the article that accompanies this video on the dronebotworkshop.com website there's a link right below the video to that article while you're on the website please consider signing up for my newsletter it's not a sales letter it's just my way of letting you know every now and then what is going on in the workshop and it's great to be able to communicate with you and all i need from you for that is your email address another thing you could sign up for if you want to discuss linear power supplies is the dronebot workshop forums and you'll find a number of like-minded people out there who can help you with your electronics projects if you're running into a problem or who would just like to hear about the latest thing that you're building and of course the forum is free to sign up for as well and finally if you haven't yet please subscribe to the youtube channel i always love getting new subscribers it's really easy to do all you got to do is click the red subscribe button and then hit that little bell notification and assuming you've enabled notifications on your youtube you'll get notified every time i make a video so until we meet next time please stay safe out there please take good care of yourself and i will see you again very soon here in the dronebot workshop goodbye for now [Music] do [Music] you

Channel: DroneBot Workshop

Views: 568,333

Keywords: Build a DC Power Supply, linear vs switching power supply, linear power supply, dc power supply, power supply, build a power supply

Id: nVpgO6xEF8Y

Length: 72min 38sec (4358 seconds)

Published: Wed Jun 29 2022