Power supply ripple and how to measure it

hi there and welcome I just received a question on YouTube how to measure ripple on an oscilloscope and I think that is a excellent topic for a video so in this video I'll be talking about the power supply design because Ribble is one of the unwanted byproducts when you do a power supply design so typically to do a power supply you would have a 220 volts in 110 for the u.s. going through a transformer and if we want 12 volt out the transformer winding ratio would be something like 1 to 20 for Europe and 1 to 9 for the US and if we look at the output of the transformer like this 12 volt AC transformer it will basically look like this if you can excuse my terrible sine wave drawing skills but the output should be a sine wave and the peak values should be plus 18 and minus 18 the reason the peak value is not 12 volt is that this sine wave here with a peak value of 18 plus minus 18 will be able to generate the same amount of power as a DC power supply with a straight 12 volt output so of course we don't want this sine wave on the output of our power supply we want to run some electronics or some other circuit from this power supply so what we want is in this case 18 volt DC on the output or 12 volt but if we want 12 volt we have to regulate it afterwards but I will talk about that later but the first thing that we would think about would be why don't we use a diode because diodes we can just get current through one way so at least that should get rid of some of this negative voltage and that's exactly what it does I have drawn the circuit here and this is called a half wave rectifier and the reason it's called that is because once there's a positive voltage here the diode of course will conduct and we will have current going through and we will see the voltage here on the output so yeah this is the output we out and this is what we're displaying here and of course over time so we will have these when the diode conducts we will have voltage coming through and when the voltage is a negative polarity from the transformer we will not have anything because the diode will block it so we will get these half waves half of the sine wave and that's why it's called a half wave rectifier now we are almost there because what we can do now is if we can store some energy between these pulses we will have a nice DC on the output and the way to store energy is using a capacitor and I've drawn the circuit here and as you see the capacitor will charge up on the first pulse from the diode and then the capacitor will keep the voltage across its terminals even when the pulse has died down so basically it will just stay here and we have a very nice DC voltage now the problem we have here is that in this circuit of course we are not driving in any electronics we are just making a nice DC supply but of course we want to run some electronics we want to have a load and I have drawn this load as a resistor but it could just as well be a LCD display or a computer or microcontroller or whatever anything that requires DC to run so what happens here is that we will of course charge the capacitor during this the first pulse the first wave and then the load was will use some current and it can't get current from the transformer it has to get current from the capacitor so the voltage on the capacitor will start dropping slowly then we have another pulse from the diode it will charge again and then it will discharge slowly and that is what we call the ripple the ripple is the noise on top of the DC so to speak the ripple is the maximum and the minimum value of this voltage drop on the capacitor now we have some conditions that must be met for this to work of course the capacitor has to be large enough we cannot accept that this is a ripple noise or the voltage across the capacitor goes down to zero then we have no power supply at all so if the load is too big or if the capacitor is too small we will have a terrible ripple on this output signal another thing we need as a condition is that the transformer must be large enough if the transformer cannot charge the capacitor during this period here then we have a problem also we see the bigger the capacitor the less ripple we will have now large capacitors are expensive so someone came up with the clever idea here and that is called the full wave rectifier and using that we use four diodes instead of one and then we have positive voltage running through there and when this point here is more positive than that one that means we need when we have the negative when we have the negative part of the sine wave then the other diode will conduct so basically what we have is a all the waveforms go through and that is called the full wave rectifier or the rectifier bridge this is called a rectifier bridge so of course with a rectifier bridge like this we will charge the capacitor twice as often if we compare these pulses to these here we will have additional pulse here from the negative period on the transformer of course you're also charging the capacitor twice as often so the ripple noise will have a frequency that is a double that of the line frequency so if you have 50 Hertz in Europe your ripple will now be 100 Hertz and in the u.s. you will have a 120 Hertz riho so the question is probably how large should my capacitor be and a good typical value is four thousand seven hundred microfarads for full wave rectifier you can go lower if you use a voltage regulator on the output but I think this is a good starting value and then now we get to how to measure ripples because typically like in our case we have 18 volts output from our across our capacitor and when we draw a current we have these little ripples here so it can be very difficult to measure because you're basically measuring something like maybe 0.2 volts on an 18 volt scale on your zero scope so basically what we have to do is if we can subtract the 18 volts the ripple will be moved down to around here and then we can amplify the signal and we will see the ripple something like that and this is very easy to do really on an oscilloscope and I will show it on two of my little scope I have a very old analog source oscilloscope and I think it's maybe 30 years old and even back then you can do this very easily ok so what I have here is a little load that I made and it's basically just consisting of some resistors in parallel I could have used one big power resistor but I didn't have that so I just put some quarter watt resistors in parallel and that is equivalent and then I have this little power supply and as you can see this is 220 volts in and it has DC out 100 milli amp and after we plug that in we will take a look at the oscilloscope and see whether we can find any ripple and also see how we can measure so let me just plug it in and then we will get started ok and now my oscilloscope is switched on as you can see this is an old analog scope from about 35 years ago and it's flickering a little bit because of the camera and the update rate of the scope and what-have-you but I think it's good enough for our purpose here and I hope you can see what's happening now on this is zero scope it has an input selector here that shows a see ground and DC and the right now is ground and you can see my amplitude per division it says 0.5 volts per division but my probe is set to 10 times so we actually have 5 volts per division so if we look at the screen itself we have 5 volt here 10 volt 15 20 25 so on and so forth so if I switch the input to DC which is the normal mode you can see that the trace is jumping up from here to here which is 5 10 15 18 volts so that follows the theory very nicely you may also be able to see that the line is jumping up and down a little bit and of course that is the ripple that we want to measure now as I said the easy part is to actually remove the DC remove that big jump here and it's really really simple you just change the input mode from DC to AC and as you can see is still moving up and down a little bit but now it's moving up and down around the zero and I can move the 0 up like that with this a little knob up here and then I can increase the gain and basically there you have it that is our ripple and there is no of course cursor or anything for measuring stuff here but we can count we can count divisions here and there's roughly 1 division and we have 0.5 here we have 50 milli volts so we have about half a volt peak-to-peak showing here so that is our ripple using this circuit and as I promised we'll do the same on a digital oscilloscope because that is a little bit more tricky most of the time the DC AC input selector is done in software ok I'm back and we now have the same on my digital storage scope and basically this is zero down here and I have 5 volts per division of course in a digital storage scope you can key in your multiplier gain in your probe and I already set up this scope for 10 time probe but let's say plug in the signal here and we see the line jump up just like on the analog scope and we have 5 10 15 18 volts on the signal here and again you can see it's a little bit unstable it's kind of jumping up and down so that is indeed the ripple now on this oscilloscope you have to find the ACDC selector in the channel 1 menu channel 1 and what can we find there there we go it says DC we can now change it to AC and you can see as soon as I did that the trace moved down and again I have a knob in this case it's here is do we have it there then I can move the signal up again and then we can use this thing to amplify so I'm moving it down and there we have it I said on my analog scope we had about 0.4 what 0.5 volts ripple and we can do the same here we can measure of course we can measure of course division so we have about 2 divisions which is 400 millivolts some scopes like this one I think it can also measure volts peak-to-peak and immediately it does that it says 430 millivolts so that is our ripple here another way on a digital storage scope is to use cursors and where do we have those moved here mode manual cursor and then we can move them up and down and we can put one cursor here at the top and we can move the other cursor up and we have again about 412 milli volt peak-to-peak so yeah basically to measure anything that is buried so to speak in DC you simply just change the input to AC and the DC will disappear and then you can scale up and down the AC part of your signal just like I did here of course if I change back now I have 200 millivolts per division if I change back to DC the 18 volts will be off screen it will overshoot the screen you can see it immediately just BAM in Heat here's the top of here this 12 volt adapter that I have here this little thing here of course it says 12 volts but actually the output as we saw is 18 volts so if we have to regulate it down to exact 12 volts we will use a voltage regulator and if that's interest I will do a video about voltage regulators otherwise - shereena who asked this question and all the people who are watching thank you for watching and see you again soon

Channel: NatureAndTech

Views: 56,146

Rating: 4.8734179 out of 5

Keywords: electronics, power supply, ripple, measure, test, PSU, theory

Id: aAfXZID5ikU

Length: 14min 47sec (887 seconds)

Published: Sat Aug 20 2016