AE#21 Building And Using A Dim Bulb Tester

greetings this is already from artifact electronics in this episode we'll be looking at this useful little device that it's called a dim bulb tester or DBT for short now the name is a bit of a misnomer because it may lead people to believe that this is a bulb tester but it isn't it actually is a device that will detect shorts in devices that you plug into here and if a short exists it will indicate so by lighting up the ball brightly to its full brightness actually and the the bulb will leach most of the power flowing through the circuit so that there's very little power left to go into the device with a short thus preventing damage in that device so whatever you have a new piece of equipment you should not plug it directly into the wall but rather plug it into this and then power it on and it will tell you very quickly if there is a short while protecting the device now notice that this doesn't have a switch on it and the reason for that is is that essentially the switch on the device under test is what becomes your switch when you plug this into the wall and there is no load plugged in it's an open circuit and it won't really do anything I mean it is a live outlet but it there is no current flowing in this and when you turn on the equipment that is when a low starts appearing here that's when this will start working then do its job of course right off the bat safety comes first this is powered by your AC voltage and even though when put together properly your exposure is pretty much limited to plugging something in here and then unplugging it but still be careful there's a lot of voltage in here and it will bite so how does this work well this is basically a light bulb wired in series with this outlet you have to use an incandescent bulb CFLs and LEDs will not work and the whole thing works on the principle of how a filament in this bulb reacts to power draw now we need to match the bulb wattage to the equipment under test and a rule of thumb there is the bulb should be approximately rated at hundred and fifty to two hundred percent of the power requirements of the equipment simply said you have a device that this is rated at 100 watts you use 150 to 200 watt light bulb and it'll work for you the way it works is when the load is plugged in here if the load is drawing excessive power basically the light bulb will draw all the power it can and light up brightly telling you that there's something wrong while at the same time starving the load from power so that the load can't self-destruct or blow up or whatever that's the entire principle that this that this tester works on and what is so good about it is it takes the ax filament about a hundred milliseconds to heat up so yes it is not ideal it will not instantaneously cut power but there's only so much damage that can be done in 100 milliseconds and that's what makes this really effective the first thing you should do is have a good understanding of how bright this bulb gets when the load is is that a dead short and the easiest way to test that is I have a test plug here that is as did the blades shorted together and when I plug it in this thing should light up at max and it does its drawing all the power it can get and that there's very there is still power running through this of course the circuit has to be complete but it's very little and even though this is hard to show pictorially I had this running before for a while and now it's been running and if you touch this or the plug there is you cannot detect the rise in temperature and that proves that essentially this is saying uh-uh you're not getting any power over here I'm using it all up for myself so how do we wire this well very simply this is the AC plug that goes to the wall and the part of the plug that has the that is fatter has the protrusions on it the a polarizing part is the neutral light and that goes straight to the outlet the slimmer plug goes through the light bulb and to the other side of the plug and that's it the only other thing here is is that you're planning on testing equipment that does employ a grounded plug and what needs to be done is your power cord will obviously have to be the ground of type and what you do is you connect the ground from the power plug directly to the ground on the outlet so called cheater plug here you should never defeat ground on a equipment that requires it and vice versa you should never force ground into equipment that doesn't have it a lot of people think that oh this is old equipment and it doesn't have a ground plug it's unsafe let me just give it a three prong plug and connect the ground to the chassis and the equipment that can be lethal to the equipment as well as yourself because there was no standard in the old days as to what potential the chassis sat at it could be at ground or it could be at 1oak directly connected to one of the legs of the AC power coming in so rule of thumb is don't play put the number of prongs on the power plug in your equipment but if you wire this in you could theoretically build this with a three prong plug and it just wouldn't come into play when you plug something in that didn't have a three prong plug the ground does not affect operation of this in any way shape or form so what do we need to build one of these well first we need a base I use the wooden base some scrap what I had and the dimensions are dependent on the size of the parts that you're going to get now what we all need is a power plug or power plug and wire and what I did was I just bought a in an expensive extension cord non grounded and snipped off the female part now remember we talked about if you are trying to use grounded equipment then you better buy an extension cord that has a ground plug and wired it properly the outlet itself you will need of course an outlet a mounting box for the outlet as well as a cover plate and if we look in here remembering the schematic is that we have one of the legs of the AC coming in and going to one end of the load then we have a wire coming out of here going to the socket and the way that works is removing the socket which is another part you're going to need to look underneath we will see that the other leg of the AC chord is tied directly to one of the contacts here and the remaining contact is wired to the second side of the outlet and that's pretty much it the only other thing to note is that I have to use spacers to mount the lamp circuit because the contacts and the wires kind of protrude from the bottom of it and you can't mount that thing flush on this baseboard but they let your imagination run wild and again it's very simple to build and you should be able to get all of these parts at your local hardware store well time to go for a test drive our subject or guinea pig is going to be a Heathkit scope it's a model ole - 12 circa 1957 and entirely tube driven now let me caution you I'm not going to repair make any repair attempts on this in this episode because I'd rather concentrate on the D BT and other methods for now but if you stay tuned long term I will devote an episode to checking this guy out and fixing it but he is functionally enough for us to do a test so as I explained the very first thing we need to do is well of course plug in the device under test which is the scope into the outlet plug the D BT into AC and insert a bulb now the largest water is bulb I have is 100 watt bulb which is a little bit low I mean this thing is rated at 80 watts well actually 480 watts I'm a 50% of 80 watts is 120 watts so yes we are low but let's give it a try anyway and if that doesn't work figure out what to do next so first we do our brightness test to get our eyes used to how bright this bulbs getting and there's the bulb lit up at a hundred watts because I just inserted a short now and you're probably gonna have to take my word for it because this bulb doesn't show up too well on video but I promise I'll tell you the truth but now it's time to turn on this guy and the ball slightly surged you could see it glow then it went off but the tubes are still warming up and as the tubes warm up now it lights up again and my best guess is is probably at about 10 or 15 percent of max capacity on this let me briefly turn off the room lights so you can see for yourself so it is leaching power but the scope came up and it put a well that's interesting look at what happens to the trace when I touch it hmm well that is something that's going to have to be left for the actual repair video but what of course disproved was that the scope is not shorted and as a matter of fact this one actually comes up and puts a somewhat something that kind of looks like a trace or a dot up there even though it also functions as a touch plate right now and that's gonna be interesting to look into however what we do want to do is get this to a point where the bulb doesn't light up at all I mean there's always going to be a little bit of power on the bulb because otherwise you know it has to complete the circuit but we absolutely want to minimize that so let's try to figure out what to do when this is the highest wattage single bulb I have available right now and what we're going to do is another little device I dealt while billed is overstating it another little device I put together from off-the-shelf parts and I think I call that the DBT expander and I'll show you what it does well first let's take this ball down and remember I spoke about electrical safety and I need to add to that be aware and practice thermal safety too especially a 100 watt bulb gets very hot even when it's only lit up partially and it's cooling down but I can still feel it so what we're gonna do here is here is the expander which looks just like a power strip which is exactly what it is what we're gonna do here is instead of using a single socket we are going to use this socket now remember it is plugged in but it's off there's no life circuit in here and to do it correctly I should unplug it but I didn't use a little adapter and you plug in the power strip and then you put a complement of light bulbs in there to make up the wattage that you're looking for and I got a little socket to outlet convert it here first I'm going to put in a hundred watt bulb and then I'm going to put in a 40 watt bulb and that basically make it look like there's a single 140 watt bulb in here of course it's never that simple but for the purposes of the test that should work so now what we should see is maybe they'll light up a little bit again but once it reaches a quiescent state of operating then these should be getting slightly warm to the touch but they shouldn't light up meaning that they're leaching a minimal amount of power from the whole thing so now that we're ready for that let's go ahead turn it up this is the intent not just paranoid but intensity too and give it some time we can see the thought appear on the screen and it's gravitating towards the center now notice my hand is on here I think I'm still changing the width of the dot yeah I am it's gets a little narrower there but looking at it I can see and again it's probably difficult to see on the video but the hundred watt bulb is lit up a little bit I can actually see filament and this one is also getting warmer so actually both filaments are lit up at a minimum of course so let's go even larger let's remove the of 40 watt bulb and put in a hundred watt bulb now that should give us a minimum of power leach on the bulbs and make the scope be able to fully power up so staring at the bulbs again I still can't see the filament light up wait for the dots to gravitate towards the center which seems to be what it tries or wants to do let's see if my hand still has an effect on it and it does I cannot see any lit filaments in the bulb they are slightly warm and again that's to be expected because the current has to flow in order to complete the circuit but they're not lit up at all and I'm assuming that the scope is getting full power right now so it works it told us that there is no short in the kind of disproved the put in a bulb that's 150 to 200 percent of the rating in there I went higher I went to what 200 I went to 250 percent of the rating would with a 200 well with 200 watt bulbs ie a 200 watt bulb and that seems to work so one last thing to do is is to take the DBT out of the circuit and plug this guy and directly into the wall and see what happens to the display then how much do these bulbs falsify operation of this device so I unplugged the scope from here plugged it directly into the wall one other thing to be aware of is on two powered equipment you shouldn't power cycle devices like this quickly ie don't turn the power off and turn it on immediately again that may result in a blown fuse because the tubes are hot and you need to give it's almost like a a computer where you want to let the reset capacitors for a processors discharge I generally turn it off wait for a little while and then turn it on instead of rapidly turning on and off so now DVT is completely out of circuit and now we can turn on this guy and see how similar the dot on the display is going to be as compared to one we had the 200 watt bulb load let's call it and we can see it's it's still a little bit different it is wider than it was before and the interesting thing would be what happens if I put my hand on it now and I don't know if you can see that but it's actually getting slightly wider when I touch it now so there is interaction with the DBT and to do proper tests of repairs you've made or if you're just testing testing component values or whatever be aware that the DBT will cause I shouldn't call it aberrant behavior but the behavior will be different when it's powered off the DBT even with the large bulbs as compared to being plugged directly into the wall I mentioned that I'll show you an alternative to the DBT and that's this guy this the instrument was featured in an earlier episode and explained pretty much in depth so I'm not going to repeat all of those explanations just give a quick rundown of it as it relates to us catching shorted devices so this has a isolation transformer followed by an auto transformer and an auto transformer looks a lot like a pot except that it doesn't have a tapped resistor but rather a tapped winding transformer winding and you can change the AC voltage that's coming by essentially going to one side of the of the winding and to the tap here and you will be able to adjust the AC voltage in this case the range goes from 0 to 150 involved and what that means is that if the line voltage here is 17 or 117 volts I can actually adjust that between 0 and 150 volts and that should immediately bring up the idea of gently increasing the voltage on an unknown device it also has a an ammeter here so what you can do is plug the device in here set the voltage to zero turn on the device and then gently increase the voltage while closely watching the ammeter here and if at any point the ammeter start succeeding where it should be at or exceeding the max that you've previously calculated if you have good reactions then you will turn back this Nile immediately the voltage will drop to zero and you just saved the instrument but if your reaction time is like mine and the instrument will probably go up in smoke if it's shorted before turning this back so this also has overcome protection here and you just set a value on this dial the thing is rated at 5 amps so basically if this is pointing straight up you just set the overcurrent protection to two-and-a-half amps and so if you're too slow to turn this back this will catch it and turn off power to the device so let's have a look at that turn the power on in here now of course we already know that that this scope is not shorted and as much as I'd love to demonstrate it I'm not gonna put a shorting plug in here I will just show you how how this would work and you go in here and start increasing the voltage notice on the left side meter you can see the voltage come up and I can now see the power light light up on the sculpt and we'll just increase it till the analog meter tells us 117 volts which it is now current is very low the dots appearing and we can see the current meter moving up and what it's telling us that's using up about half an amp here now if at any point in time if we saw this thing all of a sudden start moving uncontrollably to the right most likely the over current protection would have reacted before we could and shut off power so this is exactly the same principle in saving the scope as as a DBT I can't make a direct comparison because I couldn't find the specs on how quickly the over current protection reacts on here we know that it takes about 100 milliseconds for a ball because that's how long it takes for the filament to light up and 100 milliseconds compared to the speed of light is a rather large fraction but it is better than not having it at all I mean I'd rather have a hundred milliseconds of shorted over shorter device being powered up than having a shorted device that's powered up for a much longer time than that which is what it would usually take a person to realize that it shorted and turn off power to it so functionally this would do the same obviously this is a bigger device this is a heavier device it is a more expensive device and yeah being bigger it's going to take up more space on your bench but it does have a built-in that isolation transformer which is a great safety feature so I think what the question boils down to then is it really worth having one of these and the answer to that is a resounding yes for for just testing whenever you're working on anything electrical you should be using this even if it's only for the fact that it has an isolation transformer in it but so of what happens to the DBT now opinions differ on which one is better for short detection and which one is more effective at preventing damage from a an equipment short and I'll just add my five cents to it I think that DBT is excellent for detecting shorts and limiting current to it very quick limiting current to the device very very quickly and another thing that this is very good at is of course if you are measuring light voltages inside the instrument and you're like me and you either short the terminal strip or even worse than that you drop a small metallic tool inside it this will kick in just like it did before because it'll see the short and cut power and reduce damage now yes this does the same thing but you may notice that I'm not going to have a section on to build your own because that gets a bit more complicated expensive and and it's not a realistic thing to build at home so the final verdict is in my opinion is that this is to be used to do a quick check on when you first power up new equipment and also do a check after you do repairs if you found something that was shorted and you're working on the power supplier you're swapping tubes or whatever if you still power it up while it's connected to this to do a short test to see if your repair was effective or not after that you should most likely gravitate over towards this guy and have him on all the time because of the isolation transformer in it now one of the shortcomings of this instrument is that are the analog meters they are not very accurate I mean if we look at this right now with the scope warmed up we've got 117 volts and approximately half an amp of current being drawn so we'd have to put a meter on it and create a spaghetti mess on there on the workbench but that is a better way I found and that was also featured in the original treatment of this and that is a friend via kilowatt which is a home energy monitor but it does have settings for voltage and bridge and everything and it essentially becomes a digital meter there is also second one I have and that one out plug on this side and if I wanted to measure current of how much current is exactly being consumed and see it in digital form I plug the scope into here so again you see these are all are doing anything but that's because they're powered off of this device and the voltage is at zero so what I'm going to do is increase the voltage and they're gonna kick in at about 50 volts ac but now what I can do if I can digitally see the voltage and set it to 117 yeah and one thing you will notice very quickly is that the refresh rate of the display is not very high so if she were staring at these while turning up the voltage to catch a short it has the same that would be with the amperage here the amperage changes as slowly as the voltage so catching that manually and turning off the power when you have a short just isn't going to work but if we look at this if we remember and it would be kind of see the meter on this side it's just above half an amp but this thing tells us that we're at 0.72 amps so it is showing low and if we look at the meter from where I stand it looks like about a hundred and twenty and this is saying one hundred and sixteen point seven so the wall voltage accuracy of the meter is a little bit better but the the the amperage and the amperage accuracy is not very good but with that said you can with these two guys plugged in this does become a lot more useful albeit with a slower update rate so you don't want to be using it for short detection for short detection probably for short detection you want to leave the Volt that the kilowatt in the voltage side and see that digitally but you want to remove this one and actually observe the meter because you're looking for trends you're not looking for exact measurements when looking for a short but rather for the meter starting to deflect rapidly and it'll work for that it would in conjunction with the air current limit limiter here it will do the same job I called the component in here that lets you adjust the the AC voltage an auto transformer but some of you may not be familiar with that term because it is much more commonly known as a very AK and the variac was actually the brand name of a company that made these things or made the early versions of this but somehow it stuck kind of like scotch tape so there is a very a kin here and that's what we affect by turning this dial so now we have these two devices and I think I was vague enough about when to use which this is great for a quick and dirty for a quick and dirty short test on initial power up of a piece of equipment as well as after repairs but for ongoing repairs you should be using something like this but since you know these things cannot be built by yourself and a little bit harder to come by this is a nice inexpensive solution that should be accessible to everybody and should save your equipment should the disaster try to occur now what do we do if we want to be extra safe about shorts electrical shorts that is we use both of these together so the DBT is powered by the monitor here and then we plug in the device on the test to the DBT and now we have both the short the short circuit protection or the over current protection and here working as well as the bulb filament working for us and we have double protection against shorts and the way that works is let's make sure this is off we turn this guy on and he stabilizes at what 117 point 8 or 6 or whatever and now looking at the bulb as I turn this on the bulb surges very briefly our current rating is very low and when we saw that this meter showed about half an amp for proper operating conditions we do see the dot or the trace show up slowly drifting towards the center the current has increased a little bit but it's still way off from 0.5 amps and that's caused because you I'm using only a single 100 watt bulb and it is lit up probably again around 10 or 15 percent and this current would most likely go up if I brought back the the DVT expander and used 200 volt of bulbs but this should provide us with maximum protection against short circuit damage hope you liked it and that hope you learned a little bit from it or you know hope you were able to make sense out of this and if this was an enjoyable experience please click the thumbs up button and if you haven't already make sure to subscribe because not only will I be looking at fixing this guy up here but he has some cohorts which also needs fixing and those are also going to end up on the bench and I'm pretty sure each one of them is going to be worth an episode and incredibly interesting to watch on how they get fixed see you later

Channel: Artifact Electronics

Views: 30,348

Keywords: Dim, bulb, tester, artifact, electronics, variac, autotransformer, vacuum, tube, power, short, amp, oscilloscope

Id: k5oNQ1etN2c

Length: 35min 34sec (2134 seconds)

Published: Wed Oct 18 2017