Isolation Transformers: Internal Design and Safe Use with Oscilloscopes

well greetings out there in YouTube land and welcome to today's video in which we will delve into the mysteries of isolation transformers in this video rather than theory as was presented in part 1 we're going to actually get into practical creation of our own isolation transformer and comparing it to the performance and safety provided by a much more expensive commercial isolation transformer using some of the contributions from patreon patrons and PayPal donators I bought a pair of 4 amp transformers these are one twenty two twelve one twenty two twelve and I'm going to try to connect them sort of a tail to tail just like we did in the Rick tone power supply video where we made the tremolo boost and see if we can't end up with a decent isolation transformer in this case for a little over thirty dollars and that included shipping and all now since most of the amplifiers I work on have two amp or lower fuses I figured that four amp transformers would be able to provide a reliable source of current to those circuits that I'm testing without stressing the Transformers or causing them to overheat one concern I had was that I know there'll be a voltage drop across each of these transformers will the output voltage be too low to power normal amplifier now a sense of receptacle voltages have gone up in the United States to around 120 volts some slight reduction down to like 110 or 115 might actually be beneficial but if it drops way down like around 90 or so I don't think it's going to be usable so there's only one way to really find out and that is let's assemble these together and make our own isolation transformer okay using the color coding that was provided by the supplier of the Transformers I've connected the I have here the primary will be energized with a hundred and twenty volts AC I tied off the centre tabs and insulated the ends so they keep them out of trouble and we connected the twelve volt output from this one to the twelve volt input of the second transformer and then we have the output voltage from the primary of this transformer okay also have my AC voltmeter connected across the terminal so when we plug in the primary to this transformer we'll see how many AC bolts are being applied to it looks like one nineteen point one is the input voltage I've also taped a piece of paper here so that I can write down every one of the measurements for future reference now let's see what kind of no load output we get when I plug into the current limiter well actually not too bad one sixteen point seven or 0.8 which is still higher than the one 15 volts that we used to get from the receptacle now granted that is no load voltage once we start running current through this the voltage level will probably go down but for now I'm pretty happy that the voltage drop across the two transformers is not all that great and now for our test circuit I'm going to use a really nice original graph sixty one fifty amp this one still has the original two prong plug and I believe the death capacitor is still in circuit ooh so there should be a perfect candidate then for us to test our isolation transformer okay I've turn the little amp around I pulled that rear cover off so that we can gain access to the circuit this is an ideal type of amp for this purpose because we don't have to pull the whole chassis to gain access to the circuit just remove that back panel and sure enough there is the death capacitor oh I touch it and the light goes out the death capacitor is in place in this amp okay here's the plan I have the original two prong plug then I'm going to connect to the output of the homemade isolation transformer remember we got now around 116 volts with no load at these terminals let's plug in the isolation transformer turn on the amp and see if we have enough current and voltage for it to operate now with the amp off we're showing a hundred and seventeen point three volts AC at the plug of the amp I think once we turn on the amp that voltage is going to drop let's see if we still have enough voltage and current for the app to operate okay I turned on the app and the voltage dropped dramatically and it's only come up to 85 volts I am not sure if it's going to operate at that voltage level let's crank up the volume and see if we [Music] [Applause] seems to be working okay I've got us another idea let's check the plate voltage with this plugged in to the wall outlet and then we'll check the plate voltage with it plugged in to the isolation transformer let's see if the plate voltage drops way too much for us to be able to operate and reliably test this sir okay I have the test probes from pin three to pin 8 let's plate to cathode and with the isolation transformer providing the current the plate voltage is only 162 volts DC all right I've plugged in the amplifier directly to the current limiter and we see now that the plate voltage is has gone way up to 268 point 5 which is the normal level for the plate voltage in this amp so there was about a hundred volt drop on plate voltage because of the restriction put in the line by these two transformers even though they're both 4 am wired like this to be an isolation transformer they simply cannot provide the current and voltage necessary to operate even a basic three tube amp so I think you can save yourself the time trouble and money of trying to build your own isolation transformer using transformers like this one alternative you might consider is to get a single one to 1 transformer that can handle say 4 amps if you can find one on eBay or something like that that might work but this arrangement is not but fear not I more or less anticipated the results of that first experiment but I just had to see it for myself now that I have it's time to move on to our second alternative which I've gotten for us and that is a medical-grade isolation transformer here's Jack's new buddy a little feral cat we've named Ali lives in our back yard now and begs for food and catnip he keeps a Jack very entertained and for that we're appreciative after all he finishes his morning bath we'll see if he enjoys some of Jack's boutique catnip and as you'll see Holly loves his cot nap role in that catnap Polly he's hooked well Jack enjoys always shameless performance and this is the time we need to discuss something of great importance and that is that not all isolation transformers are equal okay there are two main types medical grade and tech grade isolation transformers now another name for the medical grade could be non tech grade or public isolation transformers now most medical grade isolation transformers the word isolation does not mean what we think it does it means that it is used to eliminate background noise electrical interference and things like that so that very delicate medical instruments like EEG s and EKGs can operate without any sort of problems that can be caused by external electrical noise well that's all just fine and dandy but it doesn't work for us because that type of medical grade isolation transformer will have an internal connection between the neutral return line and ground okay that will make it not suitable for our use with our oscilloscopes fortunately however that problem can be easily remedied so you can convert a medical grade isolation transformer that is really for isolation from electrical noise into a tech great isolation transformer that effectively isolates you from ground reference now part of the reason I bought this isolation transformer is in the ad I could read this statement output floating neutral conductor not grounded that means that even though this is called a medical grade isolation transformer it really is a tech grade transformer I will not have to modify it however I'm going to open it up and show you if you have a real medical grade isolation transformer that is not for isolation from ground reference I'm going to show you when we open it up how you can modify it and make it suitable for that purpose before I pull the lid let's see where this one came from it's the toroid corporation of Maryland okay with that in mind let's pull the lid and see what awaits us inside and what lurks within is the biggest fattest toroid transformer I've ever seen in my life okay now surely this gem is going to be able to adequately power any amplifier that we're going to encounter that draws up to five amps of current now there's a piece of protective tape here I'm going to pull it back to show you where if your medical grade transformer is not suitable for isolation from ground reference where yours will be different from this one and that is right here is the neutral line it's usually white and the hot line is black in this the hot is brown and the neutral or return is sort of a yellowish color somewhere in your medical grade isolation transformer there will be a connection between the neutral return and ground if that exists then remove it you see that that doesn't exist in this one you want to make yours like this with no ground connection between the neutral return and somewhere on the chassis it'll be bolted down okay so that's how you can salvage your medical grade isolation transformer and turn it into a tech grade one like this the second characteristic which must be discussed is if you look in here there are three dark green grounding lines and green is the normal color for grounds there's one for each Bank of output receptacles and there's one from the grounding lug down here on the power cord that we're going to plug in to a wall receptacle now all three of those are combined right down here and bolted to the chassis of this unit what this means is when you plug in your amplifier or whatever you want to test to this outlet you're going to get a ground reference to the chassis of that amplifier from this bottom lug which is continuous with this bottom log which goes directly to the grounding lug in your wall saw now the result of that is debated widely on the internet and is a huge source of confusion to people with regard to isolation transformers we're going to put that to rest with some very clear and straightforward statements about the effects of this grounding look I'm going to discuss with you the effects of leaving this in place and using it on the amplifier that we're going to be testing or if you choose to go inside and eliminate the ground connections for your output receptacles or using some sort of an adapter which turns a three prong outlet into a two prong outlet okay we'll discuss all of that just be patient let's briefly discuss these weird receptacles and you may encounter them also if you buy a medical-grade isolation transformer normal plugs of course they're not going to work with this now when I bought this I saw these and I the vendor agreed to send me three cords that have a plug at this end that will plug into this weird outlet and at the other end would be a female normal AC outlet okay but he forgot to send them okay which is always fun they're on their way to me now but I didn't want to delay the video so what I did was I made my own like remove this from an old computer cut off all the wings and extensions and the ways that it used to bolt into the chassis okay I left the three wires intact hot neutral return and ground and now this will fit in here very nicely and plug in and allow us to use and test our isolation transformer so I just wanted you to understand what this weird thing is sticking out here okay but it's going to really make it easier for us to demonstrate how this thing works fortunately he did remember to send me the power cord for the in it so I plugged one in here and as you can see the other end is very conventional and I'm going to plug that into an AC outlet okay let's discuss now how this thing works I've connected the black hot wire to a lightbulb socket and the white return wire let's flip on the isolation transformer and see that yes it works just like we would expect when something is connected in this manner okay now let's try a little experiment I'm gonna disconnect the common return from the bulb and I'm gonna touch it over here to what amounts to the ground log of the AC receptacle in my shop I put a stainless steel sheet metal screw into the ground lug hole and I've connected this jumper to it so that tip right there is a to the ground or earth ground in my receptacle now if this output right here from this receptacle has ground reference me touching this to ground will illuminate the ball let's see what happens okay I've touched it nothing okay our receptacles at this point from this unit have no ground reference okay and I proven that by touching this to the ground lug of my wall receptacle now what would have happened if you had left your return wire on your medical grade isolation transformer connected to ground okay if you had the type of isolation transformer that's really just to isolate from electrical noise not from ground reference if you touch it to this piece of metal it would illuminate the bulb so my suggestion is if you're not sure about your isolation transformer test it this way and see if you touch the metal here that is the ground lug of the AC receptacle and this lights you're going to have to go into your isolation transformer and unground that common return wire however since this particular medical isolation transformer has the output floating neutral conductor not grounded I don't have to do that this it's already been done on this one at the factory so we can move on now to step 2 now for testing purposes I'm going to use a really nice completely original gret 61 50 amp okay this one's so original that it still has the two prong power cord this is going to serve a well here in our demonstration of the difference between a grounded chassis and a non grounded chassis this is at this moment an ungrounded chassis okay there is no three wire power cord now as you can see I've connected the two prong plug for the Gretsch amp to the black and white wires coming from the isolation transformer the ground wire is not connected to anything alright nor is the chassis grounded at the moment step 1 let's turn it on and see what our plate voltage is when the isolation transformer is providing the voltage and current to make sure that the isolation transformer is adequate to power this amp we know darn well it is but let's prove it right now okay I have my multimeter set to DC voltage I have the probes connected to pin 3 which is the plate and pin 8 which is the cathode of the 6v 6 output tube this is just like I did yesterday let's turn it on and then come over here and see what the plate voltage is now when I tested the amp plugged into the wall outlet the plate voltage was 268 volts DC it appears that using the isolation transformer is actually higher it's around 278 volts so it's gain 10 volts of plate voltage by the using the isolation transformer which tells us that the wining ratio is probably not exactly one-to-one but it's real close regardless we know now that our amp can be adequate quick adequately powered by our isolation transformer unlike the two smaller transformers in the first experiment now let's see if this circuit is actually isolated from ground reference in other words if we were to think of this clip right here as the ground clip of the oscilloscope and it's equivalent to it because as we saw it goes back here and connects to the ground log of the AC outlet which is exactly what the chassis and ground lug in our Silla scope do so we'll be thinking of this then as the ground log of the oscilloscope is there any potential now between it and any point in here in the chassis in other words is it is the circuit adequately isolated then I can take my ground lug from my oscilloscope and touch it to any place in the circuit well let's find now instead of measuring voltage potential between different points in the chassis and the ground clip of my oscilloscope I'm gonna measure current flow okay and the reason being is you can get false readings with you a measure voltage potential there are things called ghost potentials ghost voltage readings where it's actually due to capacitance within the transformer and it might say 60 volts or 70 volts but it's it is unable to flow any current the ghost readings are kind of annoying but they have to be eliminated to eliminate them I'm going to measure current flow between the points in the circuit and the ground clip of the oscilloscope because that's really what causes the damage it isn't the voltage it's the idea of current flowing through my oscilloscope probe and the chassis of the oscilloscope so I have hooked up my digital multimeter here to show us if micro amps of current will flow between different points here in my amp circuit and my oscilloscope ground clip okay a micro amp is so low that it's almost well it's virtually undistinguishable from no flow okay so let's see if we can get any micro amps to flow between the circuit and our ground clip okay for our first test let's hook on here to this log which is bolted right to the chassis of the amp okay and let's see if any current will flow between the chassis and the ground clip of our oscilloscope and we see that it's zero micro amps now I'll just move down to some random point here okay I don't know what that wire goes to it's hot I'll guarantee you but this is the same as if you were moving your ground clip around in the chassis and you touch it to that point is there going to be an issue well no zero micro amps okay let's come over here to some other place let's look at the cathode okay we know that that has a little bit of voltage is that going to flow to our oscilloscope nope okay zero let's go in here and connect oh heck let's go for broke and connect to the AC primary okay that's the tail of the fuse ac primary surely that's gonna flow some okay nope zero micro amps so in other words the way we have this set up right now with the two prong plug plugged into the isolation transformer there is no place in this chassis or circuit that you can connect the ground clip of your oscilloscope and have any problems at all because no matter where you clip it even to the primary winding of the AC no current is going to flow now for those of you skeptics are saying well wait a minute that's a DC ammeter it can't tell if there's any current flowing between the primary circuit here and my ground clip on my oscilloscope well then let's change the meter to AC volts how about if I show you point zero zero four volts potential between the primary circuit and the ground clip of my oscilloscope how about those apples I just thought of one connection that might really make an impression on you and that is let's connect to the plate of the six v6 for God's sake that's the high voltage is it not what is that close to 300 volts surely that's gonna flow through my ground clip and to my oscilloscope nope absolutely no current flow whatsoever even if i hook the ground clip of my oscilloscope to the plate of my output tube now there's two points I have to make here okay and these are critical for your safety so listen up the moment that I connect that ground clip from my oscilloscope to any point in the circuit it's safe okay no current is going to flow through my oscilloscope and harm my probes or my scope but I have now given ground reference to the circuit now think about that I have done the unthinkable by hooking that clip in yes I can put the clip anywhere I want but the second I do put it in suddenly that is ground for this circuit and it has ground reference how does that affect us well two ways number one you can no longer touch anything in here because it has ground reference and it can pass through you to ground if you're grounded and kill you so when you're putting your ground clip in be very careful because the moment you attach it to any metal in this circuit becomes ground referenced and can kill you so be very careful number two if you have two probes as I do you cannot connect the ground clip from one probe say here and the other clip for the ground flow of the ground clip for the probe anywhere else because then you'll have to ground references and current will flow between them and through your scope and possibly destroy it so once you've put like one ground clip from your oscilloscope probe anywhere in here number one we can't touch any metal number two the next ground clip that we put in for our second probe has to be exactly the same spot that we hook the first one or else we're going to have a potential between them which could effectively fry our probes and our oscilloscope so please remember those two rules they are critical now let's move I think to the final point that we have to make and that is and this one is debated widely should we ground the chassis of the electronic device that we're testing in other words should i when I plug the device in should I plug in a two prong plug like I just did or should I plug in the three prong plug from the device well let's see what happens okay if we jumper from here to the chassis of the amplifier circuit which is exactly what would happen if you had a three wire plug right isn't the green wire bolted to the chassis it would come over here and plug in right down here and it would connect to that point so let's do it and see how it changes things okay here we go I've installed a jumper between the grounding log of the isolation transformer receptacle and the chassis of the amplifier now this is exactly the same is if we now had a three wire chassis grounding power cord being plugged in to that receptacle now I'm going to be using DC volts as my criteria of judgment if a potential exists simply because I don't want to take a risk of damaging my meter by exposing it to a high flow of current okay so we'll be measuring volts of potential between different places locations in the circuit and the grounding clip of our oscilloscope and I can assure you that any potentials we encounter will not be ghost voltages but will be the real thing okay our first point in the circuit that we'll check will be the ground lug here that goes to the chassis okay so you're figurin well there shouldn't be any potential because we got chassis chassis same thing right except it shows a tiny bit 0.1 milli volts of potential now what that reflects is this the connection between this lug and the chassis is not perfect apparently there's a tiny tiny bit of resistance between the lug and the chassis and that tiny bit of resistance is reflected in this voltage drop from this lead to this but I will say that 0.1 millivolts is essentially zero so we have no problems here connecting our ground clip on our oscilloscope to that look but let's move down the line a little bit here and connect to that same place that I connected before that showed zero current flow when I had an ungrounded chassis and oh my lord I now have two hundred eleven point six volts and God knows how much current that would be flowing through my oscilloscope ground clip if I had foolishly connected it right there with a grounded chassis okay let's disconnect it there and come down to the lowly cathode okay now that surely there can't be a lot of voltage potential well there is 14.3 some odd volts which means a little bit of current flowing through our oscilloscope ground clip and through the chassis of our oscilloscope I don't know if that's enough to do any harm but let's go and clip on to the plate of the six v6 and see what happens well the amp didn't like it and neither did our oscilloscope ground lead because it now is facing 284 volts and God knows how much current is coursing through our little ground clip from our probe causing probably a whole bunch of harm to the probe and maybe the oscilloscope so as you can see there's only one place in the entire chassis that we can now connect without just burning up or a probe or oscilloscope and that is the chassis itself so as you can see there is a huge difference between plugging in the electronic device with a three wire chassis grounding cord into your isolation transformer or using a two wire cord with the three wire cord the ground reference is the chassis and you cannot connect your oscilloscope ground lead anywhere in here without destroying the probe or the scope except for the chassis itself now if you're willing to let the chassis float and use a 2r cord to connect this circuit you can go in with your oscilloscope ground lead and connect any place in the chassis without a concern okay and once you've connected we know then concern begins can't touch you can't touch metal and if you hook another oscilloscope ground clip up it has to be do exactly the same spot but you have much greater safety and freedom of choice if you do not ground the chassis of the device that you are testing okay I think it's time to bring this rather lengthy and complicated video to a conclusion and I think the best way to do it is to review these safe rules for proper use of an isolation transformer with an oscilloscope I think it's important to emphasize that these rules were derived by experimentation as I showed you in the video these are not simply my opinion or some hearsay that I got off the internet every one of these statements is backed up by a demonstration within the video rule number one the chassis the device under test must be ungrounded if you are one of those individuals who refuses to work on any device that doesn't have a grounded chassis then you that's fine but you're wasting your time by using an isolation transformer the minute you ground the chassis you're losing any benefit that would have been derived from the isolation transformer now this was well demonstrated in the video where I showed you that an ungrounded chassis none of the points within the circuit had any potential to earth ground and therefore could not harm your oscilloscope or probes the minute that a ground was provided to the chassis all that changed and the circuit went back to having all of its normal operating voltages and once again a very serious hazard to both you and the oscilloscope and probes now if you have a three prong plug on the device that you're going to test you have to either use a three to two prong adapter that has the polarized two prongs on one side three receptacle holes on the other and no connection between the ground lug and the receptacle in the isolation transformer or if you don't like using adapters you can permanently disconnect the output receptacle grounds in the isolation transformer if you wish in that case you'll have to go inside and find the two green wires which provide the grounding to your output receptacles trace them all the way down to the floor where they're bolted down here to the chassis ground unscrew the nut remove the two of them tape them off curl them up so they are out of harm's way and then tighten that nut back down on the power cord ground which you will leave intact rule number two once you have plugged the device under test into an isolation transformer that meets all the criteria described in the video and the chassis is ungrounded then you are free to attach your first oscilloscope ground clip anywhere in the circuit because as was shown in the video no point in the circuit if this criteria is met will present any sort of a current flow capability that can harm your probe you or the oscilloscope okay so you can attach that first-round clip anywhere but once you have attach that first ground clip everything changes the circuit now has ground reference which means it's gone back to presenting a shock hazard to you so you have to be very careful especially if you're grounded in any way and for heaven's sakes you wouldn't be I hope he wouldn't be so foolish as to be grounded while working on electronic gear but you have to be really careful this is the one hand rule okay you can go into that chassis with one hand and not touch any metal B once you have attached a single ground clip and you recognize that a shock hazards been created the second probe ground clip that you attach has to be to exactly the same spot in the circuit as the first clip and I mean exactly the same spot or else you will create two different ground references within the circuit and therefore you may have potential between the two ground references and that could create a hazardous situation for the probes the oscilloscope and for you well that's about it for this rather lengthy video I want to thank the patreon patrons and PayPal contributors for helping me finance projects like this well which provide learning to all viewers any viewers that would like to join in with your generous brothers and become donators or patrons I will include links in the video description to help you do so meanwhile I think that's it for us right now I hope this has been a very helpful video and that you'll join me for more in the

Channel: Uncle Doug

Views: 75,236

Rating: 4.9138756 out of 5

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Id: 5SjVm-rF39A

Length: 41min 52sec (2512 seconds)

Published: Fri Jun 22 2018