Isolation Transformers

well greetings out there in YouTube land and welcome to today's video which is entitled I sinned in response to viewer demands I'm preparing a video series on how to operate an oscilloscope primarily to troubleshoot audio amplifiers and in while preparing for that series it occurred to me there are several collateral issues that have to be resolved before we could really move on to understand how to safely operate our oscilloscope and primary among selves was the isolation transformer issue now to fully understand the problem I think we need to go back more or less to the beginning and gain knowledge along the way so that finally we'll be able to understand the need and benefit of isolation transformers I think most of us know that the reason alternating current one out over direct current is that it can be stepped up to very high voltages for transmission through miles of power lines with very little loss compared to DC and then stepped down by transformers on site in different neighborhoods to levels of voltage that are appropriate for household use my example I start off at 35,000 volts AC being applied to the primary of the step-down transformer these transformers are quite large they're tubular and they're oil filled so that the core is cooled by convection currents through the oil these secondary consists of three wires that exit from the transformer there is a center tap return wire and then two hot wires each has 120 volts AC potential to the center tap return Marv the center tap is grounded by using a heavy gauge wire that runs down the pole and all the way to the tip of the pole that is buried deep in the ground to provide a good earth ground for the center town why do we centre tap it well think about why we Center Tao the 6.3 volt filament winding in our amplifier power supplies by giving a ground reference to the center tab we ensure that the voltage is equally divided between the two output wires and that that voltage remains consistent also because of the center tap the two 120 volt AC wires that come from the winding or out of phase each of the 120 volt AC wires is used to energize half of the wall receptacles in your house that means that half of your house will have receptacles energized by this wire the other half will be energized by this wire and that means that half of your sockets are out of phase with the other half however this is a not an issue this is not a amplifier circuit so for household use the out of phase condition okay here's this step-down transformer that's out behind my house and you can see the big heavy ground wire coming down the pole and going all the way down to the base tip of the pole that is buried deep in the ground so that is the grounded center tap of the secondary of the step down transform okay here's a close-up of the power lines coming to the house the two insulated 120s and the bare uninsulated common returned our wall receptacles are wired with the smaller of the two slots receiving the hot wire the wider of the slots of receiving the center tap return wire and then a circular ground receptacle at the bottom that we will plug our three wire plug into that not only is earth grounded but also connected to the center tap return this is for safety purposes this gives two routes for the electricity to return if electricity shows up on this ground log it shouldn't but if it does it can pass through the ground back here to the center tap or it can pass through the return wire to the center tap so it's like I said for safety reasons it's like a double provision here of return capability in case of a short circuit now let's look at the three wire power cord plug that we're going to plug into the receptacle right here is the narrower of the two prongs which will go into the narrow slot and the hot wire here will be connected to the hot wire here which is generally the black wire in a three wire power cord that wire will continue on through a fuse an on/off switch and go to the primary winding of our power transformer the wider prong will plug in here to the wider slot which is our return Y our Center top return wire and it will proceed down here to also to the primary of the power transformer now I've indicated an earth ground for that wire and I've done it with dotted lines because as it stands just as you see it this is not true but the moment that you plug this plug into this receptacle remember that the white return wire is wire not only to the ground slot but also directly to an earth ground before I go any further I wanted to clarify something that might be confusing to some of you and that's the symbol that I'm using here for earth ground now most of us are familiar with the symbol it looks like an upside-down Christmas tree which indicates a chassis ground we see all the time in our amplifier circuits but I'm using this symbol to indicate something completely different now we know what the chassis ground that that indicates that the wire goes to the chassis and that electricity will pass through the chassis to get to wherever it's trying to go maybe to its origin at a transformer winding or something like that but this indicates that the earth itself could serve as a chassis if you will and that electricity can pass through the earth itself to get back to say the center tap of that transformer up at the top of the pole in my backyard so two very different things this is a metal chassis that we would have in an amplifier this would indicate almost like an earth chassis through which electricity can pass I'm going to tell you a little story here which is going to I hope stick with you and keep in mind a phenomenon about the route that electrons take in a circuit years ago when I was a kid I used to go horseback riding in the desert now there were three different stables spread out over a quarter-mile area and you could rent your horse from any of those three and on any weekend there might be a hundred people riding horses from all the three different stables out in the desert everywhere up mountains down a Royals all over the desert completely mixed up but the minute that you turn those horses around and they knew that they were headed home to go get food and water they would tick off like bats out of hell and take the straightest route possible back to their own stable now there was food and water at all three stables but the horses only returned to the stable from which they had come originally now without analogy I'm going to explain to you why the primary circuit in an amp is the most dangerous part of the whole amplifier circuit and why the secondary circuit is not OK in the primary circuit we can say that it is not only ground references referenced but earth ground referenced earth ground reference means that when the horses decide to head back to the stable ok they're out here riding through the call and they can head back to the stable they can take a shortcut through earth ground if need be to get there now here is how that can happen you are standing on a cement floor an ankle deep in water barefoot and you reach in here and touch with one finger any part of bare wire that's present in the primary circuit and you are toast ok you are you're fried and here is why because those horses are galloping around here and they say hey look this idiot is standing in water which will connect to the ground to the earth ground is touching the circuit the heck with going all the way back this way let's just cut back through the ground and get back here to the centre town where there's plenty of food and water now why is the secondary circuit not as worrisome as the primary the horses over here are leaving the stable going out here and galloping all around through the circuit doing whatever it is that electron horses do and they will return only back to this winding the horses from this stable will only return here and likewise right here if you were to stick your finger in here and touch an open spot in the wiring the horses would run right by you because you cannot give them a shortcut to ground because this side of the circuit is not ground referenced they cannot take the shortcut to earth on the EMA secondary side of the circuit like they can in the primary this is not earth ground referenced now I'm not telling you to stick your finger in and find out because due to the fact that human beings are big bags of water that have a fairly high amount of capacitance if you touch in here and touch a live wire the electricity is going to charge you up it's not gonna form a circuit between you and ground but it's gonna charge you up ok and while it's doing that it is passing through the the nerve say of your fingertip and it is hurting like the devil ok so it's still painful but because a circuit between this and earth cannot be formed you are not really at risk of any sort of a fatal shock unless you commit the unmentionable and that is to touch with one finger a pore up a location that has high potential and with the other finger on the other hand touch an area of low potential then the horses can gallop through you they're not using ground to get back to the center tab they're using your body to get back to the stable quickly by taking a shortcut so that's why we tell you to keep one hand in your pocket when you're touching anything any circuit in an amp so you cannot close a circuit even in the secondary you can't if you only have one hand in use here can't close a circuit that could lead to your demise look who decided to join us here on the table during the discussion but as the master of concealment you wouldn't know he's there because he's under the tablecloth excellent work jack excellent no one would ever suspect you're under the tablecloth and now I'll probably fall asleep and will hear and snore and in just a minute or two now what if we could magically change this somewhat dangerous primary side of the circuit and eliminate this earth rounding of that it has here if we could eliminate that then both sides would be as safe as the secondary side he is already and the way that we can do this is with an isolation transformer okay here is our isolation transformer it's generally a one-to-one winding ratio because we don't want to step up the voltage or step it down we want it 120 volts ac on both sides we have a standard three wire plug for the primary side we will plug it in and we can even use a chassis ground if we want on our third grounding lug hair and say this is in a metal box we could ground the metal box for safety which would be a good idea we also see that this primary is earth ground referenced because the common ground wire is plugged in to a receptacle where the common wire has both earth ground connections and chassis ground can so this side is still very much like the primary that we're trying to get away from in our amplifier circuit but the secondary is very different because there is no connection here no direct connections between this winding and this there is no earth ground reference on this side of the transformer this is very much like the secondary was in our amplifier you see that the earth ground of referencing is limited to the left side of the transformer and not over here these horses once they leave the transformer don't want to go to ground they don't want to go anywhere except back on this return line to the stable and that's all they're going to do so we have eliminated the possibility of touching with one finger anything in here while we're grounded and being killed so now let's plug our amplifier into the secondary of our isolation transformer when we do we see that there is no earth ground reference on the common wire that we saw before when we plugged in to the regular AC wall receptacle and therefore the horses in this circuit cannot pass through us and take a short circuit back to the center tap at the top of our pole in the backyard that makes this much safer now the primary of our amplifier circuit is just as safe as the secondary was in the original circuit they're both isolated from ground reference and the horses that are galloping around and each of these circuits cannot possibly take any short circuit back to the through us if we are grounded this is why people recommend the use of isolation transformers with amplifiers that do not have power transformers if you think about it if you take away the entire power transformer all you have is AC winding through all of the circuitry of the amp and what are the odds that there may be leakage to the chassis somewhere along that route and if you touch that chassis and you're grounded what are the odds that you're going to be electrocuted in both cases the odds are pretty good once you plug a what we call series filament or power transformer less amp circuit into an isolation transformer yes all of the AC is marching all through the circuit the way it always was before but because no part of the circuit has earth ground referencing none of the horses galloping around here in the circuit can possibly take a shortcut through you to ground and back through the earth to that transformer on top of the pole in your backyard so using the isolation transformer will render then one of these rather unsafe circuits without a power transformer into a relatively safe device that you can operate without undue concern for being fried sometime during your concert at Wembley Stadium but why you may asked do we need an isolation transformer when we're working with an oscilloscope well first off you don't need it for the oscilloscope itself you need it for the device that you are checking or testing with the oscilloscope to see the benefit of using an isolation transformer on say an amplifier that we're going to check with an oscilloscope let's use our oscilloscope here on the amplifier circuit that is plugged into regular wall receptacle now let's look at how it's made first here the probe the probes got a plastic body with a little metal wire that runs down the middle with a hook that you can hook on to different points in the circuit of the device that you're checking there is a adjustable 1 to 10 million ohm resistor in series with that wire you can set it four times one in which it will be one omegle or x tan in which will be 10 megohm the reason for that will explain when we get into oscilloscopes but for now the point is there is a whole lot of resistance right here that will prevent in the case that we touch this to a high voltage spot the voltage will come raging down here and can you imagine the voltage drop across 10 million ohms this then is the buffer that pretty well protects us from harming our oscilloscope by touching the lead here to a high voltage spot you still have to be careful but this resistance actually then reduces the chances of harming the instrument we see that that hot wire comes in here and goes into the circuitry of the oscilloscope now there is a shielded cable on the outside that connects to the body or the chassis of the oscilloscope also when we plug in the oscilloscope we know darn well that the ground look will go over here and connect to the chassis and that the ground look and the return or both earth grounded therefore the chassis of our oscilloscope has is referenced to earth ground now I think that by now you should be knowing that that's a hazardous situation because with that oscilloscope test lead there's two connections you know anytime you want to test something you have to make two connections well we've got that one there at the tip that's our main connection then we've got our ground clip this will be connected to the shield of our test probe and it looks like a little alligator clip and we can clip it around in here to different places in our amplifier to see what kind of what wave form the signal is taking we can measure voltage you can do all sorts of measurements but you need to connect the ground clip somewhere well since the oscilloscope is ground referenced if you hook the ground clip up to anything appear in the AC circuit or that's exposed to the AC circuit or inadvertently connected to it or that is being leapt to by the AC circuit you're gonna have a dead short between the ground clip and earth ground and all sorts of current is going to start flowing through here very rapidly because remember it's coming down the ground clip down the shield around the chassis and going straight to ground you have an absolutely dead short for whatever voltage or current you've connected this to now that will rapidly destroy the probe and sometimes in very exciting ways okay with sparks and melting plastic and smoke and burning your hand and causing all sorts of things that will kind of ruin your day now let's use the oscilloscope on the same amplifier but in this case let's plug it in to the isolation transformer now while the amp is plugged in the horses are running and this type of a pattern they're originating over here they're running over here through the power transformer and then they're coming back and this is their stable that's the only place they want to go if you hook this little ground in here somewhere the horses will not even notice it they'll be running back because they want to get over here to the stable they have no interest in running down this wire along the shield through the oscilloscope chassis and going down into the ground and trying to get back to the center tap at the pole because they didn't originate there the horses and now in our amplifier circuit and the secondary of our isolation transformer originated right here they have no desire to go anywhere other than in this circuit therefore our oscilloscope leads or safe because nothing can entice the electrons to take this route and cause any damage and while we're talking about oscilloscope leads I'll just show you one here so you can see it's how it relates to the drawing there's a little hook that comes out that we can use to hook on to different wires and locations and the amp circuit we're studying here is that ground lead that can cause so much trouble if the device that we're working on is not plugged into an isolation transformer here's a little switch you'll see there it's a 1 mega ohm resistance to the signal and we move down here and it's 10 mega ohm resistance okay and not much else but shielded cable coming out here to a connector that attaches to the oscilloscope so that the shield's on the outside and the signal passes down a little wire in the middle and we've got this mysterious screw on top and we're gonna have you're gonna have to wait a little while to find out what that's all about if you don't already know let's take a few moments here and let me open a gift that I received through the mail just today from a very generous and thoughtful viewer I just had to share this with you a viewer sent me an amp chassis we'll take a look at it in just a minute but I thought you'd get a kick out of the address and I guess that's Jack they're looking a little doglike but not bad well this is a warning pretty ominous I think you'll get a kick out of my padding that I used but it depends on how you look at it I guess we know what its gonna be let's open it up and see yep sure enough a nice defense thrown in her for padding and thank God because I was almost I'd almost run out of them okay this is cute the tubes are stuck into foam rubber and the amp chassis is all wrapped up and some kind of oh no looks like a diaper or something let me remove that we'll see what's what it looks like yep it was wrapped up in an adult diaper thank heavens it's a fresh one not a used one but anyway here is the camp chassis it's an airline from Montgomery Ward's 6au 6 6 v 6 6 X 5 rectifier nice power transformer looks all original underneath so I've got to find a good project to use this in thanks ed I really appreciate it well that about does it for this video on a rather complex topic we started at the top of a 40-foot pole in my backyard and ended up here at the kitchen table looking at oscilloscope codes I hope you enjoyed the journey and I hope it was helpful and instructive I want to offer my sincere thanks to all the patreon people who make pledges and the paypal contributors you're keeping us on the air and advertising free and I think we all benefit from that so thanks so much to all of you for watching and I hope to see you in our next video in which we will start to understand and operate a oscilloscope and use it to troubleshoot amplifier circuits if that sounds interesting state looks like we've got a 31 Ford coupe you can tell with 31 when the area here on the grille contrast with the cone Ford engine is set way back when you set back a Chevy engine like this the distributor is way at the mirror impossible to get to or remote but the beauty of the Ford engine is the distributor's are on the news headers powder-coated look at the craftsmanship having tried to build some of these things I can tell you this isn't easy these guys are good beautiful [Applause] even as the flat headlight lenses like the originals really with halogens always behind them and now in the what hath God wrought category so black forty Willis blown big block Chevrolet this is a fiberglass replica but who cares I think I'd give you my life savings just for one ride really something very subtle if you're a 33 and 34 Ford fan this one might work for you three window coupe gorgeous wheels knockoffs small-block Chevy super-nice this car belongs to the host of the get-together today it's probably the nicest t-bucket I've ever laid eyes on I don't think I changed a thing about this wheels tires certainly not the engine I've asked him what's it like when say you out on the road and you don't see any cops around and you really stand on the accelerator and he just looked at me with that look like guys that come back from war that that hundred-yard look or whatever it is like you don't want to know I believe it can you imagine probably seven or eight hundred horsepower and about a 1500 pound car okay here we got a 1925 Model T but they would call at all teeth pickup truck and look at the starter crank is still in position that the old fashioned headlights with the grass runes which in a girl show start to get suspicious when they see the rack and pinion steering protruding element on hard to think well maybe it's really not a Model T look at those running boards well Henry never dreamed of anything like that neat ruined wheels he's got the little lanterns at the brass bar extinguisher and I'm gonna guess that those are the turret rear turn signals if they are I got a hand it to him and I'm sure the police will be handing it to him too in the form of a ticket Lantern taillights very very nicely done look at that bad timing play toolbox nice choice of Tarzan wheels overall I don't think I've ever seen another quite like this I think it's really original and really neat

Channel: Uncle Doug

Views: 38,453

Rating: 4.9654961 out of 5

Keywords: isolation transformer, oscilloscope, test probes, earth ground, ground referenced

Id: vz70GSBhYjA

Length: 32min 34sec (1954 seconds)

Published: Sun Jun 10 2018