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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 6:50 am 
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First of all, put that outlet tester away; it is the wrong instrument for these tests. Use an incandescent lamp or put a 10K 2W resistor across your voltmeter leads. Grounding the outlet should have NO effect on these tests unless the outlet itself is defective.

TPAairman wrote:
Jim Mueller wrote:
If it has a 3 wire cord, the AC line is already isolated from the chassis unless there is a defect or a previous "repairman" did something stupid. In either case you want the chassis grounded to protect yourself. Absolutely do ground the outlet.


I disagree with you on that one. Without a ground connection, current flows out of the isolation transformer, through the device, and back as a closed loop. If there is a defect or botched repair as you pointed out, it's contained so to speak in that closed loop. The whole point of the isolation transformer is that if I'm in some way grounded, and I touch a hot part of the device, there cannot be a complete circuit. If I touched the chassis and a hot lead, then I'd get zapped. By grounding the device under test back to the wall, if I'm grounded, I'm also now connected to that chassis.

So in other words, my oscilloscope is sitting on the bench, not even turned on. I happen to have my hand against the case (it's metal and grounded) and I touch a hot lead in the chassis, I'll get shocked..

OK, let's follow the current in a device with a 3 wire cord and a fault from the hot side of the AC line to the chassis. And lets also say that the circuit common is connected to the neutral side of the AC line, which is isolated from the chassis as it should be. There is a grounded test instrument connected to circuit common and the item being tested is powered from a properly operating isolation transformer.

Case 1 - the ground pin on the isolation transformer outlet is grounded: Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. From there it goes through the cord to the housing of the isolation transformer to the ground pin on it's plug. Then on through the ground wire in your bench to the plug of the test instrument, next through the instrument to the input or output connector, through the cable to the circuit common on whatever is being tested and back to the other secondary lead on the isolation transformer. That's a short circuit; there is fireworks, something smokes, a fuse blows, a PC trace vaporizes, a piece of equipment is damaged or whatever. All are bad results. But, if you touch both pieces of equipment at the same time, you will not get shocked since they are connected together by the third wire.

Case 2 - the ground pin on the isolation transformer outlet is not grounded: Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. Next through the cord to the third pin on the plug. At this point there is nowhere for it to go; the circuit is broken by the missing ground connection. There are no fireworks or other bad events.

Case 3 - same as case 2 but now you touch the case of the device under test and the test instrument at the same time. Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. From there it goes to you who happens to be touching the case Then on through you to the case of the test instrument, next through the instrument to it's input or output connector, through the cable to the circuit common on whatever is being tested and back to the other secondary lead on the isolation transformer. No smoke or fireworks but now you are part of the circuit with possibly disastrous consequences.

Note that in none of these cases does the current path include any of the AC bench wiring except the ground wire between where the two devices are connected so a GFCI connected before that won't trip.

Conclusion: it is better to burn up a test instrument than to risk being killed. Therefore, definitely ground the isolation transformer outlet.

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 11:36 am 
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Notimetolooz wrote:
criageek wrote:
The ground wire on the power plug is just connected to the variac's aluminum cabinet. The ground terminal on the outlet is not connected to anything. When I wired the outlet as in the schematic, my outlet tester says Hot and Neutral are reversed, so I changed the wires going to the outlet and now the outlet tester says all is ok.
Rich

There is something wrong here. The outlet tester I suppose is the common type that has a three prong plug and three lights on it? If the outlet doesn't have a ground connection then the tester couldn't tell if the hot and neural were switched. I wonder if the outlet in you variac/ isolation box is really not grounded to the case. Some outlets make a ground connection through the mounting.
There usually is some voltage difference between neutral and ground on a wall outlet. Sometimes there is a small leakage current between the windings of an isolation transformer but very little, enough for a VTVM or DMM to register however. It could be your isolation transformer is defective.

This article about the [SERIOUS] shortcomings of receptacle testers and correct testing methodologies might not help with your particular problem, but should be read for the purpose of increasing electrical safety knowledge. Reading it after having been in the field for over 40 years, this article was still a real eye-opener.

http://ecmweb.com/contractor/failures-o ... ng-exposed

Regards, C-N-E

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 6:37 pm 
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Hey C-N-E - thanks for posting that...very interesting article!

Ok, here are the results of my initial test. I took the isolation transformer out of the cabinet, as well as the outlet, and set them on the carpet square I use as a work surface on my workbench. Then I used jumpers to connect the pigtail ends of a power cord to the primary leads of the isolation transformer. I didn't have a 10k 2W resistor so I used two 22k 1W resistors in parallel across my test leads for all readings (it measured a little over 11k). Here is what I got:

Secondary1 - Secondary 2: 122.3vac
Secondary1 - green ground wire on power cord: 0.023vac
Secondary2 - green ground wire on power cord: 7.70vac

That 3rd reading bothers me...seems like it should be zero. I also measured from each secondary lead to the case of the isolation transformer and got less than 1vac across each.

Thoughts?

Thanks,
Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 7:26 pm 
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Jim Mueller wrote:
Case 2 - the ground pin on the isolation transformer outlet is not grounded: Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. Next through the cord to the third pin on the plug. At this point there is nowhere for it to go; the circuit is broken by the missing ground connection. There are no fireworks or other bad events.

Case 3 - same as case 2


Actually both cases 2 and 3 are wrong, and here's why.

First, for easy typing, when I say transformer, I'm talking about the device we're building, not just the isolation transformer by itself. Let's go through what you are saying here, and let's say my scope is the test equipment we're using. The second I connect the ground lead of my scope to the chassis, I've now established a ground connection. So let's say we connect the ground prong of the transformer. That means there are two parallel ground paths back to the outlet on the wall. One is the ground prong we just connected, the other is the ground lead of the scope. So in the examples you gave of case 2 and 3, you said the circuit is broken by the missing connection. But it's not missing since it now exists through the scope.

Now, if we do as I'm saying, and eliminate the ground prong on the outlet of the transformer, then at that point, there is no difference. We have a ground one way or the other because the scope is connected. So now those reading this might ask, then why not just go ahead and connect the ground to the transformer outlet? Heere's why. When you connect the scope, and establish the ground though the test lead ground, then you basically loose the isolation, so the isolation transformer becomes useless which you can't avoid. And by the way, to Leigh - this is what I meant when I said it bridged both sides together. I probably could have typed that one better.

But the reason we DON'T want the ground of the transformer outlet connected is, if I plug in my device under test, with no test equipment connected, and I reach into the chassis to make an adjustment, or I connect my handheld multi-meter (etc), if I have my hand on something grounded on the bench, I still can't get shocked if I touch a hot point in the device. As soon as we connect the ground prong of the transformer outlet, that protection is eliminated.

That said, I did say earlier to ground the case of the transformer but not the outlet. I was thinking this through as the transformer being a device, not a piece of test equipment. That being said, I'd actually suggest NOT grounding the case of the transformer, but you have the risk of the wall side having a fault to the case. But when you buy isolation transformers by themselves, they don't have a ground prong.

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 7:29 pm 
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criageek wrote:
Thoughts?


While you have it out, and disconnected, take your multi-meter, set on ohms, and connect one lead to one of the primary wires, the other lead to one of the secondary wires. You should get no reading. Try the leads on different wires to be sure. If you get any reading, your transformer is shorted inside.

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 7:52 pm 
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I measured from each primary lead to each secondary lead of the isolation transformer and got no continuity anywhere. I also checked for continuity from each primary and secondary lead to the case of the isolation transformer and got no continuity.

So I wonder where to 7.7vac is coming from??

Thanks!

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 8:25 pm 
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Is the green wire on the power cord truly grounded? Check that you don't have a floating ground on the mains outlet.

What happens if you place a 0.15uF capacitor in parallel with the resistors when you do the AC voltage measurement?


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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 8:48 pm 
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criageek wrote:
ISo I wonder where to 7.7vac is coming from??


Now put it back in the case, connected to the outlet, but don't connect the ground. Cap off the ground wire from the cord, then see if you get any stray voltages.

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 10:07 pm 
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criageek wrote:
So I wonder where to 7.7vac is coming from??

Across 11 KΩ, that's 700 µA. That seems a little high.

To couple 700 µA, the capacitive reactance would have to be about 113 volts/0.7 mA = 161 KΩ. At 60 Hz, Zc is 1/(2*pi*f*C), so C = 1/(2*pi*f*Zc) = 1/(6.28*60*161K) = 0.016 µF.

That sounds a bit high to me, but it's not like I've measured the capacitive coupling of any other isolation transformers. I suppose your isolation transformer doesn't have a Faraday shield between the primary and secondary, and that's why the coupling is that high.

Now my copy of Wedlock and Roberge's "Electronic Components and Measurements" says that a shock of 700 µA is below the threshold of sensation, so I think your isolation transformer is safe.

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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 10:20 pm 
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Ok...I reinstalled the isolation transformer in the cabinet and mounted the outlet in the cabinet. I connected the power cord pigtail directly to the primary of the isolation transformer, and the secondary to the outlet. I left the green wire on the power cord hanging. I got these readings:

Secondary1 - Secondary 2: 122.3vac
Secondary 1 - cabinet: 0.120vac
Secondary 2 - cabinet: 0.088vac

If I measure from each secondary to the green wire I get the same readings as before...0.02vac and 7.70vac. Which means I read 7.7vac between the ground on my signal generator (or any other piece of test equipment plugged in) and one of the secondary wires. So, if this is going to be a safe setup, I'll replace the power cord with a 2 wire cord with a polarized plug, replace the outlet with a 2 wire polarized outlet, and connect the secondary with 0.02vac on it to the neutral screw on the outlet.

But first I'd like to get concurrence that this will be a safe setup. I've already attached an insulator (cardboard) to the inside of the cabinet next to the variac connections, and will spend some time identifying any other places the cabinet could possibly come in contact with ac and insulate them as well in order to minimize the risk from not grounding the cabinet.

Thanks!

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Wed 29, 2017 11:48 pm 
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I see no reason to disconnect the ground wire from your cabinet. Something doesn't seem quite right, but the cabinet is safest if it is connected to a solid earth ground.

Either way, I would plug the setup into a GFCI, which will provide protection in the event that the primary side of the transformer were to somehow short to the metal cabinet.


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 Post subject: Re: Variac Problems
PostPosted: Nov Thu 30, 2017 12:18 am 
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lexrageorge wrote:
I see no reason to disconnect the ground wire from your cabinet. Something doesn't seem quite right, but the cabinet is safest if it is connected to a solid earth ground.


Connecting the ground to the cabinet is a double edged sword. On one hand, you would want it in case a hot lead from the cord to the primary failed and touched the cabinet. On the other hand, if you hare taking a measurement, and go to change the voltage of the variac, the cabinet is tied to the chassis through your test equipment, as I explained earlier, so you run a risk there. Probably the best solution would be ground the cabinet, but put a plastic front panel to make sure you don't contact the metal of the variac cabinet.

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 Post subject: Re: Variac Problems
PostPosted: Nov Thu 30, 2017 12:53 am 
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criageek wrote:
If I measure from each secondary to the green wire I get the same readings as before...0.02vac and 7.70vac. Which means I read 7.7vac between the ground on my signal generator (or any other piece of test equipment plugged in) and one of the secondary wires. So, if this is going to be a safe setup, I'll replace the power cord with a 2 wire cord with a polarized plug, replace the outlet with a 2 wire polarized outlet, and connect the secondary with 0.02vac on it to the neutral screw on the outlet.

That won't help. The 7.7 vac is between one side of the isolation transformer secondary and the power line neutral, which is connected to ground at the service entrance. So there will still be 7.7 vac between your unit under test and the test equipment ground connection, even if you omit a three-wire plug on your isolation transformer box.

However, the 7.7 vac is not a problem, because it is sourced from something that has a 161 KΩ source impedance. That means only 0.7 mA will flow through the test equipment ground connection. You won't even notice it.

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 Post subject: Re: Variac Problems
PostPosted: Nov Thu 30, 2017 1:39 am 
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criageek wrote:
Secondary 1 - cabinet: 0.120vac
Secondary 2 - cabinet: 0.088vac


I would say this is probably inductance from the transformer to the case. Think of your cabinet as another coil with a turn of 1/2 or so.

Try taking your meter probes and connect them together - either hold them or alligator clip them, but don't let any of the metal of of the probe tips or alligator clips touch anything else. Now, with the transformer on, move one of the probe wires close to the transformer. You should get a small reading.

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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 1:47 am 
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I still disagree and here is why.
TPAairman wrote:
Jim Mueller wrote:
Case 2 - the ground pin on the isolation transformer outlet is not grounded: Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. Next through the cord to the third pin on the plug. At this point there is nowhere for it to go; the circuit is broken by the missing ground connection. There are no fireworks or other bad events.

Case 3 - same as case 2


Actually both cases 2 and 3 are wrong, and here's why.

First, for easy typing, when I say transformer, I'm talking about the device we're building, not just the isolation transformer by itself.

That's what I called the "device under test" or something similar. When I said transformer I meant the isolation transformer. I tried to always include the word "isolation" but it is possible I missed a spot.

TPAairman wrote:
Let's go through what you are saying here, and let's say my scope is the test equipment we're using. The second I connect the ground lead of my scope to the chassis,

Here's a point of disagreement. In my example I postulated a piece of equipment that operated directly from the AC line so connecting the scope to the chassis would be incorrect; it would be connected to circuit common instead. For example imagine an AA5 with "floating ground" but having a 3 wire cord. Or, for a more realistic example and since, way back when, you mentioned an amplifier, how about a newer amplifier with a problem in the input side or it's switching power supply.

TPAairman wrote:
I've now established a ground connection. So let's say we connect the ground prong of the transformer. That means there are two parallel ground paths back to the outlet on the wall. One is the ground prong we just connected, the other is the ground lead of the scope. So in the examples you gave of case 2 and 3, you said the circuit is broken by the missing connection. But it's not missing since it now exists through the scope..

No the case isn't grounded since a piece of equipment should not have one side of the AC line connected to the cabinet. (Of course, before people cared much about safety things like this were done. But that was before the age of 3 wire cords so they won't have one and this discussion doesn't apply.) The cabinet isn't grounded since the third pin on the plug isn't connected. The scope only connects circuit common to ground.

TPAairman wrote:
Now, if we do as I'm saying, and eliminate the ground prong on the outlet of the transformer, then at that point, there is no difference. We have a ground one way or the other because the scope is connected.

The scope only connects AC common to ground. The case is floating.

TPAairman wrote:
So now those reading this might ask, then why not just go ahead and connect the ground to the transformer outlet? Heere's why. When you connect the scope, and establish the ground though the test lead ground, then you basically loose the isolation, so the isolation transformer becomes useless which you can't avoid. And by the way, to Leigh - this is what I meant when I said it bridged both sides together. I probably could have typed that one better.

True. Once the scope is connected the device under test is no longer isolated. It is just as dangerous as plugging it directly into a wall outlet. The isolation transformer is protecting your scope, not you.

TPAairman wrote:
But the reason we DON'T want the ground of the transformer outlet connected is,

I'll come back to this further down.

TPAairman wrote:
if I plug in my device under test, with no test equipment connected, and I reach into the chassis to make an adjustment, or I connect my handheld multi-meter (etc), if I have my hand on something grounded on the bench, I still can't get shocked if I touch a hot point in the device.

True.

TPAairman wrote:
As soon as we connect the ground prong of the transformer outlet, that protection is eliminated.

No it isn't. Since the AC line isn't connected to the case (in a properly wired piece of equipment), it is still isolated. If you touch the case while touching a live terminal, you won't get shocked because of the isolation. Touching the grounded case is no different than touching your grounded scope. But emphasis for anyone else reading this - You won't get shocked ONLY IF THERE ARE NO GROUNDED INSTRUMENTS CONNECTED.

TPAairman wrote:
That said, I did say earlier to ground the case of the transformer but not the outlet. I was thinking this through as the transformer being a device, not a piece of test equipment. That being said, I'd actually suggest NOT grounding the case of the transformer, but you have the risk of the wall side having a fault to the case. But when you buy isolation transformers by themselves, they don't have a ground prong.

If the isolation transformer is a piece of test equipment, it's enclosure should be grounded just like any other piece of test equipment and for the same reasons. As for buying an isolation transformer, they DO indeed have a 3 wire cord and outlet that are connected https://www.hammfg.com/electronics/transformers/line/171.

To get back to the point I said I would come back to, reread case 3 above and you will see why the third prong should always be grounded.

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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 5:41 am 
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We're still not seeing eye to eye on this one.

As far as your Case 3 example, actually you are correct on that one - when you said same as Case 2, that threw me off. However, we've both actually already agreed on that one - that if you connect a piece of test equipment to the device under test, with the ground lead connected to the chassis, then by touching the case of the test equipment, and a hot point in the device under test, you will get shocked.

Here's why I still disagree with you and contend that the ground wire should not be connected to the outlet. When we connect the piece of test equipment, we've established that the case of that test equipment is tied to the chassis. I think we're also in agreement in that it also means the chassis is tied to the wall ground if the test equipment has a 3 prong cord. That means that every grounded item on my workbench is electrically tied to that chassis. My refrigerator in the kitchen, on the other side of my house is now electrically tied to that chassis. So I don't have to just touch the case of my test equipment that's actually connected. I could touch the conduit between my outlets over the workbench and I'm electrically connected to that chassis.

Where we differ is, If I disconnect the test equipment, the chassis is no longer grounded through the test equipment, but it would still be grounded the exact same way if we connect the ground wire from the plug of the transformer to the outlet of the transformer. So that means if I have my amp hooked to the transformer, with the ground wire connected in the transformer, I now face the EXACT same risk as when my test equipment is connected. The whole point of the isolation transformer is to eliminate that part of the risk - to make it so that if I do become grounded and I touch a hot lead in the device under test, there cannot be a complete circuit. But adding that ground wire now completes the circuit and eliminates the protection that the transformer is supposed to provide, because I can now touch the conduit on my bench outlets, and a hot point in that amp and get shocked. If we remove the ground wire in the transformer, that can't happen.

And I will say this - Yes I know we are supposed to work with one hand in a pocket, etc, etc, but the reality is, we've all had a stupid moment at one time or another working with what we work with.

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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 6:36 am 
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Here's a couple good visuals to explain it:

On this first one, scroll to 4:55, and he addresses this.
https://www.youtube.com/watch?v=3rvEdHyAd2k

This is from Mr. Carlson's Lab, and in this one, He's showing the same danger in a different way.
https://www.youtube.com/watch?v=XBsQ3sZ45Fk

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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 7:40 am 
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Jim - let me apologize in advance a bit on all of this - it's hard for me to visualize this stuff sometimes when I'm only reading it, and not able to actually get my hands on the device.

I've gone back and re-read your examples (since I missed Case 2) and actually, re-reading #1, I think you are looking at that situation wrong as well.

Jim Mueller wrote:
Case 1 - the ground pin on the isolation transformer outlet is grounded: Current leaves one end of the isolation transformer secondary and goes through it's outlet to the device under test and through the fault to the case. From there it goes through the cord to the housing of the isolation transformer to the ground pin on it's plug. Then on through the ground wire in your bench to the plug of the test instrument, next through the instrument to the input or output connector, through the cable to the circuit common on whatever is being tested and back to the other secondary lead on the isolation transformer. That's a short circuit;


The current would not flow through the ground, then back through the test equipment. The path from the test equipment ground lead to it's own power cord wire ground is a parallel path to the ground wire in the outlet of the transformer. They both connect between the chassis and the wall outlet ground.

With a fault in the device under test, current would flow out of the transformer, to the device, through it's fault, and back to the other secondary wire of the transformer. And that would happen whether you have any ground connection or not since the ground is not an alternate path back to one side of the transformer secondary.

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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 3:37 pm 
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I do appreciate the fact that this is being discussed...everyone is making some great points and I look forward to the dust settling and learning the proper way to hook this stuff up. For the time being I've gone with this setup:
Attachment:
variac_schematic.png
variac_schematic.png [ 45.11 KiB | Viewed 158 times ]

I'm happy with the resistance and voltage measurements I've taken and it works. If it turns out this is unsafe and I need to make corrections I will. All of these components are in one aluminum cabinet. I've checked for continuity between all terminals on the variac and the cabinet and found none, so I don't think there is an issue there. Unless that point was raised in the case where something might go wrong with the variac and short a connection to the cabinet. If that is a real concern I'll try to figure out a way to isolate it from the cabinet.

Eventually I'd like to rebuild this whole thing and put it into a different cabinet which would include both an analog and digital ammeter. While I thoroughly enjoyed designing and building a separate box for these things (viewtopic.php?f=8&t=322407), they really should all be in one unit.

Thanks again guys...I'll be following this closely :)

Rich


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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 5:31 pm 
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criageek wrote:
Unless that point was raised in the case where something might go wrong with the variac and short a connection to the cabinet. If that is a real concern I'll try to figure out a way to isolate it from the cabinet.

It's safe. Don't isolate the variac from the grounded cabinet. If something goes wrong with the variac, you want it to blow the circuit breaker or GFI. The ground connection is what makes that happen.

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