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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 11:22 am 
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Mikeinkcmo wrote:
But, swap the Green with either of the other two, and you have something like this. Yes, I know, no body likes these pesky little drawings. But it would only take one episode to really ruin your day.

Image

Of course everyone can do as they wish, but I still believe its better with "NO GROUND AROUND".


So the radio has no power going to it as the output is open. So what's the problem? You can touch any part of the radio and not get shocked.


Look at the schematic.
The Radio's cabinet, the box, will have one side of the ISO transformer's secondary tied to it.

When the power switch is closed, (ya gotta have power to test it) the other side of the ISO output is tied to ground through the primary of the receiver's power transformer.

So now, the cabinet is HOT, with respect to ground, by virtue of the very "SAFETY GROUND" pin (as so many love to call it) at the power socket. You're better off with "no ground around".

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 11:58 am 
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Joined: Feb Wed 07, 2018 6:52 pm
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Location: Stone Mountain, GA
Ok, no connection dot made confusing.

You can find a shock hazard in any situation. I prefer the ground, besides it is real difficult to isolate in a metal box with a standard outlet. I was going to use a GFCI but I wanted two different outlets (dim bulb plugs in one, DUT in other.) A power cord that miswired would be real rare and would probably be found under initial inspection.

I really only use the isolation transformer on transformerless devices. Sometimes use it on switching supplies so I can put the scope ground on the rectified AC. But then usually remove supply and use own cord.

The only time I needed an isolated ground was diagnosing a cable issue. Ended up ungrounding scope. I had 14VAC on the signal ground at a considerable current. Turns out to be a mis-designed power strip.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 12:38 pm 
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Quote:
...A power cord that miswired would be real rare and would probably be found under initial inspection...
Oh, I agree, but the goal is to be unconditionally safe even if one were to forget the initial inspection, NOT just safe except on those rare occasions when you're not.

BTW here is a more "correct" drawing.

Image

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Last edited by Mikeinkcmo on Jun Sun 06, 2021 1:02 pm, edited 1 time in total.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 5:53 pm 
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Joined: Jan Tue 10, 2012 8:39 am
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I don't see the problem here. The ONLY reason one would use an isolation transformer would be to isolate. In other words, the secondary of said transformer has no connection with the primary unless one creates one.

So you connect the primary to the power line and the secondary to the load. You can put a variac in between anywhere.

If you add a ground, no problem.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 7:01 pm 
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Location: Auburn, AL
I understand mike’s reasoning, that you should only ground something intentionally because you need to, and not by accident. BUt if I were to take it that far, I’d also fill the ground connection hole in the killawatt, so that there was no chance of anyone thinking they were grounding something through the safety ground plug at the outlet, when in fact they were not. Go to plug a three wire cord in, you’ll realize you can’t, and have to notice that the case is not grounded. I’d also label it as being isolated, along with the max watts/amps it’s fused for.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sun 06, 2021 1:24 pm 
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Bob91343 said;
Quote:
...I don't see the problem here. The ONLY reason one would use an isolation transformer would be to isolate. In other words, the secondary of said transformer has no connection with the primary unless one creates one.

So you connect the primary to the power line and the secondary to the load. You can put a variac in between anywhere.

If you add a ground, no problem...

You are absolutely correct, SO LONG AS....YOU, determine, where it is, Ground will be connected, and, ground IS NOT an integral part of the connection to power.

This is why I use a 3 wire connector, but the ground pin is not connected. This allows me to plug in anything with a power cord, but the ground is an entirely separate issue, which I control.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sun 06, 2021 9:44 pm 
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Ground control to Major Mike.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jul Tue 06, 2021 12:57 pm 
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Joined: Jul Tue 06, 2021 5:49 am
Posts: 6
SHenion wrote:
I was going to use a GFCI
Kinda curious why youse guys don't have your entire bench powered from a GFCI? I do.

BTW I have a used 250VA constant voltage transformer from fleaBay as my isolation transformer because they are cheap used - and also have the interesting property of being inherently current limited. BUT - you have to put it first before your Variac, not the other way around.

BTW youse guys probably already know that the cheap Chinese Variacs all have the fuse on the primary side which is WRONG - move it to the secondary before you cook your Variac.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Sep Wed 15, 2021 9:16 pm 
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Joined: Nov Sun 19, 2017 4:24 am
Posts: 25
Location: Rossland BC
I've got a Variac in a custom metal case with analog amp and volt meters and with a modern duplex receptacle (grounded) on the front. Plugged into one of the front outlets is an old metal-cased Triad N-54M isolation transformer--I say "old" because that same model nowadays sports a polarized receptacle. Into that transformer I would plug my DUT and be confident that I'm protected.

I just replaced the frayed twisted-pair power cord on my '31 cathedral "project" with a two-prong polarized plug/cord, ensuring that the hot lead was connected correctly as per the schematic. When I went to try it out I realized that my new polarized plug won't fit the old ISO transformer receptacle. What to do?

Is a polarized receptacle on newer ISO transformers merely a plug compatibility feature? Does the output of an ISO transformer have a hot and neutral?

I'm a chronic over-thinker. All my sets have power transformers with switched hot primaries. Do I need to use an ISO transformer?

Chuck


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Sep Wed 15, 2021 10:16 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2806
An isolation transformer, if it truly isolates, has no preferred polarity. At least not so you can figure out which side is which. For any normal use of such a transformer, it doesn't matter.

If your new style 2-pin plug won't fit both ways, I suggest file it down so it will. The wide blade is the cold side, normally white if you are an electrician. Since you are probably interested in preserving the polarity then it's wiser to make an adapter using an old style plug connected to a modern outlet. I wouldn't attempt replacing the outlet on the transformer unless it's clear that the new part will fit and not require major surgery.


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