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 Post subject: Variac Problems
PostPosted: Nov Mon 27, 2017 12:28 am 
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Posts: 190
Location: Cedar Rapids, Iowa
I have a variac that I obtained many years ago from someone who had put it into an aluminum cabinet along with an isolation transformer and voltmeter and a switch to switch the isolation transformer in and out of the circuit. I rarely used it until I got into the radio restoration hobby 7 or 8 months ago. Recently I noticed that when I went to connect the ground from my signal generator to the chassis of the radio I'm working on it arced a little bit. So I investigated and measured the voltage between the two when not connected and saw 20vac. The power plug on the variac was non-polarized so I reversed it. Then when I connected the signal generator ground to the chassis it was more than an arc...it was more of a bang!

So I took the variac cabinet apart and decided to rewire it, including installing a new 3 wire power cord. It was wired so the isolation transformer was first in line, then the variac. I've seen it both ways in various places on the internet. In a recent video from Mr. Carlson's Lab on YouTube he showed his setup and the isolation transformer was after the variac so I rewired it that way. I drew up a schematic for my current setup:
Attachment:
variac_schematic_r.png
variac_schematic_r.png [ 33.9 KiB | Viewed 531 times ]
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. But when I measure between the neutral of the outlet and the ground on my signal generator I get 35vac. When I measure between the hot of the outlet and the ground on my signal generator I get 100vac.

What the heck is going on? I've checked the outlets that both the variac and the signal generator are plugged into and the outlet tester says they are ok. When I plug the radio I'm working on straight into an outlet I get 0vac from the chassis (which is connected to the neutral of the ac line) to the ground of the signal generator. All of my tests thus far have been with the switch in the isolated position and the variac set to about 115v.

Any ideas what's happening and how I can correct it?

Thanks,
Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 12:35 am 
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Hi Rich,

Did you check your wall outlet? It may well be wired wrong. That's rather common.

1st suggestion... get rid of the switch.
The isolation transformer should ALWAYS be inline before the radio being serviced.

It does not matter whether the iso or the Variac is connected to the wall.
I usually connect the one with the higher VA rating first, then the lower one, then the radio.

- Leigh

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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 12:52 am 
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Joined: Mar Thu 02, 2017 1:37 am
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Location: Cedar Rapids, Iowa
Thanks Leigh! Yes, I check the outlets that both the variac and the signal generator are plugged into and they are good. They are both plugged into power strips...I checked the power strips as well as the outlet they are plugged into.

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 2:25 am 
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Joined: Apr Sun 23, 2017 11:22 pm
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Location: 44035 (Near Cleveland Ohio)
I second Leigh's advice on getting rid of the switch. One less device to go bad. Usually in radio restoration work, you'll probably want to use both in series with each other anyhow. I suspect the problem might be within your signal generator - what model do you have?

Dan


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 2:35 am 
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Location: 44035 (Near Cleveland Ohio)
Rich - - what type of radio are you working on? Many non-transformer AA5 variants call for the signal generator "ground" to be connected to the B-, not the chassis ground. Just a thought.

Dan


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 3:00 am 
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Quote:
It does not matter whether the iso or the Variac is connected to the wall.
I usually connect the one with the higher VA rating first, then the lower one, then the radio.

Both the Variac and the isolation transformer have losses. Connecting the higher rated one direct to your AC outlet allows the maximum usable output from your device.

If you had problems initially, either the switch was in the wrong position, there was a wiring error, the switch or solation transformer is defective, or there was a short somewhere. Since some of these involve things that you are reusing, make sure they are all in good shape or your rebuild may not help.

I agree, get rid of the switch. If you want to use the Variac without isolation, put it in a separate box so that you have to deliberately connect it that way and there is no chance of accidentally bumping the switch and getting non-isolated power.

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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 4:54 am 
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Location: Cedar Rapids, Iowa
SparkyDan wrote:
I second Leigh's advice on getting rid of the switch. One less device to go bad. Usually in radio restoration work, you'll probably want to use both in series with each other anyhow. I suspect the problem might be within your signal generator - what model do you have?

Dan

Thanks for the input Dan! I used the signal generator as an example...I get the same reading if I measure to the outside connection on the BNC connectors on the frequency counter and oscilloscope.

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 4:55 am 
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Joined: Mar Thu 02, 2017 1:37 am
Posts: 190
Location: Cedar Rapids, Iowa
SparkyDan wrote:
Rich - - what type of radio are you working on? Many non-transformer AA5 variants call for the signal generator "ground" to be connected to the B-, not the chassis ground. Just a thought.

Dan

I know what you mean Dan...and this radio does have the low side connected directly to the chassis. It's a Fada 148.

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 5:20 am 
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Location: Dallas, TX
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.

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 1:46 pm 
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The fact that your outlet tester did what it did tells me the ground slot on the outlet is connected. If the ground slot of the outlet is not connected, then the red light should never be able to come on, nor should the right hand yellow light. Those two can only light up because current is flowing from the hot side to the ground slot. (When it showed reversed, that was current flowing from the neutral slot to the ground slot.) The fact that either is lighting up says your ground slot in the outlet is connected.

To double check this, (Obviously unplugged) put one meter probe to the ground screw on the outlet, and the other probe on the case, then one at a time on each of the plug prongs. You should get no reading. If you get anything at all, then the ground slot on the outlet is connected somewhere.

The ground screw on the outlet should have no wires going to it. Also look for damaged insulation on all of your wires, and make sure there are no strands of wires from any of the connections that might be sticking out, touching something. I'd also disconnect all the wires from the outlet, and use the ohms meter between the ground screw and both hot and neutral. If you get any reading, then change the outlet.

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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 3:01 pm 
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Joined: Mar Thu 02, 2017 1:37 am
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Location: Cedar Rapids, Iowa
I think you guys nailed it! I just did a quick test this morning and will do more testing later. I removed the outlet from the mounting point and let it sit to the side. Now the tester shows "open ground" which is what I would expect, and I read virtually zero volts ac from neutral to the ground on the signal generator.

Now my question is, how do I handle this? I see a couple of options and wonder which will be best.

1. Find an outlet that does not have the screw mount grounded. Do these exist?
2. Replace the incoming power cord with a two wire cord and eliminate the ground (or just don't connect the existing green wire to the cabinet). The cabinet was not grounded prior to my modifications, but I think it should be grounded so don't really like this idea.
3. Somehow insulate the mounting screw for the outlet. Maybe put a thin insulator between the outlet and the inside of the cabinet (I can easily make one on the 3D printer) and use a plastic or nylon screw?
4. Use a 2 prong polarized outlet and use an adapter when I need to plug in something with a 3 prong power cord.
5. Any other ideas?

Thanks for all your help guys...I appreciate it!

Rich


Attachments:
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IMG_20171127_074025.jpg [ 209.43 KiB | Viewed 410 times ]
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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 3:29 pm 
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The grounded ground pin on the outlet is not what is causing the fireworks between your signal generator and the radio.

When the outlet is in the cabinet (and so the ground pin is grounded), then if the outlet tester doesn't indicate a problem with both the hot and neutral connections, you do not have a truly isolated isolation transformer. If so, you need to find and fix that problem first.

Since in your earlier post you said your tester indicated that hot and neutral were swapped, and you swapped them and then the outlet tester said "good", that tells me that neutral is connected to ground. That connection is what the isolation transformer is supposed to break. If that connection isn't broken, you risk fireworks whenever connecting test equipment to a transformerless radio.

Your tester should show "open ground" if everything is working correctly and the ground pin is in fact connected to ground.

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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 3:57 pm 
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Posts: 538
Location: Dallas, TX
criageek wrote:
Now my question is, how do I handle this? I see a couple of options and wonder which will be best.

1. Find an outlet that does not have the screw mount grounded. Do these exist?
2. Replace the incoming power cord with a two wire cord and eliminate the ground (or just don't connect the existing green wire to the cabinet). The cabinet was not grounded prior to my modifications, but I think it should be grounded so don't really like this idea.
3. Somehow insulate the mounting screw for the outlet. Maybe put a thin insulator between the outlet and the inside of the cabinet (I can easily make one on the 3D printer) and use a plastic or nylon screw?
4. Use a 2 prong polarized outlet and use an adapter when I need to plug in something with a 3 prong power cord.
5. Any other ideas?

Thanks for all your help guys...I appreciate it!

Rich

I would keep the three wire power cord. Check out the isolation transformer, there should be an open circuit between the primary and the secondary and the transformer core/case. If the transformer is fine, I would have it permanently wired into the circuit. The three prong ungrounded-to-mount must exist because I built a isolation case with one, mine is round, maybe black Bakelite. I don't think you can find the right kind at Home Depot, try electronics supply companies. A two prong with adapter could also work.
EDIT:
I searched for the kind of outlet I used and discovered that it is a vintage Amphenol. I found some at Nebraska Surplus for $ 30!! I didn't know I had a gold mine in my junk box!
However this type seems to be the current style. I've used them also, be very exact with the panel cut out though.
https://www.mouser.com/ProductDetail/Qu ... 252bdoU%3d

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 6:19 pm 
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If your iso (isolation transformer) is a modern product rather than a vintage product, that's your problem.

A vintage iso has a secondary that's completely isolated from everything else.

Modern isos only isolate noise, preventing it from flowing from one side to the other.
They connect the supply neutral through to the secondary neutral.

That neutral connection is usually a jumper within the iso that can be removed.
Doing so provides complete isolation of the secondary, which is what we want.

Once you've made that change, check the secondary to the primary and to chassis with an ohmmeter on its highest range.
You should read an open circuit.

- Leigh

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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 7:18 pm 
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Joined: Mar Thu 02, 2017 1:37 am
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Location: Cedar Rapids, Iowa
Thanks again guys! I ran up to Lowe's to get a few things, including a two prong polarized outlet. But I think I'm glad they didn't have one. The more I think about it the more this from stevebyan makes sense:

"Since in your earlier post you said your tester indicated that hot and neutral were swapped, and you swapped them and then the outlet tester said "good", that tells me that neutral is connected to ground. That connection is what the isolation transformer is supposed to break. If that connection isn't broken, you risk fireworks whenever connecting test equipment to a transformerless radio."

I'm in the process of doing what several have suggested...removing the switch so the isolation transformer is always connected. Besides the safety issues it will simplify the wiring. I don't think the isolation transformer is the problem. It's not what I would call a 'modern product'. I don't remember when I acquired this variac/isolation transformer, but I'm certain it was at least 30-35 years ago. I have no continuity between the primary and the secondary, none between the primary and the case, and none between the secondary and the case.

Am I right in thinking that when all is working properly there really isn't a 'hot' and a 'neutral'?

Thanks again guys...I'll update you when I have the re-wiring done.

Rich


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IMG_20171127_121156_r.jpg
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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 8:40 pm 
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criageek wrote:
I don't remember when I acquired this variac/isolation transformer, but I'm certain it was at least 30-35 years ago. I have no continuity between the primary and the secondary, none between the primary and the case, and none between the secondary and the case.

Am I right in thinking that when all is working properly there really isn't a 'hot' and a 'neutral'?
Hi Rich,

It sounds like your transformer is fine.

On the secondary side there is no "hot" or "neutral".
Those terms are only relevant to AC service lines like at the wall socket or fusebox.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 11:06 pm 
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Location: Cedar Rapids, Iowa
Well, if I wasn't confused and frustrated before I certainly am now. I've completed the rewiring and this is my current circuit:
Attachment:
variac_schematic_wo_switch.png
variac_schematic_wo_switch.png [ 25.19 KiB | Viewed 344 times ]

Seems simple enough. But here are the results of my testing:

Outlet not mounted (nothing on ground lug):
Variac: 125vac
Outlet Tester: Open Gnd
Neutral to Sig Gen Gnd: 0.76vac
Hot to Sig Gen Gnd: 123.8vac

Outlet mounted (ground connected):
Variac: 125vac
Outlet Tester: OK
Neutral to Sig Gen Gnd: 40vac
Hot to Sig Gen Gnd: 108vac

Here are my connections on the variac:
Attachment:
IMG_20171127_153702_r.jpg
IMG_20171127_153702_r.jpg [ 243.03 KiB | Viewed 344 times ]

The black wire on the left is line in. The one on terminal 3 is the wiper and goes to the primary of the isolation transformer. One of the white wires is line in, and the other goes to the other primary lead on the isolation transformer.

Any thoughts? I'm considering removing everything from the cabinet and setting it on the workbech, with the green ac line wire connected to the ground lug on the outlet, and see what I get that way.

Thanks,
Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Mon 27, 2017 11:18 pm 
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criageek wrote:
Outlet mounted (ground connected):
Variac: 125vac
Outlet Tester: OK
Neutral to Sig Gen Gnd: 40vac
Hot to Sig Gen Gnd: 108vac
Hi Rich,

What meter are you using?
It's common to get arbitrary readings when using a 10 Megohm input meter.
That's due to capacitive coupling within the transformer. It's not real.
If you connect a 10K resistor between those two points, the voltage across it it will be quite low.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Variac Problems
PostPosted: Nov Tue 28, 2017 12:06 am 
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Location: Cedar Rapids, Iowa
Thanks Leigh - I'm using a Fluke 77 to take my measurements so that may explain the goofy readings. But that still leaves stevebyan's assertion:

"Your tester should show "open ground" if everything is working correctly and the ground pin is in fact connected to ground."

Rich


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 Post subject: Re: Variac Problems
PostPosted: Nov Tue 28, 2017 12:20 am 
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I just realized what the problem is. Other than I need more sleep before I post, though I still stand by what I said. But it just occurred to me. Your variac is in a metal case, which you've grounded. If the outlet is mounted right to the metal case, it now becomes grounded since the metal frame of the outlet is part of the ground. You need to mount it so that no part of the outlet touches the metal.

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