Meter for an (isolated) variac

[romberg]

I'm a little late to the party in wanting to mount one of those Chinese voltage/current/power meters onto my variac as discussed in this thread about a year ago:

https://ampgarage.com/forum/viewtopic.php?t=32576

I've picked up a couple of pzem-061-ct meters as mentioned in that thread and mounted one in a little power strip as seen below. It works pretty nice for quickly (and safely) measuring what is going on with what is plugged into the strip.

meters.jpg

Now I want to attach the second meter on top of my variac and modify it to run off of a separate ac source in order to read sense voltage down to zero as others have done in the above thread. However there is a snag. My variac has isolated hot and neutral output:

variac.jpg

The hot and neutral really are isolated. I checked them both for resistance/continuity to the hot/neutrals of the mains side and there are four (not three) wires running into coils in the variac. I'm fairly sure there is a concentric (and isolated) primary coil under the outer secondary coil as show in these shots. Note that there are two hots and two neutral wires running to and from the variac guts. Sorry about the blurry pictures but I think they will still work well enough to show what I'm talking about.

guts1.jpg

guts2.jpg

Now my problem. The power meter has only one neutral ac connection. I'm close to certain that I can't do the zero mod by connecting 120VAC mains hot to the clipped cap and have everything turn out ok. The neutral to the meter would be coming out of the secondary side of my variac and the hot powering the meter would be on the primary side (with no reference to the secondaries neutral). Either the meter will not operate or it will destroy my isolation or both.

So, I think I need to do the zero mod by tapping 120VAC or so off of the secondary side of the variac. Splice a Y in somewhere near the point on the secondary coil where the wiper would send 120V to the outlet. This way the meter will all be running off of and sensing the secondary side.

My first plan was to scrape off some insulation near the high end of my secondary and solder on a 22awg wire for the zero mod there. But now I'm wondering if soldering a wire on there is the best approach? I wonder if there are clamps or connectors or some other method that would be better for tapping this coil. I fear soldering may risk melting something I don't want melted . I'm kinda looking around at things called magnet wire terminals. Which look promising. But I'm not sure. I'd appreciate others thoughts about this.

Thanks!

Mike

[sluckey]

If you don't mind giving up the isolation feature I think you could simply jumper the "N" and "n" connections. See linked pdf...

https://www.alltronics.com/alltronics_a ... 28N051.pdf

[sluckey]

Should have added... If you are looking for my variac pdf that info has been added to my amp scrapbook pdf, pages 4 and 5...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[romberg]

Thanks Steve! Your (excellent) drawings were my motivation for this mini project. I made sure to buy the exact same meters you did to ensure that the zero mod had the best chance of working .

Tying the neutrals togeather might be something to consider. The earth grounds on this are tied together, This is probably a good idea and can easily be defeated with a 3->2 prong adapter. I could add a switch to the variac to connect the neutrals I guess. Of course the meter would not work or read crazy when in the isolated mode.

In the end I do plan to use this to work on an old 1947 tube radio I dug up. So, the isolation feature does sound like something I would like to keep. But adding a switch has got me thinking a bit...

It does not look too difficult to tap into the secondary coil. The winding is right on the outside. And I think the meters will run fine all the way up to 240VAC. It would have been really nice if they just had a tap already installed on that end .

Mostly I'm wondering if using something like this would be a better method of connecting to that end of the magnet wire rather than trying to scrape off some insulation and solder? I might be able to slip one of these compression connectors under the last winding. I just don't know how well that will work or not. I've never played around with inductors much

https://www.standardelectricsupply.com/ ... 966-220019

[sluckey]

In the end I do plan to use this to work on an old 1947 tube radio I dug up. So, the isolation feature does sound like something I would like to keep.

That 1947 radio will most likely have a power transformer in it. There is no benefit to using an "isolated" variac to work on that 1947 radio. You may never need the isolation feature unless you work on one of those cheap transformerless amps (Valco, Harmony, Kay, etc.), or an All American 5 radio, or a transformerless tv set from the '60s.

Examine your variac closely. The secondary coil has two ends even though only one is connected. If you can find the other end you can solder the "fifth" wire to it. Or you can carefully scrape off a bit of varnish near the hot end of the secondary winding and carefully solder directly to the coil wire. Remember, the wires that connect to the pzem-061 can be tiny. I used 24AWG teflon stranded. I would brush on a bit of corona dope to reinsulate the solder connection. This would allow you to keep the isolation feature, even though you'll probably never use it. It's worth doing IMO.

[romberg]

In the end I do plan to use this to work on an old 1947 tube radio I dug up. So, the isolation feature does sound like something I would like to keep.

That 1947 radio will most likely have a power transformer in it. There is no benefit to using an "isolated" variac to work on that 1947 radio. You may never need the isolation feature unless you work on one of those cheap transformerless amps (Valco, Harmony, Kay, etc.), or an All American 5 radio, or a transformerless tv set from the '60s.

Alas the radio in question is an AA6. Found it in the parents garage (probably from the gradparents) and can't stand the thought of tossing it out without at least an honest attempt to recap it and see if it can be made operational again. You are probably right in that if I'm careful, install a polarized power chord, and move the goofy power switch off the neutral then I will probably not kill myself. Still this is the first circuit of this kind I've tinkered with. So, having the isolation seems like a good idea.

R1229-schematic.pdf

Examine your variac closely. The secondary coil has two ends even though only one is connected. If you can find the other end you can solder the "fifth" wire to it. Or you can carefully scrape off a bit of varnish near the hot end of the secondary winding and carefully solder directly to the coil wire. Remember, the wires that connect to the pzem-061 can be tiny. I used 24AWG teflon stranded. I would brush on a bit of corona dope to reinsulate the solder connection. This would allow you to keep the isolation feature, even though you'll probably never use it. It's worth doing IMO.

Thanks for the ideas! In particular, I was unfamiliar with corona dope. I'll check into that. I think I will probably also add a neutral "bond"/isolate switch while I'm at it. I agree that most of the time having the secondary isolated is not worth it. With a switch, I could have both in one package.

Mike

[romberg]

It works! I soldered the second hot wire right onto the secondary coil (and am getting about 140VAC on it). It is just alligator clipped onto the cap right now. But the meter reads right down to zero now.

meter-zmod.jpg

Next I'm going to mount the meter into a little aluminium enclosure I bought for it and stick that on top of the variac.

Thanks again Steve for your excellent drawings and advice!

Mike

[turbofeedus]

Just chiming in because I started the original thread that was evoked. I've been using that setup with the current sense loop successfully since it was completely, apparently for a year now. Works great.