Power Meter for Variac

[JayMan07]

WOW that pic turned out huge let me post it again like this.

[IanB]

There was a picture showing a circuit diagram, and you didn't include it?

[floobydust]

It looks like I've already drawn 
Confirm with an ohmmeter, that voltage-sense resistor R13 one end, the trace goes to terminal block "Line" and we want to cut that trace with a decent say 1mm gap. Then run a small wire from that end of R13/trace to the variac's hot output. Should have no continuity under 1MEG from that wire to anything if you found the right trace.
Incoming AC power goes to the Line and Neutral terminal block pins, so the power meter runs off that.
Variac Neutral you just tie to the neutral somewhere.
The current-transformer I think goes over the Hot output wire, unless you want to include the variac's losses and have it upstream on the incoming Line wire.

I would take q-tips and IPA and clean up the board a bit, by U4/C13 it's high voltage and I've never liked flux there.
Also a couple solder balls stuck to the board, a toothbrush works or the q-tip. Let it dry off a while.

The backside doesn't show the Line/Neutral labels. Line is the outside terminal.

[JayMan07]

There was a picture showing a circuit diagram, and you didn't include it?

Sorry about that!  I took several pictures and didn't realize the one I uploaded had the diagram cropped off.  Floobydust got it though!  That's the correct diagram he posted!

[JayMan07]

It looks like I've already drawn 
Confirm with an ohmmeter, that voltage-sense resistor R13 one end, the trace goes to terminal block "Line" and we want to cut that trace with a decent say 1mm gap. Then run a small wire from that end of R13/trace to the variac's hot output. Should have no continuity under 1MEG from that wire to anything if you found the right trace.
Incoming AC power goes to the Line and Neutral terminal block pins, so the power meter runs off that.
Variac Neutral you just tie to the neutral somewhere.
The current-transformer I think goes over the Hot output wire, unless you want to include the variac's losses and have it upstream on the incoming Line wire.

I would take q-tips and IPA and clean up the board a bit, by U4/C13 it's high voltage and I've never liked flux there.
Also a couple solder balls stuck to the board, a toothbrush works or the q-tip. Let it dry off a while.

The backside doesn't show the Line/Neutral labels. Line is the outside terminal.

Awesome thank you!!!  I should have a chance to do the mod today and I'll report back!

[JayMan07]

Ok well it worked perfectly    Big thanks to Floobydust   

Here's where I added the volt sense wire




I first tested the voltage and current at 120VAC with a small load (I wanted to see if the clamp was capable of measuring low amperage accurately).  Note PF is accurate the light bulb was a florescent and not a filament.




It was very accurate at 120VAC so I tried the current again at 24VAC.  It was a little less accurate but still plenty good enough!

[floobydust]

That's great it worked out  I wasn't sure if the IC's firmware would be OK down to low voltages. These energy metering IC's have an "anti-creeping accumulator" which I think can be set to register no energy/no load below a certain drain, so there is a cutoff but likely a few mA.

I have also done the voltage-sense mod with low cost 22mm mains-powered AC voltmeters, they use a capacitive-dropper inside.

[bdunham7]

How low will it read the voltage?  All the way to zero?

[JayMan07]

That's great it worked out  I wasn't sure if the IC's firmware would be OK down to low voltages. These energy metering IC's have an "anti-creeping accumulator" which I think can be set to register no energy/no load below a certain drain, so there is a cutoff but likely a few mA.

I have also done the voltage-sense mod with low cost 22mm mains-powered AC voltmeters, they use a capacitive-dropper inside.

Yeah it's an extremely useful mod for all of these ac meters.  I'm sure I'll find use for it again.  You would think that some company out there would just make these with another port on the terminal block to hook up a voltage sense wire, but I guess it's a lot more common for people to want to measure the supply voltage.

[JayMan07]

How low will it read the voltage?  All the way to zero?

Yes it does basically read all the way down to 0.  The resolution is low.  It reads in whole numbers and my variac only goes down to 0.6VAC which it reads as 1

[igor78]

I think the variac mod would go like this for a PZEM-0022, cut one trace and add a wire from R13 to the variac's mains output.

I have variac with truly isolated output.

I'm a bit confused wrt isolation.
Mods like this seem to create a galvanic coupling between secondary and primary, either creating a flying lead to sense voltage on the variac output or powering the PZEM-* from the input.

Is this correct understanding?
I'm trying not to blow up my oscilloscope with this mod.

[SilverSolder]

I think the variac mod would go like this for a PZEM-0022, cut one trace and add a wire from R13 to the variac's mains output.

I have variac with truly isolated output.

I'm a bit confused wrt isolation.
Mods like this seem to create a galvanic coupling between secondary and primary, either creating a flying lead to sense voltage on the variac output or powering the PZEM-* from the input.

Is this correct understanding?
I'm trying not to blow up my oscilloscope with this mod.

The "belts and braces" approach might be to have a separate small transformer just to power the meter and ensure it is galvanically isolated from mains.

[floobydust]

A variac does not provide galvanic isolation, because they are auto-transformers and constructed as a single wire wrapped on the core.
I've never seen one that isolates, unless there is also a separate isolation transformer before or after the variac.

[TimFox]

Staco division of ISE (q.v.) makes variable transformers that have an isolated primary and sliding-tap secondary, but normal Variacs (originally a trademark of General Radio) are single-winding auto-transformers with no isolation (common neutral connection).

[bdunham7]

The less commonly found Chinese-made SC10T variacs are purportedly isolated.  Available on eBay, etc.  I haven't hi-pot tested any myself...

https://www.parts-express.com/10A-Variac-0-130-VAC-120-844?gclid=Cj0KCQjw9YWDBhDyARIsADt6sGaogGsY6oGico96wDlW89l-NueSjApkoz_HcaB3A8bQRtyNDrB2juwaAnooEALw_wcB

[igor78]

I have this https://www.metrel.si/en/shop/LABS/power-supply-other-units/ma-4804-power-supply.html

It says that it's isolated and it is indeed isolated


Manual  https://www.metrel.si/assets/Metrel/Navodila_instrumentov/Instruments/MA_4804_Power_Supply/Ang/MA_4804_Variable_transformer_ANG_Ver_3.0_20_750_280.pdf

[igor78]

I think I found the answer I was looking for. I need to find a way to power the PZEM from the top of the isolated secondary coil. That way it will see full voltage and stays fully on the isolated side.

[floobydust]

If you use the top secondary-end to power a voltmeter, it's 245VAC with 230VAC input. So the voltmeter might get powered with say 256VAC with 240VAC input. I'm not sure what power system your PZEM is using, if that would be OK.
I would add a small fuse for the voltmeter power, or connect it after the 4A fuse.

[igor78]

My mains are 120Vrms
PZEM says at the back 80-260V
What is the motivation for the fuse? it's not going to protect against voltage spike right? Are you thinking of shorting the secondary coil via the PZEM voltmeter?

[floobydust]

With 120VAC mains and the variac top tap at 128VAC, the PZEM's SMPS or capacitive-divider will be fine. Some have no fuse onboard, others do.
Yes, I like to protect the variac (they're expensive) - as long as there is a fuse somewhere upstream to protect the primary.

[james_s]

You have to mod a power meter.
What I did on my variac is take the meter apart, pick off the power feed to the meter (capacitive dropper) and run that to incoming mains. The voltage-sense resistor I left alone and ran that terminal to the variac output. Or you can pick off the voltage-sense (resistor) and run that to the variac's output.

If you have pics of your meter's pc board, it's not hard to do.

The Ningbo Peacefair PZEM-022 power meter that you pictured, I think it has a small SMPS. Easier to pick off the voltage-sense (resistor) and run that to the variac's output.

That's exactly what I was going to suggest. I played with a similar meter once (friend blew it up on accident so we were repairing it) and found it just used a very basic transformerless power supply with a capacitive dropper.

[electro-56]

I have dismantled a number of similar metering devices to see if one of them could replace the analog metering I used for my Variac bench supply back in 1972. I had no problem breaking off the sensing input from the power path, and they seemed to work down to fairly low voltages and currents. I did note a significant (~2-4V) offset at 0V in some of them, and the current readings seemed to suppress low values. Still, this is largely cosmetic and I could live with it, but there was one deal-breaker that kept me from using any of them. All of them had fairly low update rates, which is irritating when trying to adjust the Variac output. If anyone stumbles across a model with reasonable update rates (~5Hz?), I would be very happy to hear of it.
Thanks

[ledtester]

The Peacefair PZEM-004T is a AC power meter module that has a serial interface. Detailed review here:

https://youtu.be/qRsjsenvlJA

[floobydust]

I also find the chinese power meters have very long averaging time in firmware, it takes a few seconds to get a settled reading with a constant load. They aren't useful for fast response unless you communicate directly with the IC.

[Zero999]

I also find the chinese power meters have very long averaging time in firmware, it takes a few seconds to get a settled reading with a constant load. They aren't useful for fast response unless you communicate directly with the IC.

I wonder if that's because they use cheap/slow parts and a low sample rate?