Power Meter for Variac

[JayMan07]

I can't seem to find a simple power meter that I can mount on my variac that allows me to see watts.  All of the ones I've looked at say they are capable of reading AC ~80-240V.  I can't find one that will read at low voltage range.  If I'm powering up a piece of equipment I want to see if it's pulling excessive wattage at low voltage.

Here's an example of one I have mounted on my generator panel, but again it says it doesn't read below 80VAC.


https://www.amazon.com/gp/product/B07JB9B2QL/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1

Does anyone know of a meter that will work or have another suggestion for how to do this?  Thanks!!!

[bdunham7]

I don't know of one offhand, but you need a meter that is powered separately from the circuit that it measures.  Mechanical meters might work better, but those digital modules are so cheap and tempting...

[JayMan07]

Yes I had thought about that.  I would power the meter off the input to the variac (120VAC) and then the meter would be monitoring the output.

[IanB]

I can't seem to find a simple power meter that I can mount on my variac that allows me to see watts.  All of the ones I've looked at say they are capable of reading AC ~80-240V.  I can't find one that will read at low voltage range.  If I'm powering up a piece of equipment I want to see if it's pulling excessive wattage at low voltage.

Why don't you put the power meter on the input side of the variac where the voltage is constant? It may not be 100% accurate, but it will give you a pretty good indication. If you do that you could even use a Kill-A-Watt kind of device to get the reading without needing anything special.

[bdunham7]

Why don't you put the power meter on the input side of the variac where the voltage is constant? It may not be 100% accurate, but it will give you a pretty good indication. If you do that you could even use a Kill-A-Watt kind of device to get the reading without needing anything special.

The problem is that this method is inaccurate and unstable at the very low end of the variac, for example when bringing up a DUT to power when it is in fact shorted or nearly so.  For example, an audio amplifier with a shorted rectifier after the transformer will suddenly start to draw huge currents at a very low voltage, but what that current is compared to the input current will vary wildly with only a small movement of the variac control.  This is why properly designed variac systems have input and output current protection.

[schmitt trigger]

How low do you require to go?

I would use a transformer which has 120V/240V windings as a 1:2 autotransformer to double the input voltage to the meter.
IMPORTANT: you will only be doubling the voltage to the meter, not to the load.

That way, your meter will be able to read down to 40V, and you simple divide by two the watts readout.

There are (more expensive) machine tool transformers with ratios as high as 100V/600V, which would allow you to sextuple the output voltage, and read all the way down to 13 volts.

CAREFUL when turning the voltage back up!!

[IanB]

The problem is that this method is inaccurate and unstable at the very low end of the variac, for example when bringing up a DUT to power when it is in fact shorted or nearly so.  For example, an audio amplifier with a shorted rectifier after the transformer will suddenly start to draw huge currents at a very low voltage, but what that current is compared to the input current will vary wildly with only a small movement of the variac control.  This is why properly designed variac systems have input and output current protection.

I see, but surely in this case you do not need to see power, you need to see current. A clamp-on current meter on the output side of the variac will be able to show excessive current draw with no constraints on the output voltage?

[JayMan07]

Why don't you put the power meter on the input side of the variac where the voltage is constant? It may not be 100% accurate, but it will give you a pretty good indication. If you do that you could even use a Kill-A-Watt kind of device to get the reading without needing anything special.

The problem is that this method is inaccurate and unstable at the very low end of the variac, for example when bringing up a DUT to power when it is in fact shorted or nearly so.  For example, an audio amplifier with a shorted rectifier after the transformer will suddenly start to draw huge currents at a very low voltage, but what that current is compared to the input current will vary wildly with only a small movement of the variac control.  This is why properly designed variac systems have input and output current protection.

Yes exactly!  I need something that will show that low voltage/high current state if it's present!

[JayMan07]

The problem is that this method is inaccurate and unstable at the very low end of the variac, for example when bringing up a DUT to power when it is in fact shorted or nearly so.  For example, an audio amplifier with a shorted rectifier after the transformer will suddenly start to draw huge currents at a very low voltage, but what that current is compared to the input current will vary wildly with only a small movement of the variac control.  This is why properly designed variac systems have input and output current protection.

I see, but surely in this case you do not need to see power, you need to see current. A clamp-on current meter on the output side of the variac will be able to show excessive current draw with no constraints on the output voltage?

I could look at current but it would really be nice to see wattage.  Quite a few of the variacs that I've seen that have meters built into them (besides the obvious voltage meter) use wattage when monitoring output.  A watt meter seems like such a simple thing to incorporate into a variac...that's why I was just especially frustrated when I looked and looked, and could not find an add-on meter capable of something so simple.

[wizard69]

Isn't that an indication that a power meter is the wrong solution.   If you are concerned about excessive current draw it would seem that ammeter is the better choice?

Why don't you put the power meter on the input side of the variac where the voltage is constant? It may not be 100% accurate, but it will give you a pretty good indication. If you do that you could even use a Kill-A-Watt kind of device to get the reading without needing anything special.

The problem is that this method is inaccurate and unstable at the very low end of the variac, for example when bringing up a DUT to power when it is in fact shorted or nearly so.  For example, an audio amplifier with a shorted rectifier after the transformer will suddenly start to draw huge currents at a very low voltage, but what that current is compared to the input current will vary wildly with only a small movement of the variac control.  This is why properly designed variac systems have input and output current protection.

[TimFox]

For your application, is it important to know the actual power (as in the meters used by your electrical utility), or only the VA (volts times amps, ignoring phase angle)?

[bdunham7]

I could look at current but it would really be nice to see wattage.  Quite a few of the variacs that I've seen that have meters built into them (besides the obvious voltage meter) use wattage when monitoring output.  A watt meter seems like such a simple thing to incorporate into a variac...that's why I was just especially frustrated when I looked and looked, and could not find an add-on meter capable of something so simple.

If you are using this for repair work, wattage will just be distracting and misleading.  A large current in quadrature, as you might have with a failed half-wave mains-direct rectifier with  filter capacitor, will not show up as high wattage but will still melt your variac or DUT--or explode that filter cap.  If you want measure power for devices that aren't broken, just plug a Kill-a-Watt into the unit. 

[floobydust]

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.

[bdunham7]

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.

Do they then read the AC voltage accurately (as far as they are accurate) down to zero volts?  If so, this sounds like a winning plan.

[JayMan07]

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.

Do they then read the AC voltage accurately (as far as they are accurate) down to zero volts?  If so, this sounds like a winning plan.

Yes that would be awesome if that works?  So were you able to get it to read AC volts and watts down to 0!?

[JayMan07]

I could look at current but it would really be nice to see wattage.  Quite a few of the variacs that I've seen that have meters built into them (besides the obvious voltage meter) use wattage when monitoring output.  A watt meter seems like such a simple thing to incorporate into a variac...that's why I was just especially frustrated when I looked and looked, and could not find an add-on meter capable of something so simple.

If you are using this for repair work, wattage will just be distracting and misleading.  A large current in quadrature, as you might have with a failed half-wave mains-direct rectifier with  filter capacitor, will not show up as high wattage but will still melt your variac or DUT--or explode that filter cap.  If you want measure power for devices that aren't broken, just plug a Kill-a-Watt into the unit.

Thank you and you are making a really great point!  I think focusing on current is a good idea, but I just really like that style of meter I posted and how it's got all the various measurements including current right there on the screen.  It's just crazy to me that I cannot find one that reads down to 0VAC.  Why do they limit them all to 80VAC...why, why, why lol   

[TimFox]

The voltmeters with a high lower limit are designed for users who need to adjust the voltage to a precise value near 120 V.  Your application is different.  With analog meters, we called this an "expanded scale" movement.  By the way, "true rms" meters do not measure accurately down to zero;  you need range switching to read values well below 10% of full scale.

[floobydust]

With the mod, I found them accurate right down to a few VAC and good enough for me. The main thing is the energy meter always stays powered up instead of conking out below 80VAC.
I trace the pc board and voltage-sense, a voltage divider to an active rectifier (then MCU A/D) or straight to a  power meter IC. It depends on the make/build.

PZEM-004+LED display board using popular SDIC SD3004 IC datasheet which shows the basic circuit. You find the 1MEG resistor pcb R3/datasheet R9  and cut trace, and connect to the variac output.
OP's is the LCD display variant, which I have not seen inside but likely just a capacitive-dropper instead of SMPS.

pic taken from from http://pdacontrolen.com/electricity-consumption-meter-peacefair-pzem-004-esp8266-arduino-nano/

[IanB]

It's just crazy to me that I cannot find one that reads down to 0VAC.  Why do they limit them all to 80VAC...why, why, why lol

Because the meter needs power to work? It's not that it can't measure down to 0 VAC -- it most likely can. It's just that it can't be powered from a 0 V supply. It may accept a wide range of input voltages like 80 V to 240 V, but there is still a lower limit.

However, give the power meter an external power supply and the limitation disappears. This is what floobydust suggested--separate the power input from the measurement input so that the meter has a stable power supply to work from. The only issue I can see here is one of isolation: the unit may not be designed to have a large voltage difference between the power input and the sense input. It might be good to power the meter from a small isolation transformer so it's floating rather than powering it directly from the mains side of the variac?

[JayMan07]

With the mod, I found them accurate right down to a few VAC and good enough for me. The main thing is the energy meter always stays powered up instead of conking out below 80VAC.
I trace the pc board and voltage-sense, a voltage divider to an active rectifier (then MCU A/D) or straight to a  power meter IC. It depends on the make/build.

PZEM-004+LED display board using popular SDIC SD3004 IC datasheet which shows the basic circuit. You find the 1MEG resistor pcb R3/datasheet R9  and cut trace, and connect to the variac output.
OP's is the LCD display variant, which I have not seen inside but likely just a capacitive-dropper instead of SMPS.

pic taken from from http://pdacontrolen.com/electricity-consumption-meter-peacefair-pzem-004-esp8266-arduino-nano/

Alright awesome!  I ordered the meter (from the link I posted above).  It should be here Thursday evening.  I'll take it apart and post a pic of the internals so hopefully you will be able to tell me where to feed in the variac output!  Thanks this will be pretty cool if I can get it to work!

[LateLesley]

I did a quick google, and found this, if it helps??

https://www.aliexpress.com/i/32983209104.html

[bdunham7]

I did a quick google, and found this, if it helps??

https://www.aliexpress.com/i/32983209104.html

That looks good, although you would need to add a small DC power supply.

[ledtester]

Have a look at TheHWCave's video on this module, especially around the 3 min mark:

[floobydust]

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.

[JayMan07]

Alright I got it!  I believe it's exactly the one that is in the video (a PZEM-022).  The main chip is a V9811A.

So what do I do now to make it read low VAC lol 

See next post for the photo of the board!

[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?

[ledtester]

The Peacefair units use the V98XX series of energy metering SoC chips:

https://youtu.be/G0AlH3vtT0k?t=9m12s

http://www.vangotech.com/int/uploadpic/157234475389.pdf

Update: In section 18.3 (physical page 275) of the V98XX datasheet there are some tables showing the update and settle times of the metering registers along with this note:

... In 60Hz power grid, the time for updating and settling is 1.2 times of that for 50Hz power grid. ...

[coppice]

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?

Its because they use the cheap, but quite accurate, devices made for the utility energy meter market. That market cares about accuracy over a wide dynamic range, but not about the update rate.