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!