Using a fixed range AC voltmeter with variable input [SOLVED]

[enut11]

I want to monitor the output voltage and current of a variac using a digital panel meter.
The meters available appear to work over a small voltage range (eg 80-200vAC)
My need is for (close to) zero and all the way up to 300vAC
Current will be around 2A max with a resolution of about 10mA

Has anyone modified one of the fixed range panel meters for variable input?
I guess the first step would be to isolate the meter power circuit from the voltage measurement circuit. Ideas?
enut11

https://www.ebay.com.au/itm/1x-LCD-Panel-Monitor-Current-Amplifier-AC-Combo-Meter-Voltage-Current-Accessory/193163573647?hash=item2cf972398f:m:mLZiHZlTM9LnQukgkhrbDFw

[intabits]

Yes, they can be modified to do what you want.
As they sit, the input to the unit's internal power supply, and the voltage sensing input to the meter at connected together internally.
Separating them allows the unit to be powered from the 240V input to the variac, while measuring the variac output voltage via the meter voltage sense input.
Basically it involves disconnecting the large capacitor (or the resistor in series with it, whichever one it is) from the meter's input terminal and running it instead directly to the mains input.

I did exactly what you are looking at two years ago, and made a couple of videos about it:-



though I don't think I went into specific details on the mods required to the meter. I may have got them from this video, or one of several others out there covering the procedure:-
https://www.youtube.com/watch?v=wPFaEFEembc,

But your meter is a different model, so you'll need to nut it out for yourself, though I'm sure the essentials are the same. (I've got one of your type of meter on order, so I'll be looking into this again sometime also)

[enut11]

Thanks @intabits, you have put me on the right track.

I ordered this meter and it has just arrived:
https://www.ebay.com.au/itm/AC-Digital-Multi-function-Meter-Power-Energy-Voltage-Current-V-A-W-KWh-Monitor/251878747833?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648

Mods
I removed the back cover, snipped the input end of the 1uF cap and soldered a wire to it. Drilled a hole in the back cover to pass the wire through.
This (brown) wire goes straight to 240vAC to power the panel meter.

You can see the steps I took in the pix.
enut11

[enut11]

More pix

[enut11]

More pix

[enut11]

These panel meters can be powered from 80vAC to 260vAC.
When operating at 240vAC, the internal 50 ohm series power resistor gets hot.
Rather than move this resistor out of the meter box, I elected to use another 1uF/200v capacitor in series to drop the voltage that powers the meter.
It works well.
enut11

[intabits]

I see why you didn't go with the meter you first showed.


No large capacitors or resistors, but it does have a transformer - looks like it has a "proper" power supply.
But that makes it a lot harder to find where the voltage sensing and power supply input join together so that they can be separated...
(maybe the sensing is done on the secondary side of the transformer, so no separation would be possible)

[enut11]

@intabits
Good luck in solving that one!

I bought this one as well as it has extra resolution for current. I hope that it has accuracy to go with it.
enut11

https://www.ebay.com.au/itm/6in1-AC-60-500V-20A-Blue-Backlight-LCD-Digital-Multifunctional-Panel-Meter-gib/193212177949?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648

[enut11]

New YB5140DM has arrived. This has higher resolution for both volts and amps - good for low power AC monitoring.

The new meter is similar to previous models on how the internal electronics is powered.

As received, the meter needs more than 60vAC to power the internals. In order to read down to zero volts, I isolated the meter power from the monitor input.

This involved cutting free the 100 Ohm power resistor.

[enut11]

Next step was to solder a (brown) wire to the loose end of the 100 ohm resistor and drill a hole in the case for the wire to exit.

[enut11]

This pic shows the meter power mod and rear case.

Counting screw terminals from the left:
1) Identified as "OP" is load return and connected to "N IN" via a 5mOhm shunt.
2) Identified as "IN" and marked also with an "N" is the neutral AC input.
3) Identified as "IN" and also marked with an "A" is the monitor AC input. This input will read down to zero vAC.
4) Identified as "OP" is load active and internally connected to (3).
The brown wire is the meter electronics power lead.

[enut11]

As this meter will be used with 240vAC, the final mod is to reduce voltage into the meter circuits by using an external series capacitor.

The large internal (red) capacitor is 0.33uF so it makes sense to use the same value externally. Voltage rating of at least 200v is recommended.
enut11