External power supply for 80-260V AC panel meter

[noluck]

I have a panel meter that only starts measuring once it sees 80–260V AC input. It uses that AC line to power its own internal circuitry. I want to bypass this and instead power the meter from an external power supply.

The goal here: to start voltage measurements from 0V. So I could see full range of my variac output.


I measured the voltages for voltage regulator (U3). The input is 12V and the output is 3.3V.

Disconnected this panel meter from mains and fed into the input of the voltage regulator. The IC works and the panel meter lights up. To do that, I used a bench PSU it's positive and negative terminals were earth referenced.


How do I proceed now? Do I connect this panel meter to the mains (or variac) and use and non earth referenced power supply to power it up? Would this work?

Thank you.


Took inspiration from this person, https://lajtronix.eu/2021/04/29/diy-variable-isolation-transformer/
He used the same exact panel meter as I did. Unfortunately the unit I have, has a different PCB. He removed an internal diode and soldered in a 1117-5V voltage regulator.

[ledtester]

The goal here: to start voltage measurements from 0V. So I could see full range of my variac output.

There might be an easier solution... see the video below.

The video references an image at https://i.postimg.cc/hPwdXhV7/image.jpg -- It takes a long time to load so I attached it below. Here is what is said about the picture in the comments:

I forgot to add the diagram to the video: https://i.postimg.cc/hPwdXhV7/image.jpg There are two options for proper connection. If you mix them up, it's not dangerous; only the wattmeter's supply voltage will change depending on the position of the meter's knob.

The Russian text in the pic says "Output from the LATR" -- LATR refers to the autotransformer. If you look carefully you'll see that the black cable carries connections to both screw terminals labelled "AC in". The yellow wire is the one soldered to R15 in the video.

Disclaimer - I have not done this mod. I'm just trying to describe what's going on in the video.

[noluck]

Thank you, but this won't work. It's a completely different pcb.

[PCB.Wiz]

How do I proceed now? Do I connect this panel meter to the mains (or variac) and use and non earth referenced power supply to power it up? Would this work?

My educated guess :
Largest visible SMD resistor is R13, looks to be the measurement dropper. Maybe a tad over-rated there with one part on mains ?
You could check it connects direct to the AC in ?

R15,D1 etc power the electronics, non isolated.

If R13 is the measurement dropper, when you split off R13 by carefully cutting and removing part of the trace, that give you the 0-240V, if you power R15 from a fixed supply.

[theHWcave]

I agree that R13  (and R12) are the volts measuring inputs forming a divider. You should check that the left side of R13 is connected to one volts terminal  block and the bottom side of R12 to the other. 
The large 47 Ohm R15 may be fusible (acting as a fuse), at least it acts to limit inrush to the power supply which  by the luck of it is a buck converter to step down the voltage for the regulator.

Solutions
1 .In theory all, you need to do is disconnect the left side of R15 from the terminal block and connect it to a supply that is always at mains voltage. BUT (!!!!) this must be done in such a way that you do not create a short on the mains side. Say the terminal that used to feed R15 was Live and the other Neutral, then that Live terminal is the output of the VARIAC and the now disconnected R15 must instead be connected to the LIVE input of the VARIAC. There is a small possibility if that doing this will back-feed some voltage into the measuring side but I think it may work. If I were testing this I would run the input into the VARIAC through a current limiter using a 60 or 100W incandescent (!!!) light bulb wired in series. If the bulb lights up brightly, you have a problem that needs fixing. This is of course only for testing with no load connected to the VARIAC output. An example of a light bulb current limiter (super useful device to have) is here https://youtu.be/EgRs2InB3_o

2. To play it safe, an alternative is to disconnect R15 so the buck converter is now dead. Then build a second small power supply with a proper (isolated) transformer of say 12VAC, bridge rectifier and small smoothing cap of 220uF 25V and connect the output to the black and red leads in your picture to the regulator input.  In either case, no earth is ever connected to the module.