Voltmeter / Ammeter / Frequency display

I have a variac, or variable transformer which I use to supply variable AC to projects, or testing switch mode power supplies. Normally I just connect a DVM to measure the volts and don't worry too much about the current. I recently saw these incredibly cheap panels meters that show volts, current and frequency - typically 2€. You can get them in singles too, but I bought a combined one as it came duty paid for ~4€. You try buying one in France and they start at 10€. Mine is red.

Part no AD16-22FVAH 60-500V, 0-100A, 20-75Hz

It's supposed to work at 60V. Lol. 80V min.

When I connect a motor, the frequency changes from 50 to 53Hz.

The resolution of the current is 0.4A, so I've put 10 turns on the CT. Didn't modify the DP as I'd put the unit back together. I suspect the LED DPs are all wired together as per the VK1640 data sheet, so not possible anyway.

The CT wires are live, so take care as the insulation is inadequate even if it's installed. This alone makes me suspect the validity of the CE marking.

Time to hack it! To get it apart, remove the red LED cover and unsolder to 2 wires on the terminals. Live goes to the cap/res.

The chip on the right is a VK1640, I2C to 7 segment driver. IC on the left is sanded off. Probably a PIC!

The yellow and blue wires are the mod. Originally the 1k resistor on the yellow was soldered to the 330n cap. The cap is a simple wattless psu. On reflection, as the voltage input range goes to 500V, reducing the cap to day 220n would have reduced the input voltage, so long as the zener current isnt exceeded. However, this mod allows it to read from 6V. It consistently reads 2V high above 20V, and 1V below, so the error ranges 10% to 1%. Current is spot on.

20/9/23 Testing on a cheap 2-stroke genny, the volts and frequency shot up. I saw volts at 300V, and the 56R resistor glowing. Took it apart to check and it is a little burnt, but 57R. Have left it.

I'd guess that the 2 feeds top right are for volts and frequency.

Update 4/4/20. Got another one! Very similar externally, but just volts/amps. I've installed this onto the variable trafo, and the one above will be made into a lead for the genny to show the frequency (only shows +/- 1 Hz)

It's just 1 digit difference on volts and starts at 3V.