70 to 500V panel LED voltage display (with schematic)

This is the first of two very similar looking units I bought to compare the circuitry.

The most notable features of this one are the tri-state multiplexed display and slightly over minimalist power supply and circuitry.

As with most of these generic grey import items, the design does not really take electrical separation seriously, so I'd recommend a suitable HRC fuse to protect against failure.

It seems accurate enough, and does have a calibration potentiometer on it with a sweep covering 130V which makes it a bit too wide for easy calibration. But I did get it spot on, using a Fluke meter as a reference. All the circuitry is live when active, so care should be taken when calibrating it. I recommend a plastic trimmer adjustment tool.

The minimalist circuitry means that the software is probably looking for a peak voltage value on the pulsing input signal and basing its reading on that.

Technical data:-
Power 0.3W
Current 27mA
Power factor 0.04 - which is normal for this type of power supply and low voltage circuit

The 330nF capacitor will be a higher value than needed for anything other than the lowest supply voltage. For higher voltages the capacitor value could be reduced, which would reduce the power and take the strain off the zener diode.


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