by pebe » Sat Nov 29, 2014 9:26 am
Digital ammeters work by passing the test current through a sensing resistor and measuring the voltage across it. The one you are using has a built-in shunt across the sensor to give an increased current range. The LEDs and chip can be powered by an input of 4V – 30V so the display must contain an internal voltage stabiliser to keep the LEDs supply at ~4V
I think you have it wired as in the RH circuit shown in the ebay advert, ie. the white wire of the sensing resistor is connected to the +ve of the variable power, and the black wire of the sensor is connected to the +ve of the load, with the –ve of the load being taken to the –ve of the variable power.
I think your problem is the use of a 12v buck converter for powering the ammeter. Although not stated in the advert, none of the buck converters I have seen are isolated; the –ve input is the same as the –ve output (check the underside of the PCB for verification). I cannot be certain how it is connected because you have not shown the colours of the meter’s wires, but I think that loop currents between the various –ve connections are flowing through the sensing resistor and giving a reading, because on the PCB of the ammeter the two black leads will almost certainly be common.
You can check if that is your problem by disconnecting the 12V buck converter and temporarily powering the ammeter with a 6V battery. If that is the problem, then there are two ways round it.
1. Wire it up so that the variable supply powers the ammeter as in the LH supplied circuit (but that way has the limitation that its negative terminal is at a different voltage than the other –ve terminals on your panel), or
2. Power the ammeter with a separate 5VDC supply removed from a mains ‘wall wart’ 5V charger (the better option).
I would be interested in your findings.