Using eBay Digital Ammeters to measure current in a bipolar power supply

Discussion in 'General Electronics Chat' started by Mark Colan, Feb 10, 2016.

  1. Mark Colan

    Thread Starter New Member

    Feb 18, 2012
    I have a bipolar power supply with a common ground. I would like to add digital ammeters and have equivalent outputs - plus, common ground, and minus.

    The inexpensive digital ammeters available on ebay seemed to be a good match, at first, but the problem is that their design APPEARS to be limited to measuring current in the ground branch. This makes it impossible to measure current on both the plus and minus branches. Conventional ammeters do not have this problem, probably because they do not require a source of power in order to operate.

    When circuits are provided at all on ebay, they seem to take one of two forms: the first, in which the power supply being measured also powers the ammeter, and a second, which uses a separate, "floating" power supply. The first pair is the same as you will find on ebay, though I re-drew them in a way that makes more sense to me. Also, on ebay, they often refer to a Thin Red and a Thick Red lead, but all the meters I have bought have a Thin Red and a Thick White lead; it is clear from experimentation that the Thick Red and Thin White wires are connected to the same thing inside the meter.

    I have confirmed that these circuits work to measure current for the positive branch of the circuit, alone.
    da1+.png da1-.png

    They can also be used to measure the negative branch of the circuit. I have simply changed the labels from the pictures above to show different connections to the power supply:
    da2+.png da2-.png

    The problem I have is that I want to use them both at the same time, and still have a common ground AFTER the meters. That is, it still looks like a bipolar power supply, but now it has meters, too. I have not found a way to combine one of the top two with one of the bottom two. This is because the circuitry of the meter is such that it can only used in the negative branch (relative to the upper branch). Conventional ammeters can be used in either. I suspect the problem is that providing power to the LED display inside the unit is not isolated from the measurement wires (White and Thick Black leads), guessing from the right-most pictures, which show that the Thin Black wire is not even required when the power supply being monitored also powers its own LED display.

    Put simply, what I need is a three-wire output for plus, common/ground, and minus. The power supply is like that (and I cannot change it), and I need the outputs from the meter circuits to work the same. In the diagrams above, you will see that I labeled the outputs as plus and minus, because there is no way I have found to provide a common ground for a three-wire output.

    Does anyone have a suggestion on how to do that with these ammeters?
    Have you found any affordable digital ammeters that don't have this problem?
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
    Use an independent and isolated power supply for each meter then you can put them anywhere in the circuit.

    If they can run on 5 volts cheap mini USB power adapters work great for this sort of applications. ;)
  3. Mark Colan

    Thread Starter New Member

    Feb 18, 2012
    I thought I had tried all possible combinations, but I guess I missed one, or had a wiring error.

    The first observation is that using a floating power supply enables you to move the meter into the positive branch, BUT you have to switch the polarity of the meter by switching the White and Thick Black wires. Here is the circuit:

    From here it is easy enough to combine the circuit above, which measures current in the positive branch, with one of TWO circuits that measure current in the negative branch. This allows to be ground to be ground in both the bipolar power supply and the loads that attach after the meters.

    In this circuit, I have chosen to use ONLY ONE floating power supply (for the positive branch) rather than an additional one for the negative branch:

    Thanks very kindly to tcmtech for pointing me to the correct path, and I hope these circuits will help someone else.
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    So how are you powering the ammeter in negative supply rail?

    You need to power it from it's own independent supply.