High voltage current shunt gain stage

Discussion in 'The Projects Forum' started by bart_dood, Mar 19, 2011.

  1. bart_dood

    Thread Starter New Member

    Jul 14, 2010
    Hi folks, there is probably an easy way to do this but I don't know how. I have a 144 volt DC battery pack that I want to add a small resistor as method of measuring the current being drawn.

    This current measuring shunts are available online pre-made, I'll be buying one that has a 50millivolt drop across it at 200 amps.

    I want to input the value into a microcontroller so I need a gain stage to take the 50mV and add 80x or so, so that I get around 4 volts at 200 amps. This gives me some headroom in case the current goes above this, my microcontroller analog input is 5volts max.

    At first I thought this would be trivial with an opamp non inverting amplifier, but the problem is that I seem to need a floating gain stage, if I reference ground I end up dumping the high voltage to ground at some point. Here is an example of my schematic that is obviously incorrect.

    Do I perhaps add some 1megaohm resistors to the shunt either side to severely limit any currents from +144volts to ground?

    Thanks a lot for your help!
    Last edited: Mar 19, 2011
  2. SgtWookie


    Jul 17, 2007
    Use the current sense shunt on the ground side instead of the high side.

    Use an instrumentation amplifier to sense the voltage drop across the shunt and amplify it.
    Just off the top of my head, an INA122 might work for you; it's a single-supply opamp with common mode that includes ground.

    Have a read-through of this application note: http://www.intersil.com/data/an/an1298.pdf
    It does a good job of explaining why you should use an instrumentation amplifier rather than trying to make your own.