Current Sensing

Bob Blick says:

All you need is seven resistors(six if you have a great collection) and an opamp to do high-side current measurement. If your opamp is a rail-to-rail output type, you can run it on the same supply as your [processor] (5 volts):

Note: I give myself an "0x41" in ascii art!

        0.1R
24in-*--\/\/\/\--*---24out
     |           |
     \           \
     / 1M    1M  /
     \           \
     |           |     10M
     |           *----/\/\/\/----|
     |           |    |\         |
     |           *----|-\        |
     |           |    |  >-------*----  5v=5A
     *-------------*--|+/
     |           | |  |/
     \           \ \
     / 100K  100K/ / 10M
     \           \ \
     |           | |
  ---*-----------*-*-- ground

The voltage dividers(1Meg,100K) lose 11 to 1, but the opamp gain is set at 110, net gain is 10. The opamp's inputs are sitting at about 2.4 volts, so they are quite happy. Use one of them new-fangled Microchip-brand opamps and you're sitting pretty. Expect a little offset error using a circuit with this much gain.

Roman Black says:

Here's a simple suggestion, run two simple resistor dividers from each side of the resistor to GND. These can feed 2 ADC inputs on the [computer]. So instead of measureing 0.5v across the resistor you are measuring 24v and 23.5v both with reference to ground, this is very low parts count and will work fine. Just subtract them in software.

Resolution will be a bit lower, and it uses two ADC inputs, not one. But it might do what you need. :o)

Comments:

• Current shunts are a great application for a 4 terminal resistor.
  4 terminal resistors are a bar of some slightly resistive stuff,
  with a connector at each end for the high current. They have a small tap contact at a point near each end. It's the resistance between these tap contacts that's well known.
  Since the tap contacts are in a high Z circuit, the contact resistance is negligable. Since the high current contacts are in series with the measurement shunt, they don't affect the reading.
  
  Anne Ogborn+

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