Quote:
Originally posted by verdeyw
Here's a circuit I banged out that might be able to help you.
It has a small current sensing resistor in series with the battery and circuit breaker. The voltage across the resistor, which is proportional to the battery current, is determined and amplified by a difference amplifier circuit that has a gain of 25, which allows the 1 milliohm resistor to fill the available range on the A/D input. (200A through the circuit corresponds to 255 A/D counts). And the wheel on the bus goes round and round.
You might want to throw on an output filter to remove the PWM ripple, and perhaps another gain stage to fine tune the circuit's range, but it should work reasonably well now.
Please note that Digikey doesn't have any 1 milliohm resistors that can handle enough power to work here - so you have to 'make' your 1 milliohm resistor from five 5mOhm precision resistors, or ten 10mOhm precision resistors (in parallel). Five of these would be able to handle 24W continuously, which is short of the 40W required for the anticipated 200A spikes, but I think they would hold up in practice, because the average power dissapated would be well below 24W.
If you have any questions, let me know. I know that I didn't document the image too well, but it shouldn't be too hard to get the general idea.
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I like your idea, but I think the circuit might have a little problem if you use the 741 OP-Amp.
The problem is that of common mode voltage of the inputs. The circuit you have shows the 741 sensing close to it's power rails, and I am pretty sure it is NOT capable of that.
I'm also not sure, but it looks as though you have it powered off of a 2nd battery, which might make it worst if it's voltage is below that of the 12v robot battery.
We have actually built a circuit to measure motor currents, and it looks like it is working. We do have one little secret, which I will NOT devulge at this point...,and you have to trust me, I'm sure that others will be doing the same....but I don't want to "crow" about it until I get it in it's final form and working. (which is always a challange)
I will post a PDF of the schematic when it is finally all up and working.
Anyways, I like your difference amplifier circuit with minor change.
-Quentin