Odysseas

Alright, now I think I understand what you mean. But that would still only allow me to sense the current for all motors (at least when they're running in in one certain direction), not for each individual motor.

Let's say two motors run forward (the first relay provides GND to the common wire, and two of the other relays provide +12Volts), then all the current would flow through the left relay, meaning I can only read the current drawn by both motors. If the motors run backwards (the left relay provides +12Volts, the other two provide GND), then I can read every individual current drain, but that's only 50% of the proper solution...

Please correct me if I still don't get it.

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ericgibbs

hi,
This is an option, ref attachment.

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Odysseas

You once said that I could also build an instrumentation amp from simple OP Amps. How do I do that? Because the plain inst amps are fairly expensive, especially considering that I would need four of them...

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ericgibbs

Hi,
Example, ref gif

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Eric
"Good enough is Perfect"

PIC tutorials:
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Ambient

The 6134 op-amps are kinda pricey, but the ones I have go rail-to-rail very nicely. It seems like you will need a rail-to-rail amp, as the input on one end of the resistor will be either 12V or 0V. So not using a r-r op-amp will possibly give you some offset from 0 or 12V. Some op-amps will give enough offset to really screw up the final output to the uC. They usually go for around $4 per package of dual amps. So they are not too bad. But Someone else may know of another r-r that is cheaper.

Is r-r needed here? Am I totally off?

eblc1388

Hi Odysseas,

I looked again at your motor control schematic and cannot be sure it will work as you would have intended.

Supply of each motor is controlled via a single 1P2T relay but then you switched the common of all the motors via another relay.

This will not allow individual motor to rotate in either direction. You have created a situation that a motor can either run in a certain direction or just stop, depending on the state of its control relay.

In order to run motor in both directions, you would need a 2P2T relay for each motor to reverse the current to the motor.

If you place the current measurement resistor of each motor before this relay then all the current will be in a single direction and you don't have to use differential Opamp for voltage measurement.

Is this really what you have wanted?

edited: correct bad spelling.

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Odysseas

Yes, this is fine. I am aware of the restriction that all motors can only run in one direction or be off, but, as I said, I cannot modify the wiring of the motors, so I have no influence here to change this.

I am presently testing a different solution, without precision current metering... I'll post again here once I know whether or not it works

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speakerguy79

Bidirectional current sense amplifier? Try MAX4081.