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Hi all, I'm using a high-side shunt resistor to measure the current through a load (I'm using an amplifier to measure the voltage across the shunt). From my understanding, the voltage across the shunt is proportional to the current passing through it, but when the load's current increase/decreases, I don't see any change in the shunts voltage value. I'm using a bench power supply, which connects 12v through the shunt, to the load. I can see the current value change on the power supply, but the voltage never varies. If I vary the votlage from the supply, I can see the voltage drop across the shunt resistor changing. Would this be because I'm using a bench power supply, and if for example, I used a battery to power the shunt/load I would see a change in voltage when the loads current draw changes?

How are you measuring the voltage drop on the shunt resistor? Do you have a multimeter connected directly to either end of the shunt resistor?

Drawing a schematic might be helpful.

Many problems,

1- Your battery negative terminal is connected to the circuit while the positive terminal is free (like an antenna). Nothing supply power directly to your circuit, in the model. Move the battery to the side, connect its negative terminal to the ground and its positive terminal to the top of the circuit.

2- The output of the op amp is free (like an antenna) since the voltmeter is not really part of the circuit( unless you consider it as a large resistor.). Add a resistor between the output of the op amp and the ground to "close" that loop of the circuit (since the voltmeter DOESN'T close the loop).

3- The op amp amplification is not limited and the result will likely be the supplied voltage (if the op amp is a rail to rail one) that you supply to the op amp.

To remediate to point 3, use negative feed back with a control gain.

Hi, I'm using a current sense amplifier chip due to the low resistance of the shunt (30mΩ). I'm viewing the output voltage of the amplifier with a multimeter.

Interesting. It sounds like the current sense amplifier may not be working as intended. What current sense amplifier chip are you using? What current levels are you experiencing?

I note the voltage either end of the shunt will be very close to 12v with respect to ground. What is the power supply voltage to the op amp? If it is less than 12v the op amp will simply saturate one way or the other. For this application an isolating op amp connection should be used but the voltage restriction still applies

I put he circuit together in the CL editor, shown at thttps://www.circuitlab.com/circuit/8zm7w2m48fjh/opampexp/ I'm very puzzled as the model does not seem to care if the differential inputs have a common mode voltage greater than the power supply but this was an absolute nono when I made a living at it. In any case, the circuit shown is what is normally used in for a differential input and should work.

Well, this one seems to work. Are you sure that your "circuit" is intended to work or is just intended to be only very "schematic", "illustrative" ?

Further thoughts: Since the voltage at the resistor to be measured is in the range of the electrical noise, you may have to add a filter to remove the high frequencies (since "noise" has a tendency to occur at very high frequency) or to use an integrator circuit ( since average "noise" over time has a tendency to be close to zero. You may have to add some other touches, but that would greatly depend on the exact op amp that you will be using. But since your question was about how to make the simulator working fine, and not to realize physically the circuit itself, those thoughts are probably out of concern, for now.

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