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Russell H.
Russell H., Service Tech
Category: Consumer Electronics
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Experience:  8+ years of professional experience
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I would like to measure the voltage drop across a shunt resistor

Customer Question

I would like to measure the voltage drop across a shunt resistor to calculate amperage. I know the calibrated voltage drop across the shunt at a given amperage. Its .075mV @ 100 A.
I would like to take the two different voltages and send them into an op amp. I want the op amp to compare the two voltages and turn anything it sees (0-0.075mV) into 0-3.3v proportionally at the output of the op amp. I will be using this amplified voltage as in input to an ADC then process the data in a micro controller. If anyone knows what kind of feedback, configuration, necessary passive components or or active components will work well for this I would really appreciate some fed back. Thanks.
Submitted: 5 months ago.
Category: Consumer Electronics
Expert:  Russell H. replied 5 months ago.
Hi, thank you for contacting My name is***** will do my best to provide the right answer to your question.For the op-amp to amplify the signal, or differential voltage, it needs to have the negative input fed back from the output... this is standard procedure enough.How to make the response curve flat... that's not so standard. Having the op-amp take a differential voltage signal rather than just a relative-to-ground signal, is also non-standard. One of the two voltages across the resistor might have to serve as the 'local ground' for just the op-amp's positive input - and that's alarmingly different in point of design, to say the least. .075 mV (milliVolts?) is astonishingly little to serve as input, also. To use one of those two voltages as the ground, it would be best if that side of the resistor being measured across were Ground, period. It would make the circuitry much more workable, and more standard, and more likely to actually work.
Customer: replied 5 months ago.
Thank you for your answer I appreciate your response. Would you be able to show a diagram of the circuit?
Expert:  Russell H. replied 5 months ago.
Here is a source for such a diagram: go to Question 6 there, and click on Reveal Answer also below that question, and you will have a diagram of a voltage amplifier, though not with the resistor values you want. The Reveal Answer will show the formula for the resistor values relative to the amplification factor, and you can use that to calculate the resistor values that will give you the amplification factor that you want.

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