Hi I need some advice . The voltage across my shunt (0.1 ohm) when amplified works fine between 0 Amps and 2 Amps. When I replace the shunt with a much bigger one (15A 50mV) the amplification is not linear. At 1A I get 400mV then 2A should be 800mA but I get 980mA. Should I use a better op amp …

mikedb: There is still some confusion on my side. Why are 0.21V and 0.018V amplified to almost the same value?? Something is wrong, it looks like op amp is working as comparator, however with 5V on pin 8 the voltage on pin 1 should be also 5V not 3,7 V. 1.op amp is defected 2. wiring problem

Are we still talking about the diagram from post#10, with the LM358. Then why trying to lift the inputs to mid-voltage (10k, 75k, etc.). This family of opamps has a common mode range that includes ground. Opamp offset with the 101x gain and the 5mV minimum output swing could create a shunt curren…

The circuit is working OK when you put bottom of R3 back to ground ( the same as pin 4), change 75K to 100K , R3 and R5 so for 0.21V you going to have 2.1V on pin 1. Use separate 6V for op amp - 4x1.5V battery, do not connect to them anithing else.

Wawa: Are we still talking about the diagram from post#10, with the LM358. ? ? ? [191f9f95f36a62393b91c22a20e4be3e8b800cea.jpg] I think we need an updated circuit? Shouldn't you have a 1K in series with pin3? Where are the resistors you are talking about? Thanks.. Tom.. :slight_smile:

ted: Post # 32 Okay, what OP-Amp? [OQOIt.png] Tom.... :slight_smile:

Hi, Okay. I think you will find if you put R6 in circuit, it may perform better. Operational Amplifiers operate in neg feedback mode by trying to establish a Virtual Gnd at its inputs. [OQOItedit.jpg] Have you grounded the other two inputs of the LM358? Tom... :slight_smile:

R6 - maybe, grounding maybe also. OP has another problem, I think he is using not isolated power supply for load and for op amp, probably common ground, so complete circuit is limping

Hi, Yes, not sure, will readup, but I don't think the LM358 are keen to behave when the inputs are near signal gnd. R6 I've added may help, you will have to suck it and see.. [UNO_CurrentAmp_Shunt.jpg] Tom.. :slight_smile:

Current shunt op amp not linear

Other Hardware General Electronics

TomGeorge June 1, 2018, 9:35am 63

ted:
Post # 32

Okay, what OP-Amp?

Tom....

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Current shunt op amp not linear

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