Voltage controlled high current source problem with low value sense resistor

Is it oscillating/unstable? ...Jim Thompson

Check for oscillation first, as Jim wrote. With the cap from OUT to IN- it shouldn't happen but depends on your load. Which opamp did you use? Does its input CM range include the negative rail? What's the offset spec?

Also, mind that cell phone signals and such can recitify in many opamp inputs, so if you have long wires there you might at least want to consider a cap from IN+ to ground. Hoping it's a ground plane ...

Either your 0.1 Ohm resistor is high, or more likely, trace resistance hasn't been accounted for.

Maybe arrange PCB traces for Kelvin sensing? ...Jim Thompson

The error seems to be mostly gain, not offset, so the problem is probably that the sense resistor doesn't behave like 0.1 ohms. It calculates to more like 0.14 or so. If it's a 2-wire resistor, resistor leads and pcb traces may be making up that extra 0.04 ohms.

You could add a resistor from opamp pin 2 to ground to correct for the error, or use a 4-wire sense resistor.

If you see an offset error near zero, you might consider using a chopper opamp. They have microvolt offsets.

John

Thanks. Looks like it probably is unaccounted connection resistance. Rearranging things has eliminated most of the error. I'll solder it all up with heavy gage wire and try again.

I have had issues in other projects where I've used alligator clip jumper wires whose thin wire is simply folded over and held against the clip by the clip's crimp.

Schematic looks ok and it's not unstable, so have you run all the critical voltage sensing wiring to a star point ?...

Regards,

Chris

Are you using Kelvin sensing or just the grunty single wire sensing?

...THAT would be the first thing to do. Convert from a grunty single wire feedback th true Kelvin sensing.