High side current indicator/

Question

^{simulate this circuit – Schematic created using CircuitLab} Hi! I am in need to make a high-side current monitor for detecting whether load (R1) is present or not. As currents running through my system are in range of few amps, I have tried my best with a small shunt resistor to keep voltage drop below 0.3V and used comparator in hope to get binary output. Unfortunately the output of the system is also high in absence of load.

Could you suggest me some simple solution to this (potentially) trivial problem?

Answer 1

Overall the simplest solution would be to deploy one of the high side current sensor chips from TI or other vendors of such parts.

That said the problem with the solution that you have tried is that at no current the inputs of the IC chip are at 0V. There is always an offset voltage in the differential input section of the device which may be negative or positive. In one polarity the offset voltage make the differential section look like it should drive the output high. In the other polarity the offset voltage would make the output be high.

Answer 2

Your basic approach is OK, but you've ignored input offsets. Specifically, you've got a chip with a positive input offset, so when there is no current the comparator still thinks the + input is more positive than the - input. To counteract this, you need a different reference voltage than the +12 line. Since at full power your sense resistor will drop about 0.24 volts, this circuit

^{simulate this circuit – Schematic created using CircuitLab} should do the trick.