High Side Current Monitor

Monitoring current on inductive loads is a common task. It's easier to do on the low side of a driving circuit, because a simple resistor can be used with an op-amp on the ground rail. The high side often demands that an expensive current sensing transformer or Hall effect sensor be used.

The circuit below is desirable because it uses a simple current sensing resistor instead of an transformer, and then shifts the output voltage to a ground referenced value. The zener circuit allows the op-amp circuit to float under the positive voltage rail by an amount equal to the zener voltage of D1. Transistors Q1 and Q2 are part of the feedback loop, and drive the ground referenced output resistor. The bias resistor should allow enough current to flow for the op-amp circuit.

Rbias=(Vs-Vz)/Iop-amp.

High Side Current Monitor Schematic