Magnetic probe amplifier

Magnetic probe amplifier connected

Magnetic probe amplifier connected

Recently, I was researching low-noise DC-DC converters and while reading Linear Technology Application Note 70 found this clever and useful circuit, designed by Jim Williams. The idea is to sense current in power inductor of the converter with another inductor, placed within short distance from the first one. The sensing inductor is connected to a circuit which amplifies and conditions the signal and generates nice clean square wave pulses which can be used to trigger oscilloscope sweep. The probe is isolated from the circuit preventing measurement corruption. As a bonus, analog output of probe amplifier allows observing current waveform through power inductor.

As is often the case with application notes, circuit description and build details are somewhat brief; I’m posting my notes hoping that the information will be helpful for other builders. Also, since BatchPCB doubled my order, I have extra PCBs; if anyone wants to build this circuit on a professionally made PCB with just couple green wires, e-mail me – the PCB can be yours for the price of postage.

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