How do I protect a sensor IC from overvoltage when not in use?

Question

I am attempting to use an IC to monitor the value of an inductor in my circuit. During normal operation a voltage higher than the maximum tolerated value will be present across the inductor. There are intervals of time when the signal through the inductor will be turned off, and I would like to take inductance measurements during these times.

My question is this: How can I isolate the IC from the high voltages present when I am not actively measuring? Would a BJT work for this, or potentially cause interference with my measurements in some way? Is there a standard way to handle this sort of situation?

Answer 1

A relay is the easiest solution.
Either DPST to switch sensor onto the inductor in circuit.
or DPDT to switch the inductor out of circuit and onto the sensor.
The latter arrangement gives full isolation of the inductor during measurement but the contacts are now part of the main circuit.

A relay is probably about as good as any for "on resistance".
Cost will usually be about as cheap as any alternative.
Isolation when not measuring is probably superior.

Add a pair of LARGE zeners in series opposed arrangement across the sensor for when you make a mistake with the relay.

This circuit may also be able to be implemented with MOSFETS or bipolar transistors but a relay utterly trounces them for ease of use.