The Case of the Unknown Source Impedance

By Jim Mettler

Years ago, a customer needed a transformer for a battery-driven muscle stimulator. The transformer stepped up the voltage from the battery and drive circuitry and then presented it to adhesive pads on the skin where the current would cause the underlying muscle to contract.

The “defined” muscle load was 1k ohms and we were asked to create a transformer with as high an output as possible. This appears to be a classic case of impedance matching to transfer the most power, but the difficulties stemmed from the fact that the battery, PCB traces, and transistor impedances were unknown and were likely to remain so. How would we optimize the transformer for maximum power transfer with an unknown source impedance?

Ah, but we did know the exact parameters of the transformer, so I suggested that we test the circuit with a varying output load.  By determining the optimum output impedance for maximum power transfer and applying the transformer parameters we could empirically determine the total aggregate source impedance. Once the source was known, tweaking the transformer to give the maximum output into a 1k ohm load was simple (relatively speaking). Sometimes you need to fall back to the very basics of power transfer to see the big picture.

Jim Mettler is the engineering manager at Triad Magnetics. His career has spanned 27 years of magnetics design in applications from doorbells to satellites and everything inbetween.