Noncontact device tests power supplies

Alberto Ricci Bitti, Eptar, Imola, Italy -- EDN, 3/1/2001

The probelike device in Figure 1 comes in handy as a quick go-no-go test for step-down power supplies. You can build it using a very bright surface-mount LED and an inductor of the same type as in the power supply, which in this case is 100 µH. Placing this probe close to a working step-down power-supply coil lights the LED. The probe lights when the distance from the step-down coil is as much as 1 cm, making the probe capable of testing even plastic-encased or epoxy-filled power supplies. Industrial engineers will particularly appreciate the capability of not touching the circuit, which is also a useful feature when testing boards that operate without insulation from the mains.

For optimum performance, use a very bright-red LED. Other colors feature greater forward voltages, which reduce the sensitivity. You are not restricted to surface-mount LEDs, although this type helps by keeping the probe small and rugged.