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 |xH. 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 partic- ularly appreciate the capability of not touching the circuit, which is also a useful feature when testing boards that operate without insulation from the mains.
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