Schematic Reference

DMM Fuse Protector

Typically the input protection fuse of your DMM will blow in the middle of a demonstration or an exciting phase of your construction work. Spare fuses are always hard to find, and if available take a lot of time to install. This circuit replaces the fuse by a 500 mA current limiter. When resistor R1 passes about 500 mA, it will drop 0.75 V which is sufficient to switch on T2. With the buzzer acting as a pull-up resistor (and, of course, as a very loud acoustic warning device), the voltage at the gate of power FET T1 will drop to a level at which the drain-source current is limited to a safe value of about 500 mA. Of course, the excess energy caused by the current limiting action is dissipated by the FET.

Cooling is required in all cases where the dissipation can be expected to exceed about 1-2 watts. After all, without cooling, the voltage allowed to occur across the FET will be just 4V (2 W = 0.5 A × 4 V). Although an IRF740S is indicated in the circuit diagram, almost any power FET may be used. The popular BUZ10, for example, is a good choice when a lot of power has to be dissipated. If a 12-V mini battery is used then the buzzer should also be a 12-V device. However the circuit will also work fine from the more commonly found (and certainly less expensive) 9-V battery and a matching buzzer. If the latter is not required it is simply replaced by a 10-kΩ resistor.