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.


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