- on Test & Measurement
- Published in issue 7/2003 on page 0
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.