Enhance Your Multimeter Using a Polyfuse

The Problem : Using multi-meters as ammeters in a school lab always concludes in burning the internal fuse in order to protect the ammeter. The internal fuse of 250mA - 250V is of course replaceable but you usually have to open the multi-meter to replace it. Doing this for 12 multi-meters for several times drives you crazy and finally you decide not to measure the current in circuits.

Why : In a DC circuit with power supply and resistors, the current is inversely proportional to the resistance of the circuit. If you short circuit the power source then the current is growing to an enormous value. When you switch the multi-meter to ammeter, its internal resistance is 0 Ω , so its like a short circuit. If you then put the probes of the multi-meter directly on to the power source the current grows over 200mA and the internal fuse blows.

Solution : Replace the internal fuse with a resetable polyfuse.

1. Open your multi-meter and locate the fuse. Take it out of the socket.

2. Bend the polyfuse as in the picture to solder it easier.

3. Put some solder paste on the outside of the socket and solder the polyfuse to the outside of the socket.

I preferred to solder it on the outside because if sometime i wanted to revert my changes (why?) i could easily cut the polyfuse and put the old fuse in its initial place.

The old fuses were 250mA - 250V but i chose to use 200mA - 250V polyfuses for three reasons. First, they are much cheaper than the 250mA, second i couldn't find 250V but only 72V and third my multi-meters never showed anything above 200mA because the display could not handle numbers above 200 so even if i could find polyfuses of 250mA-250V there would be no difference.

4. Close the multi-meter.