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.

Step 1: Parts

We have chosen a 200mA - 250V polyfuse which is extremely cheap. You can buy 50 pcs for less that 4$

from ebay

Step 2: Open Up and Replace

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.

Step 3: Final Thoughts

A polyfuse has some resistance which is not 0. You can see how is the resistance in some datasheets, so it is not a perfect solution for everything.

Also to be on the safe side it is better, if you are using your multi-meter with very high voltages, not to retrofit it.

For a school lab though it is better to have meters that working all the time, than meters that are accurate but usually not working. And for statistics, from my last six multi-meters only one fuse was ok, the other five where blown and i have replaced the fuses twice in this semester.

So, Hurray!!! for the polyfuse.



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    6 Discussions


    2 years ago

    I wouldn't call the poly-fuse (or PTC fuse as i know them as) a enhancement for multimeters. The fuses in multimeters serve to protect the user above all and the old fashioned fuses are more reliable in that task. Polyfuses (or PTC Fuses as i know them as) are namely a bit slow in tripping and are more likely to fail catastrophically in the face of nasty current/voltage spikes. My own multimeter uses special HBC fuses designed to withstand getting hit with lightning!

    Now for low voltage DC circuitry with low power consumption and such a PTC will do perfectly fine, but the moment you start stuff that involves AC, higher voltages, higher current and such i wouldn't trust a Polyfuse at all. I prefer a multimeter that doesn't explode over one i don't have to replace the fuse of.

    Only when you know the multimeters aren't going to get used for anything bigger than a small DC circuit and one is expecting the fuse to be blown often by neglect would i consider polyfuses. for everyone else...just buy a small carton of simple fuses.

    2 replies

    Reply 2 years ago

    Thanks for the information. You are probably right. But as I stated in the introduction this solution serves the need of replacing the fuses all the time in a school lab.
    Also I have seen nowadays multimeters that come out of the box with polyfuse.
    Finally the specifications of the polyfuse that I have used are lower than the original fuse 200mA instead of 250mA.
    Of course I wouldn't touch my very expensive multimeter (if I had one).


    Reply 2 years ago

    Yes and it will work great in such a environment, but it is best to state that it is not a good idea to do this on every multimeter. It works great in a area of low DC and neglect, but it doesn't enhance every multimeter and is risky. if someone were to follow this instructable assuming it will work for any purpose will find themselves with a fried meter quickly. :<

    Just because a few come with PTC out of the box doesn't mean it is good. There is very little control on multimeters so there are a lot with claims that aren't even remotely true. when looking for a good quality safe meter i found that majority of the sub 100$ meters aren't safe at all and plenty aren't truthful about their specs (don't trust Uni-T nor Voltcraft!!). Took me months to find one that fit my needs.

    Tripping of PTC is also a bit more complicated than just derating the current being equal to safer, it won't make it able to resist a transient any better. never assume a lower current rating = safer.

    I'm sorry if i'm sounding a bit critical, but when it comes to safety and protection-circuitry i find it important to mention the risks involved with modifying them.


    2 years ago

    Never knew of poly fuses. Saw these in my new alarm system


    2 years ago

    Thanks. I never really knew about polyfuses before. How long do you have to wait for it to reset after the polyfuse trips so you can use the multimeter again?

    1 reply

    Reply 2 years ago

    Thank you. It is resetting in milliseconds.