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Enhance Your Multimeter Using a Polyfuseby gkal on December 2, 2016

Table of Contents

Enhance Your Multimeter Using a Polyfuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Intro:   Enhance Your Multimeter Using a Polyfuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 1:   Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 2:   Open up and replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Step 3:   Final thoughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Related Instructables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Advertisements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

http://www.instructables.com/id/Enhance-Your-Multimeter-Using-a-Polyfuse/

Intro:  Enhance Your Multimeter Using a PolyfuseThe Problem : Using multi-meters as ammeters in a school lab is always concludes to burning the internal fuse in order to protect the ammeter. The internal fuse of250mA - 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 andfinally 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 thecurrent 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 probesof 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 polyfuse.

Step 1: PartsWe have chosen a 200mA - 250V polyfuse which is extremely cheap. You can buy 50 pcs for less that 4$

from ebay

http://www.instructables.com/id/Enhance-Your-Multimeter-Using-a-Polyfuse/

Step 2: Open up and replace1. 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 could find 250Vbut 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 polyfusesof 250mA-250V there would be no difference.

4. Close the multi-meter.

http://www.instructables.com/id/Enhance-Your-Multimeter-Using-a-Polyfuse/

Step 3: Final thoughtsA 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.

For a school lab though it is better to have meters that working all the time, than meters that are accurate but usually are not working. And for statistics, from my last sixmulti-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.

http://www.instructables.com/id/Enhance-Your-Multimeter-Using-a-Polyfuse/

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Comments

2 comments Add Comment

 caitlinsdad says:  Dec 2, 2016. 8:36 AM  REPLYThanks. 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?

 gkal says:  Dec 2, 2016. 8:41 AM  REPLYThank you. It is resetting in milliseconds.