This document describes enhancing multimeters used in a school lab setting by replacing the internal fuse with a polyfuse. The internal fuse often burns out when measuring high currents, requiring opening the multimeter each time to replace it. A polyfuse acts like a fuse but resets itself in milliseconds, allowing continued use of the multimeter without frequent fuse replacement. The instructions show how to open the multimeter, bend and solder a 200mA polyfuse to replace the internal fuse, then close it up for protected but reusable current measurement.

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

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

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

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

5. 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 REPLY
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?
gkal says: Dec 2, 2016. 8:41 AM REPLY
Thank you. It is resetting in milliseconds.