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Yesterday, I did something dumb, in a hurry. I wanted to check the AC voltage at the terminals of a hair-dryer, to determine the voltage being applied to the terminals of the AC fan behind it. However, in a hurry, I plugged in the positive lead, to the unprotected 10Amp AC socket on the DMM, instead of keeping it in the regular 5Amp protected, 750VAC socket. While I do think the DMM showed me right (or expected) values of AC voltage (between 12-15VAC for fan), and I kept it connected only for about 5-10seconds at max., it apparently drained the 9V battery in DMM, and all other measurements started showing "1" or "Err". There was no smoke or anything, but no correct readings, or even a valid continuity test. I thought that it must be just the exhausted battery, so I replaced it with a brand new one from a pack, and still got same results. Then I tried yet another new battery, and still same. I checked batteries with 2 LEDs, and they were glowing pretty bright, and very feeble on the first battery I'd replaced, indicating that the new batteries were probably fine. I left the DMM there, and went ahead and ordered a new replacement (yet to arrive). Now all this was about 24hours back.

However, just before dumping the DMM, about an hour back, I thought of doing the continuity test one final time. Voila, it worked, as did every other measurement I took. While I am happy to know that my trusted DMM isn't dead after all, I was surprised that this seemed like some safety-fuse which had blown, but auto-reset after some time. Is this really true / possible ? This is a very cheap ($5) small basic DMM, bought from SEA, shipped free kinds.

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  • \$\begingroup\$ You were doing voltage measurement and the two choices of connection on the DMM were 5A protected and 10A unprotected??? Sounds like it should have been in the ACV/DCV input for anything to work as expected. Something in your description doesn't add up. \$\endgroup\$ – HikeOnPast Jul 29 '12 at 20:53
  • \$\begingroup\$ @DeanB, you are right. It should indeed have been in the ACV/DCV input, not the "5A protected". I believe the ACV/DCV input is protected and can withstand upto 5A (but maybe not). However that is what I intended to refer to. \$\endgroup\$ – bdutta74 Jul 29 '12 at 23:58
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[...] auto-reset after some time. Is this really true / possible ?

PTC fuses exhibit a behavior where they reset themselves over time. When the current become too high, these fuses heat up and increase their resistance dramatically. They become almost open circuits. The fuses themselves do not get damaged by this, though. When high current is removed, the fuses cool down over some time and return to the original low resistance.

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  • \$\begingroup\$ Thanks for answering. That can certainly explain. I shall open my DMM again and check to see if there's anything that looks like the PTC fuses. \$\endgroup\$ – bdutta74 Jul 30 '12 at 0:03
  • \$\begingroup\$ Wikipedia article shows PTC fuses in throughole packages. They come in surface-mount packages too. Sometimes, they look not unlike a 1206 resistor. \$\endgroup\$ – Nick Alexeev Jul 30 '12 at 0:06
  • \$\begingroup\$ Ah. That would make it harder to figure out, if not impossible. \$\endgroup\$ – bdutta74 Jul 30 '12 at 0:12

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