DMM or VOM with fuse in the resistance measuring circuit. - Page 1
Forum section: electronics / testgear

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 06, 2021, 12:55:41 pm
content: With a multimeter if you by mistake measuring voltage while it's in the resistance mode wouldn't get damage but some do. However, does any of them has fuse in the resistance measuring circuit? I don't think so but some guy told me that his meter has fuse to protect all the measuring function. There are other circuit protection but not fuse for resistance measurement I don't think so.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2021, 01:28:34 pm
content: Some meters have fusible resistors as a last line of defense. One can consider them a relatively low current fuses with a high resistance.Quite often there are PTCs, which are sometimes also called self resetting fuse.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2021, 10:57:17 pm
content: Old multimeters are more likely to protect the ohms converter with a fuse. Some use a small high voltage low current incandescent bulb as a combination of a PTC and fuse. A good modern meter might use high voltage depletion mode MOSFETs to protect the ohms converter.PTCs are too slow to protect an ohms converter.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2021, 05:27:07 pm
content: Relatively high resistance PTCs are not so bad. For the fast part they work as a resistor, limiting the fault current and the PTC action is at least fast enough that the PTC itself does not overheat itself. So a good PTC at least can protect itself up to some votlage.The voltage reading path of the ohms circuit is relatively easy - this is like the voltage input, often it is actually the same path.The current soruce for the ohms part is the tricky half. One polarity can use a diode to block at least that half. For the other side the constant corrent source can be made to also work for a relatively high external voltage (like a few 100 V). This can use high votlage MOSFETs or BJTs in series. This is the usual way the protection is done in bench top meters - in handheld meters this is rare, as it takes up quite some space and it is still not as reliable to really high spikes and not good for a CAT II or III rating.High voltage (depletion) MOSFETs are a nice protection, but as semiconductors not good as the only line of defence. So they should still have something like a fusible resistor to limit the damage in case the MOSFETs fails.In most cases the FETs work and no damage done even from quite some excess voltage.

author: Fungus
author_rank: Super Contributor
timestamp: October 07, 2021, 06:52:43 pm
content: Quote from: BeBuLamar on October 06, 2021, 12:55:41 pmAnswer: Buy one that doesn't.Quote from: BeBuLamar on October 06, 2021, 12:55:41 pmI don't believe him. It would be a pain in the ass to constantly replace the fuse.Ask him to show you the fuse. Then take it out and see if resistance still works.

author: J-R
author_rank: Super Contributor
timestamp: October 07, 2021, 10:03:15 pm
content: Could be fused test leads.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 07, 2021, 10:35:58 pm
content: The HP3456A for example. And the fuse (FF 1/16A, 1.25'') is internal and requires removal of the top cover and the internal screen. You don't need to ask me how I know.CheersAlex

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 07, 2021, 10:41:33 pm
content: Quote from: Fungus on October 07, 2021, 06:52:43 pmYou didn't get my idea. In the other forum the guy said that if you set the meter on ohm and try to measure voltage it would blow the fuse. I told him none of that happens to any of my Fluke meters and there is no fuse in the resistance measuring circuit. But he said his meter has fuse and they would blow if you do that.

author: Fungus
author_rank: Super Contributor
timestamp: October 08, 2021, 07:47:03 am
content: Quote from: BeBuLamar on October 07, 2021, 10:41:33 pmI got the idea, I just don't believe it.OK, maybe there's some old pre-1975 meter that does it that way. Ask him (a) What the meter is, and (b) if he uses it regularly.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2021, 07:07:56 pm
content: Quote from: BeBuLamar on October 07, 2021, 10:41:33 pmFor some multimeters that is the case, but it depends on how it was designed. There should be a specification for maximum input voltage on ohms ranges:DM501 bench with fuse protection - No specification; Based on schematic, 1/16 amp fuse should open above +/- 20 volts. I replaced fuse with lamps to fail above +/- 56 volts.DM502 bench with lamp protection - 130 volts RMS indefinitely, 250 volts RMS for 1/2 hour.RMS225 handheld - 500 volts RMS indefinitely.DMM916 handheld - No specific specification; might be same as voltage ranges or 1000 volts DC and 750 volts RMS.