Can measurement of voltage on line with high current damage dmm ? - Page 1
Forum section: electronics / beginners

author: openthomas
author_rank: Contributor
timestamp: October 02, 2012, 07:19:51 am
content: Ok so I am trying to measure a power supply output. With my DMM on volts, can the current damage the DMM if it exceeds the specifications ? In this case I think its 10A. Or does that only apply if I am trying to measure current ?

author: Rerouter
author_rank: Super Contributor
timestamp: October 02, 2012, 07:27:51 am
content: in general most multimeter voltage ranges use a 1 megaohm resistor between its positive and negative, in this case it generally takes about 1000 volt to permernantly damage most (not all) from overheating the resistor or just the voltage getting somewhere it shouldnt,on the current range it uses a shunt, a trimmed piece of metal to give a voltage drop, usually in the ballpark of 0.1 ohm, this is what is rated for 10 amp, and is only used on the current range,so to answer your question while in voltage range, the multimeter should not pull more than 1mA by design and you dont need to worry about the current rating,

author: openthomas
author_rank: Contributor
timestamp: October 02, 2012, 09:38:35 am
content: Perfect Thank You

author: PA4TIM
author_rank: Super Contributor
timestamp: October 03, 2012, 09:47:00 pm
content: Most DMM ' s use 1 MOhm between plus and minus , Is that not a bit low ? I have none lower as 10M, i have seen some very cheap meters that are 1M, and there are low R meters for power electrics ( the guys who do the electric plumbing in your house)But bisides that, if your meter has a 1000V range you can measure upto 1000V, it does not metter if the circuit had 1mA flowing or 10 TWatt. The meter only measures the voltage. Through an ideal voltmeter there flows no current but ideal does not exist, best meters are > 10 GOhm so at 10V there is a few nA flowing. But the Rin depends on the range. The higest ranges are most times 10M.

author: Unknown
author_rank: Unknown
timestamp: October 03, 2012, 10:02:39 pm
content: The only time when the maximum current of the circuits enters the pictures is in case of faults. If a short develops in your DMM, then the maximum current (output impedance) of the circuit might determine the size of the spark/flash/bang. This is partly what the CAT I/II/III/IV IEC61010 ratings are about: the amount of energy available in the circuit.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 03, 2012, 10:35:51 pm
content: Yes, that is very important, safety. I has a Voltcraft scopemeter. It was in the 1000V range but the voltage turned out to be 1200V, the meter exploded inside. Really everything was gone, opamps in peases ( even the ones that had nothing to do with volt measuring, displaydriver, even the internal voltage regulators ( is was used with a adapter) were blown so I was lucky the housing stayed in one piece.That was the last not A - brand meter I bought. For me sincethen only Keithley, Agilent and Fluke and I never killed a meter from those brands ( and I sometimes do really stupid thingslike last week I was working on a esi decavidr coupled to a Fluke 332. I meaured the output resistance with the Agilent. Then I went on and forgot about the meter still on the output. i switched on the 332 that wad still at 500V from calibrating. I stared for minutes to my meter, checked wires ect because the data did not make sense until i noticed it was still in the ohms position. But it still works like it should do.Thats safety. I had it in my hand st that time !