Testing DMMs on current - 400 mA didn't seem that much...... - Page 1
Forum section: electronics / testgear

author: jpb
author_rank: Super Contributor
timestamp: July 14, 2014, 10:03:19 pm
content: I was comparing my multimeters current readings, I hooked three of them up in series with a bunch of small resistors which is all I had to hand (I have some power resistors on order but they've not come yet).Without doing the calculation, I just assumed I could go up to a few hundred mA ...The picture shows what happened somewhere between 300mA and 400mA, the sad demise of my nice test clips not to mention an unpleasant burning smell!All slightly embarrassing !

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: July 14, 2014, 10:10:59 pm
content: 30 odd ohms total would burn 5W at that current, mighty toastyyou might by now have realized that, if you have a constant current supply and you just want to test the shunt in your meter, wire the supply straight to it in CC mode and the power will be near-zero. Constant current + nonzero resistance is bad.

author: Matje
author_rank: Regular Contributor
timestamp: July 14, 2014, 10:21:57 pm
content: Quote from: jpb on July 14, 2014, 10:03:19 pmOuch.But at least you learned something from it: do not only think about current, voltage or power, all of these are important. And you are supposed to stay within all the limits at the same time. I too melted some plastic clips, attaching them to a halogen bulb and giving enough voltage to make the bulb get really hot wasn't that clever either... Right of passage or something I'd guess.Hey, at least you only burned some test clips, could have been worse.

author: rolycat
author_rank: Super Contributor
timestamp: July 14, 2014, 10:23:54 pm
content: Since it's confession time, I have to admit I did something similar with my nifty 3.5kV power supply.I'm used to thinking of a megohm as having vanishingly low power dissipation, so I hooked up a 0.5W resistor divider to measure the voltage with a meter and cranked it up.At 3.5 kV a megohm dissipates 12.25 watts. Cue the magic smoke...

author: jpb
author_rank: Super Contributor
timestamp: July 14, 2014, 10:26:48 pm
content: Quote from: alexwhittemore on July 14, 2014, 10:10:59 pmConstant current mode perhaps would make more sense - I was a little reluctant to connect my meters with no series resistor in case the constant current mode didn't work as well as it should.I have some 200W 10.7 ohm resistors on the way from e-bay, if I connect those in parallel and attached to a heat sink I should be able to reach 3A without melting anything.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: July 14, 2014, 10:34:03 pm
content: Except that the leads may well get hot enough to melt those thin clips.

author: gaijin
author_rank: Contributor
timestamp: July 14, 2014, 10:56:27 pm
content: I've damaged a few clips because of hot resistors.A box full of alligator clips was my solution.

author: jpb
author_rank: Super Contributor
timestamp: July 15, 2014, 09:55:07 am
content: I've got some solid copper crocodile clips which I can use to replace the now melted clips. But perhaps I should use the ones on my car jump leads!