Shorting a circuit using multimeter - Page 1
Forum section: electronics / beginners

author: isaacsong23
author_rank: Contributor
timestamp: April 12, 2017, 02:33:49 pm
content: I'm currently using fluke 115 true rms multimeter and my question is, will shorting a circuit (limit 5A on my PSU) cause my fluke 115 multimeter's fuse to spoil ? even though it is rated at 10A. Wanted to know haha Thanks!!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 02:41:23 pm
content: It depends.If it is a lab supply, there shouldn't (note that I'm saying *should not* and not *will not*) be a problem - they are built to go into current limit mode when you pass their limit.If it is just a standard supply, it's impossible to say without any more information. If your supply has short circuit detect/protection, it shouldn't break the fuse. If it does not, and it is capable of currents (even just very briefly) of way over the rated 5A, who knows what will happen.In any case - Why would you want to short out a supply?

author: Seekonk
author_rank: Super Contributor
timestamp: April 12, 2017, 03:14:10 pm
content: Even if a supply is current limited, excessive capacitance on the output (cheaper supply) and in the tested circuit can supply excessive current and blow the fuse in the meter.

author: isaacsong23
author_rank: Contributor
timestamp: April 13, 2017, 01:14:34 pm
content: Thanks for the reply!!I am planning to adjust the current limit on the below circuit by shorting the outputs using my multimeter for a short while. I understand that it will get very hot but it should be fine for a short while right?Sent from my SM-G935F using Tapatalk

author: Shock
author_rank: Super Contributor
timestamp: April 13, 2017, 04:20:09 pm
content: Do these have OC protection? I don't have one of these nor can see the converter part number, but I'd measure the voltage output and whip out ohms law and a resistor first. You can then determine current, adjust it into constant current mode or if you're satisfied it's not pulling more than 5A put your multimeter in series with the resistor and measure then set the current.The Fluke 115 says:DC current from 0.001 A to 10 A (>10 to 20 A, 30 seconds on, 10 minutes off). > 10.00 A display flashes. > 20 A, OL is displayed.So 5A looks fine to me, but don't use a multimeter to test current on an unknown circuit and if there's the possibility of creating a high current short, well I wouldn't do that.

author: isaacsong23
author_rank: Contributor
timestamp: April 13, 2017, 05:58:13 pm
content: Yup!!They have OC protection, i guess i'm gonna short it using my cheap chinese multimeter rated 10A and adjust the current limit from there, thanks for the advices !Sent from my SM-G935F using Tapatalk

author: mmagin
author_rank: Frequent Contributor
timestamp: April 13, 2017, 11:11:28 pm
content: Quote from: Seekonk on April 12, 2017, 03:14:10 pmThis. Depending on the parameters of the multimeter fuse, you may even be in for an unpleasant surprise with a pretty good quality supply. Or at least plenty of old bench power supplies I've seen (HP and others).The key is to have the terminals shorted when the meter is connected and then remove the short so this capacitor cannot charge. Or connect the meter before powering on the supply. And use some firmly wired leads, no fumbling around with loose meter probes.(Embarrassing story: I recently blew a multimeter fuse for the first time in my life. It was the 400 mA range on the fluke 179 and an old HP 0-320 V power supply, with the current limit set much less than its 100mA maximum limit. But then the load (a CRT G1 and cathode) arced over and the output capacitance blew the meter fuse. After this, partially to protect the CRT, I added a 5K power resistor in series for current limiting and just measured the voltage across it to monitor current.)