Misuse of a Cheap Multimeter - Page 1
Forum section: electronics / general

author: faultywarrior
author_rank: Newbie
timestamp: July 25, 2012, 01:43:06 am
content: So first off, I'm Jim. I'm a computer geek at my core and have been pretty much since I could walk.I'm a systems administrator by day (and often by night as well) with cars, electronics, photography, and all of that mixed in together as my hobbies.With that out of my way, I decided to throw up a quick post about what happens when you use cheap multimeters...and more specifically, what happens when you use them incorrectly.I happen to have a quite cheap ($30 us or so?) RadioShack meter that I just use for dirty stuff especially when I'm working on my cars as it doesn't need to be all that accurate.Anyhow, with a recent rash of dead batteries in one of my cars, I decided to probe each fuse without the key in the ignition to see which circuit was drawing power. I set my meter to current measure mode. After getting some weird results that didn't make any sense (43 milliamps across the main battery circuit even with the car running); I decided to see what my battery voltage was...well, I forgot to set the meter to voltage mode.As you can guess, I was found that out when the huge spark I got when I touched to probes to my battery terminals scared the crap outta me.My meter says it's cat 3 rated...not sure if I believe that or not, but nothing blew up and I didn't get fried; so it ended fine.The main lesson here? When working with cheap tools,ALWAYSdouble-check your mode selector dial.

author: FenderBender
author_rank: Super Contributor
timestamp: July 25, 2012, 01:59:05 am
content: Well always check your selector dial with any multimeter. Whether it's a Mastech or Fluke, no one wants their multimeter to turn into a bomb.Glad you're not hurt. Those batteries can put out some serious juice.

author: David_AVD
author_rank: Super Contributor
timestamp: July 25, 2012, 02:33:26 am
content: Quote from: faultywarrior on July 25, 2012, 01:43:06 amSo you removed the fuses one at a time and measured the current across where the fuse would normally be?

author: IanB
author_rank: Super Contributor
timestamp: July 25, 2012, 02:43:19 am
content: Even a $500 multimeter will be unhappy if you connect it across a car battery in current measure mode. The best you can hope for is an expensive replacement fuse. But worse consequences could be that the calibration is thrown off or even permanent damage to the meter.Don't do that.Also, if you want to measure power drain across fuses start out with the meter in voltage mode and try to measure a voltage drop with the fuses in place. Fuses have a certain resistance and a current through them will produce a voltage difference of mV or uV. If the leakage current is enough to drain a car battery you have a good chance of measuring a voltage.Lastly, CAT III rating has no bearing on what you did. A CAT III meter will make a bang just as well as an unrated meter if you connect the amps range across a car battery.

author: FenderBender
author_rank: Super Contributor
timestamp: July 25, 2012, 02:53:12 am
content: That's true. A little ohms law action would work. I= V/R

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 25, 2012, 05:19:16 am
content: Quote from: IanB on July 25, 2012, 02:43:19 amDoesn't CAT II and above require a minimum breaking capacity of the fuses for the current ranges?

author: T4P
author_rank: Super Contributor
timestamp: July 25, 2012, 10:28:53 am
content: Quote from: NiHaoMike on July 25, 2012, 05:19:16 amSure, but they still can go bang

author: Simon
author_rank: Unknown
timestamp: July 25, 2012, 11:58:34 am
content: This is the one situation no one can protect for, if you put the meter in amps mode and then put it across a power source there will be an almighty bang, hopefully the meter is fused, sound like it is not if the fuse did not blow (meter stop working).CAT ratings are more to do with spikes and surges above the voltage you are measuring, like say if a 1KV spike came down the mains line that you expect to only have 240V on.

author: Bloch
author_rank: Unknown
timestamp: July 25, 2012, 12:09:33 pm
content: Quote from: faultywarrior on July 25, 2012, 01:43:06 amYou can allways buy a new multimeter.But be careful next timeMost car batteries is Lead ACID - Nasty as hell - Always use eye protection - it also destroy cloth.

author: Unknown
author_rank: Unknown
timestamp: July 25, 2012, 04:02:15 pm
content: Quote from: Simon on July 25, 2012, 11:58:34 amIEC 61010 species a transient voltage and output impedance. You can calculate the transient current across a low impedance load based on this data. This is only a short transient, however. Note that IEC 61010 requires the equipment to be safe in the case of transients, not necessarily working or in spec.

author: Neilm
author_rank: Super Contributor
timestamp: July 25, 2012, 06:28:08 pm
content: Quote from: alm on July 25, 2012, 04:02:15 pmI think 61010 says that any transient protection devices should be OK at the required level. The actual surge is a separate standard.The supply impedance is specified as a function of the required test.A well designed multimeter should at most blow a fuse if measuring the short circuit current from a battery. Replace the fuse and everything should be just as it was. I have seen a very cheap meter that did something similar and it vaporised large portions of the meter. To really see the dangers, search for Arc flash explosions in a search engine. There are videos out there of what happens if you try measuring the short circuit current on an industrial supply.YoursNeil

author: IanB
author_rank: Super Contributor
timestamp: July 25, 2012, 08:06:33 pm
content: With a car battery the short circuit current before the fuse blows could be enough to damage the probe tips. Also I think the instantaneous power in the milliseconds before the fuse does blow could be enough to damage other parts of the meter, perhaps. A car battery is a hefty power source.

author: Neilm
author_rank: Super Contributor
timestamp: July 25, 2012, 08:21:42 pm
content: Quote from: IanB on July 25, 2012, 08:06:33 pmAssuming that the cheap meter you are using actually has a fuse in the ammeter line.If you do short out mains that can cause real problems as a fuse blowing with high current can cause large voltages due to the inductance of the test leads. This can cause an arc which could extend the time the current is flowing. That is why you tend not to find cheap glass fuses in decent meters, the arc of the fuse blowing could be extended and cause a Very big explosion.YoursNeilYoursNeil

author: hun_yeti
author_rank: Contributor
timestamp: July 25, 2012, 11:29:26 pm
content: I've got a cheap Chinese DMM, I bought it for about 10$ and surprisingly, if the dial is not in current mode, the current input is disconnected, so no accidental shorts!(But it still just a crappy meter...)

author: Jimmy the Squid
author_rank: Regular Contributor
timestamp: July 26, 2012, 01:31:51 am
content: Quote from: hun_yeti on July 25, 2012, 11:29:26 pmI think that in this case, the OP forgot to switch out of current mode at all. Not any way to protect from that.