Never swap the leads on your multimeter - personal rant. - Page 1
Forum section: electronics / general

author: Kibi
author_rank: Unknown
timestamp: January 05, 2013, 07:01:43 pm
content: I have a lot of choice words to accurately describe myself, some of which I made up on the fly. Alas, none of these words in any combination can be used appropriately on a public forum such as this one.For reasons I am still yet to fathom out, I plugged the red lead into the black jack and the black lead into the red jack on my multimeter. Not much harm can come to the multimeter this way, but in my case I was testing the polarity of a jack plug and I thought it was correct.I now have a £300 personal bill for a new Intel motherboard, i3 CPU (3rd Gen), 8GB memory, 64GB mSATA SSD, 128GB SSD and DVD writer.I just can't understand why I plugged them in the wrong way, we all make mistakes sure, but WTF! I have never done this before, why should I start doing it now? I am familiar with the particular meter I was using, I've had it for 20 years. Despite the age of the meter, the terminals are still clearly marked and colour coded.I was not under the influence of anything other than my own naturally occurring stupidity.Making mistakes is acceptable as a process of learning and it can even be fun at times.However, I learned how to wire up a multimeter when I was six years old, how on this earth did I manage to stuff this basic procedure up? It's simple, red to red and black to black, as far as I can remember, assuming I still have a brain between my ears, it has been that way. Two year olds have grasped this basic colour co-ordination concept.I just wish I had the ability to effectively kick my own arse.Anyway, I guess what I'm trying to say is, don't do it ever! It's not worth it. Most digital multimeters can cope with it by displaying a - sign, but when you need to perform polarity checks you can end up in a bad way.

author: Wytnucls
author_rank: Super Contributor
timestamp: January 05, 2013, 07:14:56 pm
content: Hey, don't worry, at least nobody died.Here is the perfect meter for you:

author: Lightages
author_rank: Unknown
timestamp: January 05, 2013, 07:26:44 pm
content: A costly mistake.... We all do something like that. I always wondered why there aren't different connectors for the different probe wires so you can't make an error like that.

author: Kibi
author_rank: Unknown
timestamp: January 05, 2013, 07:31:55 pm
content: Quote from: Wytnucls on January 05, 2013, 07:14:56 pmLol, I had one of thoseI lost it when some oake stole my backpack.Quote from: Lightages on January 05, 2013, 07:26:44 pmYes, my brother mentioned that to me too.

author: SLJ
author_rank: Frequent Contributor
timestamp: January 05, 2013, 08:06:47 pm
content: Sorry to hear that. More costly than leaving a shell off the cord when wiring connectors, which I seem to do more than I want to admit.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 05, 2013, 09:38:18 pm
content: Quote from: SLJ on January 05, 2013, 08:06:47 pmWhy is it always as you're wiring the last pin you notice this?!?

author: larry42
author_rank: Regular Contributor
timestamp: January 05, 2013, 10:28:16 pm
content: Sorry to hear that but mistakes happen, don't beat yourself up over it. My colleagues are much worse. They wilfully wire up lab power supplies with random lead colours - sometimes black is positive, sometimes blue is. They never bother to look for a red lead, and think I'm OCD when I complain.Sent from my HTC One S using Tapatalk 2

author: Kibi
author_rank: Unknown
timestamp: January 05, 2013, 10:42:51 pm
content: Quote from: SLJ on January 05, 2013, 08:06:47 pmOh, that's a common one for me tooI'll feel so proud of myself when I do remember to put the shell on first only to find the the gland had to go on before the shell.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 06, 2013, 02:47:01 am
content: or when you're using a waterproof housing and the cable has to go through the housing before you solder it on, because the only way the cable goes through the housing without the connector on... the number of times...

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2013, 02:58:03 am
content: Quote from: Kibi on January 05, 2013, 07:01:43 pmI'm slightly confused about what you did here.Swapping the black and red jacks on a multimeter is no different than swapping the positions of the red and black probes when probing a circuit. I am not aware of any way a circuit can be harmed by swapping the position of the red and black meter probes, so what actually did you do? I feel you must have done something other than what you think you did. Did you perhaps try to take a voltage measurement with the probes set for current measurement, and thus short out something vital?Or was it more that you assumed the polarity was X when it was in fact Y and you wired something up backwards in consequence?

author: cwalex
author_rank: Frequent Contributor
timestamp: January 06, 2013, 03:10:28 am
content: Sounds like he wired a DC jack on a notebook backwards. But yeah, I was wondering what could be the problem too.If the reverse polarity on the m/b has damaged it maybe it could be repaired instead of replacing the m/b.

author: Pentium100
author_rank: Frequent Contributor
timestamp: January 06, 2013, 03:40:30 am
content: Also, do not leave the wires connected to the "current" sockets and then measure a car battery or mains (or some power supply that has no overcurrent protection)...But that was a crappy laptop if reversing the polarity blew up pretty much everything. I would expect it to blow up a protection diode or at most the DC-DC converter (since PC components usually do not run at 19V or so).

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: January 06, 2013, 03:53:47 am
content: Sorry to hear of your troubles.However, as far as the gear, you shouldn't need to go much further than replacing the power supply or the motherboard at worst. The chance those other components were damaged is very small.

author: Unknown
author_rank: Unknown
timestamp: January 06, 2013, 04:21:28 am
content: Peripherals like optical drives often have a crowbar diode across the rails, if you're lucky it might have failed shorted. Otherwise reverse polarity may certainly take out peripherals beyond the mainboard. Only the memory and CPU might be OK, depending on the design of the VRM.I would test anything you save for shorted power rails, though.

author: Kibi
author_rank: Unknown
timestamp: January 06, 2013, 06:57:31 pm
content: OK, so to clarify. My motherboard is a Mini -ITX. The power supply is one of those picoPSU's which runs off 12V via a regular DC jack. For those who don't know, the picoPSU plugs directly into the motherboard and is not much bigger than the ATX power connector it's self. Perfect for low energy computing. I have a 90W picoPSU which is fine for me as my PC will peak at about 40W with the i3 3220T (low TDP, 35W max) CPU.Here is a picture of a picoPSU. The image has just been linked through to one of google images results.I had soldered a DC jack on to the end of the cable. Tested, oops, the polarity is wrong. Un-soldered the connector and soldered the wires back on the other way. Tested, good, it's the "correct" polarity. Plugged it in.........I had calmed down enough to test some things.The Pico PSU is still working, I tried on an older ATX board.The SATA SSD is working, tested with a USB caddy. So the DVD drive is probably still OK, not too fussed.I have no means of testing the CPU or memory until tomorrow.I have no means of testing the mSATA SSD until I can fix / find / buy a new board with an mSATA connector on it.All of the fuses that I can find on the (small) motherboard are closed circuit. I need to know the status of the CPU / memory before I spend any more time on the board.

author: Gridstop
author_rank: Contributor
timestamp: January 06, 2013, 07:30:44 pm
content: If the picoPSU is a fairly normal buck converter wouldn't the reverse body diodes in the switching elements eat most of the current?On the other hand, I suppose the 12v passes straight through to the motherboard/peripherals, since it takes 12v in. But once again, really nothing in a computer actually RUNS on 12v, so I would expect the reverse body diodes in the motherboard's buck converters' mosfets would have absorbed the vast majority of the power. I guess I wouldn't be too surprised if the motherboard was damaged, but I'd be surprised if anything else really was (I'd bet money the CPU is fine).

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: January 06, 2013, 07:39:41 pm
content: Quote from: Gridstop on January 06, 2013, 07:30:44 pmI'll take it a step further and offer 10:1 odds that the cpu is fine. RAM is probably ok too but not offering odds on that one.

author: Kibi
author_rank: Unknown
timestamp: January 06, 2013, 08:26:27 pm
content: Quote from: Gridstop on January 06, 2013, 07:30:44 pmI initially thought this, but there is no 12V present on the output of the PSU when it's off. Makes sense really, because the PSU must switch the main power rails off when instructed to by the motherboard just as a regular power supply would. Even if the 12V rail was not bucked or boosted, it'd at the very least need to be switched through via a transistor of some sort. This would not pass reverse voltages.With my more optimistic view than I had yesterday, I'd like to think that the CPU is fine. Memory is probably fine too, if not it's the cheapest bit.