Multimeter reading mishap - Page 1
Forum section: electronics / beginners

author: zz
author_rank: Newbie
timestamp: March 10, 2025, 07:57:31 pm
content: Hi all,maybe a stupid question, I am still learning.I was attempting to repair a dead Lenovo ThinkPad motherboard. While testing input voltage on the DC jack (the yellow usb-a type charging port) I put the black probe on the housing of a nearby ethernet port and my red probe on the second lead connecting the port to the pcb. My multimeter was set to DC measurement. When I did that I got sparks and a bit of the lead was actually eaten out (as you can see on the picture), as well as a bit of the probe. I have tested input voltages many, many times and never faced anything similar. The only similar thing to have happened was when I once accidentally placed a probe so that it touched both live and ground leads - this was not the case here, I was careful only to touch one lead at a time.I am wondering why this has happened so that I can prevent it from happening again. Any help would be appreciated.

author: benj38
author_rank: Frequent Contributor
timestamp: March 10, 2025, 08:37:15 pm
content: The following options come to mind:You were in DC current mode instead of DC voltage mode;your multimeter has more than two jacks for the probes to be plugged into, and you had the probes plugged into the wrong jacks (i.e., instead of the voltage measurement jacks, you used one of the jacks used for current measurements;your hand slipped and you did short out two connections with the tip of the probe without noticing;your multimeter is defective.In short, if you are in DC voltage mode with a good multimeter and probes connected correctly on both ends this kind of thing will never happen.

author: zz
author_rank: Newbie
timestamp: March 10, 2025, 10:36:32 pm
content: Thank you for the swift reply.1. If I understand my multimeter correctly then I should have been in the correct mode - I am attaching a picture of exactly how it was set up.2. Same as above.3. I though about that, but as I said as it has happened to me before I am extremely careful regarding this. The dc jack's leads are nice and far apart and I think this is not the case. What I have not mentioned before is that I actually tried this on two exactly same "daughter" boards with exactly the same result - Sparks on the second pin and damage as seen in the original photo. After it happened to me the first time I suspected a shorted dc jack (or smth like that) so I tried with another one as I had the same type from another leftover laptop.4. Is there anyway to check this? I have not experienced any other problems with it and I have been using it a lot.Thanks for the help!

author: mwb1100
author_rank: Frequent Contributor
timestamp: March 10, 2025, 10:57:11 pm
content: What make/model is the meter? (I see now it's an MD-420). Some pics of the inside might be telling.Your meter is one of the sort that uses the Volt/Ohm input jack to also measure the lower current range(s). When in current mode there will essentially be a short between the red input jack and the black common jack. Maybe there's some fault inside the meter that causes that short to be in place even when the selector is in the DCV position mode?Update: it looks like it's the same as the ALLSUN EM420B that has a teardown here (heads up - Darren Walker review video):https://youtu.be/MA9fnWCFcRo?t=894

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 11, 2025, 12:00:49 am
content: Quote from: zz on March 10, 2025, 10:36:32 pmYou should check and see if your meter is still working correctly. If it caused a short due to the fact that it was in current mode there would be damage to the meter. If the meter had fuse it would be blown.

author: u666sa
author_rank: Frequent Contributor
timestamp: March 11, 2025, 10:43:21 pm
content: You touched ground and positive with your probe, that's all.

author: golden_labels
author_rank: Super Contributor
timestamp: March 12, 2025, 12:21:46 am
content: There is always such a possibility, but zz clearly indicated their belief is this isn’t the case here.Speaking of OP, I hope they finally reply to the questions. I’m interested if the fuse is ok. A short eating out metal should easily kill a 250 mA fuse.I’m not willing to reverse engineer this PCB at this stage, in particular with the switch layout unknown. But in the video linked by mwb1100 I can see a PTC directly on the input (the yellow ball). If that was somehow damaged earlier, my guess is it could be responsible for a short.I’m also unsure what R23 (south of the fuse) and R22 (west) are. Those appear to be very low value resistors. But in the video their first bands look black too, which can’t be right. If we assume brown, then we get a bunch of 1s and 9s. Oddly specific.