measuring lead resistance on a multimeter? - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: June 28, 2017, 08:49:36 pm
content: I'm wondering about some of the simplest ways to measure lead resistance on a multi-meter.

author: Audioguru
author_rank: Super Contributor
timestamp: June 28, 2017, 08:58:46 pm
content: Plug in the leads and short the probes together. Then measure ohms on the lowest range. The leads of my multimeter measure 0.2 ohms like that.

author: MosherIV
author_rank: Super Contributor
timestamp: June 28, 2017, 09:38:35 pm
content: An old colleague of mine said to me "dont trust a dmm that shows 0ohms when you short the probes"Absolutely right.0.2- wow you got very clean probes. ALL of mine show between 0.4 and 0.8I beleive most of the resistance is down to contact resistance at the probes.Most hand held dmms are pretty inaccurate below 2You need to go to high end bench dmm with 4 wire measurement to do that kind of measurement accuratly

author: jimmeh11
author_rank: Newbie
timestamp: June 28, 2017, 11:02:29 pm
content: A more accurate way to measure the resistance would be to put a known (small) current through the lead and measure the voltage across it. Usually voltage and current measurements at low values are much more accurate. Then V = IR.

author: kalel
author_rank: Frequent Contributor
timestamp: June 29, 2017, 02:38:21 am
content: On the DT meter 830B (factory fused version of it), improvised clamp probes I made show 0.4 on the 200 OHM range. Now, how much that can be trusted, who knows. Although, I'm only using those leads to measure resistance, so if that's anywhere near accurate - an addition of 0.4 shouldn't be an issue.I had a beloved 830D version as well (it has a beeper and it sounds good, no issues) but I noticed some problems with the contacts recently, and then I saw this:Now, I'm not blaming the meter on that, I did use probes from another meter in it, which might have slightly wider contacts or such, and when adjusting them to fit, I definitely plugged the probes in/out many times. Well okay, perhaps on a higher quality meter this wouldn't happen, but I still really like this one, it worked well for a while, so I hope to fix this somehow.Does anyone know if a replacement part might be available form China, or should I try to solder some bridge and glue the part to the plastic? I'mnotusing this for any high voltage testing, and I'm not putting my life at risk if the quality of contacts is not perfect. I would rather have it back for continuity testing, low voltage and current tests, than throw it away, if possible.

author: IanB
author_rank: Super Contributor
timestamp: June 29, 2017, 04:51:04 am
content: Quote from: Audioguru on June 28, 2017, 08:58:46 pmQuote from: MosherIV on June 28, 2017, 09:38:35 pmI just measured 0.04 to 0.06 ohms on various test probes I have. Upwards of 0.4 ohms seems rather high.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 29, 2017, 05:21:33 am
content: I have measured mine with a milliohmmeter (4 wire), pretty simple.

author: Rick Law
author_rank: Super Contributor
timestamp: June 29, 2017, 05:33:47 am
content: Quote from: kalel on June 28, 2017, 08:49:36 pmI am just a hobbyist. I went though the same thing. Eventually, I conclude there isn't much point in trying to get down to < 1ohm. Too many factors can add/subtract small amount of resistance. To get consistent measurement, you will need very consistent everything once you are down at sub 1ohm.Touch the end of the probe is the best way to find the resistance of the probes - but then if you squeeze the two probe-ends together real hard, the resistance will drop. If you clean the probes/jack/sockets well, the resistance will drop again. Spin the jack (while in the socket) around a bit, the resistance will change again...Play around with it a bit, you will get a feel for it and you can decide how accurate is accurate enough for you. By then, you will know the likely best-case (best contact/least resistance) would be for your probes.My meter is UT61E and typically I use gator clip for probes so I can clip it on what I am testing. Best for me is clipping them together at 0.08ohms. If I hold the clips together by hand, it will depend on contact pressure. 0.22ohms typical and 0.08 if I press them together really really really hard. Probe with sharp ends has similar variation. So, in real life, it is meaningless because I will never be able to press that really really really hard against whatever I am testing.

author: BravoV
author_rank: Super Contributor
timestamp: June 29, 2017, 05:40:19 am
content: Why this matter, as long the DMM has the NULL function, the parasitic resistance at the lead doesn't mean anything, right ?Also using 2 wires at ordinary handheld DMM, to measure accuracy < 1 Ohm is basically pointless as well.CMIIW

author: IanB
author_rank: Super Contributor
timestamp: June 29, 2017, 06:01:23 am
content: Quote from: BravoV on June 29, 2017, 05:40:19 amIf you are making current measurements it would be ideal if the test lead resistance is as low as possible, perhaps? So it's nice to know what that resistance actually is.

author: BravoV
author_rank: Super Contributor
timestamp: June 29, 2017, 06:06:58 am
content: Quote from: IanB on June 29, 2017, 06:01:23 amAssuming we're talking "common" DMM that has relatively high burden voltage already, this is not a big deal I believe.Of course we're not talking the lead that is so crappy say like it has several Ohms magnitude.