Measuring resistance with multimeter - Page 1
Forum section: electronics / testgear

author: engineheat
author_rank: Frequent Contributor
timestamp: October 13, 2019, 05:51:04 pm
content: Hi, I have a significant voltage drop across a power supply to the load, and I want to measure the resistance of the connections. There is about 6 ft of wiring from the power supply to the load, and with some connections/junctions along the way.I calculated roughly 0.3 ohm of resistance (based on supply voltage, voltage and current at load) which I believe is way too high. The copper wiring has a cross section of 0.75mm^2.I want to measure the resistance using this multimeter:I wonder if the fact that the leads on the multi-meter being so long would give me inaccurate measurements? Would the resistance of the leads be added to the actual resistance of whatever you are measuring?Thanks

author: engineheat
author_rank: Frequent Contributor
timestamp: October 13, 2019, 05:58:18 pm
content: Quote from: engineheat on October 13, 2019, 05:51:04 pm

author: hexreader
author_rank: Frequent Contributor
timestamp: October 13, 2019, 06:07:11 pm
content: Yes, lead resistance will add to real resistance.Short multi-meter probes to measure probe/lead resistance.Contact resistance is variable and much harder to measure.Volt drop and current measurement is often a better indication of wire resistance, as you did already.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2019, 06:47:48 pm
content: Some multimeter (especially higher resolution ones) can zero the Ohms reading with shorted leads and this way subtract the part from the leads.For low resistance it may be more accurate to have a known extra current source and measure the voltage drop. This is kind of similar to a 4 wire ohms measurement, though with an independent source.

author: Gregg
author_rank: Super Contributor
timestamp: October 13, 2019, 09:26:12 pm
content: If you are looking for voltage drop, just use the meter in the appropriate voltage range (set the meter on a low range rather than auto-ranging) and test the drop on each connection and each length of wire. This is the simplest method and a little math will tell you the resistance. The connection or part with more resistance will most likely become hotter than other similar places.

author: Fungus
author_rank: Super Contributor
timestamp: October 14, 2019, 10:55:48 am
content: Quote from: engineheat on October 13, 2019, 05:51:04 pmYou cannot measure that easily but you can measure the current and voltage drop on a wire and use Ohm's law.(assuming current is less than ten amps)

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 14, 2019, 11:49:31 am
content: If you made those cables yourself and have a reel of that, then measure the resistance of the whole reel. This will be higher due to the longer length. Then scale it by the ratio of the lengths to get the resistance of the 6ft cable.

author: tooki
author_rank: Super Contributor
timestamp: October 15, 2019, 12:17:20 am
content: Quote from: engineheat on October 13, 2019, 05:51:04 pmPlain and simply, that meter is not made to accurately measure sub-ohm resistances.You could tell us the voltage and current involved (expected, measured, measured at other end, etc) from which we could calculate whether it’s the expected result or not.

author: Fungus
author_rank: Super Contributor
timestamp: October 15, 2019, 07:04:04 am
content: Quote from: engineheat on October 13, 2019, 05:51:04 pmSet your meter to mV and measure the voltage drop across each part of the system. It shouldn't be difficult to find.DC voltage losses are often higher then you expect though. You only need 1 Ohm to cause big losses.Quote from: engineheat on October 13, 2019, 05:51:04 pmYou simply can't measure resistances that low with a multimeter set to Ohms range. You need to apply a load then measure current and voltage across the points of interest.(preferably with two multimeters simultaneously)

author: engineheat
author_rank: Frequent Contributor
timestamp: October 15, 2019, 09:31:13 pm
content: Thanks.That meter doesn't offer the resolution needed for sub-ohm measurement (only 1 decimal place). I did calculate the voltage drop across multiple points and calculated the resistance.As a follow up question, if I use the meter is voltmeter mode, will the long lead on the meter sway the result?Thanks

author: Fungus
author_rank: Super Contributor
timestamp: October 15, 2019, 11:03:29 pm
content: Quote from: engineheat on October 15, 2019, 09:31:13 pmNo.