Why do DMM leads have such high impedance? - Page 1
Forum section: electronics / testgear

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 04:38:34 am
content: Why do DMM leads have such high impedance?What is the resistance of your DMM test lead? Mine (UT61E) lead is 0.6ohms. I recall in other comments, the commenter said his test probe is merely 1.1ohms; another comment in another thread has his leads at as high as 2.1ohms. I recall in a youtube video by Martin Lorton on constant current drain, while discussing the need of his 1ohm-50W resister, he said his CrocClip cable is 0.5ohms. The GatorClip cables (5 pairs) I got all measure in the 0.5ohm range.According to standard, the 18awg wire is only about 0.02ohm per meter.So I wondered how a production DMM lead has such high impedance. I removed the two clips from a single GatorClip cable and made a pair of Banana-Plug to GatorClip DMM cable (about 12 inches each) replacing the stock cable with some 18awg (300volts) cable I have laying around. The not very well made homemade clip-to-DMM impedance flexes between 0.01ohms to 0.02ohms. The cable itself is the bulk of the 0.5ohms!That really brings my puzzlement to the forefront. How come production DMM cables have such high impedance? My not very well made DMM clip-probe is 0.02ohms. Why is the production DMM leads in the 0.5ohms range? That is a good magnitude difference!Rick

author: cyr
author_rank: Frequent Contributor
timestamp: March 26, 2013, 05:39:59 am
content: Why? Because copper is expensive.Those leads probably have a few tiny strands in them, or even be made of copper-plated iron.FWIW I quickly checked a couple of DMM leads I have:Fluke TL175: 31.5mohm/lead.Crappy noname: 210mohm/lead.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 26, 2013, 06:33:22 am
content: What instrument did you use to measure such low impedance?

author: cyr
author_rank: Frequent Contributor
timestamp: March 26, 2013, 06:49:46 am
content: I used a bench DMM with 4-wire offset compensated ohms, works pretty well as a milliohm-meter.You can do the same with pretty good accuracy with a current-limited power supply and two regular DMMs though, use a high enough current to get a voltage drop you can measure accurately.

author: BravoV
author_rank: Super Contributor
timestamp: March 26, 2013, 06:58:00 am
content: Quote from: cyr on March 26, 2013, 06:49:46 amMost of the times when it comes to measure lead resistance, I used my bench psu, at CC mode, hook the lead and let the psu toast it say like at 1 Amp, then read the volt reading or use normal handheld dmm which good enough at mV level to measure at the psu terminals.Yeah, sometimes powering up the bench dmm alone just for this is too complicated and I was lazy.

author: Simon
author_rank: Unknown
timestamp: March 26, 2013, 07:11:14 am
content: remember the meter will make an error, nothing to do with the leads

author: Wytnucls
author_rank: Super Contributor
timestamp: March 26, 2013, 08:01:03 am
content: I measured a few test leads with a 2-wire UT612 LCR meter (not perfect, but good enough for a ballpark figure).The Fluke TL175 lead appears to have an impedance of 32 mOhm on this meter, which is consistent with your figure.I don't see much difference with other manufacturers' leads, except for those with smaller gauge wire:UT71 18AWG 34 mOhmUT61E 34 mOhm (Not 600 mOhm!)CEM 18AWG 38 mOhmUT204 20AWG? 80 mOhm (lead unmarked)Contact point resistance can introduce some extra impedance, if the connections at the tip and jack are loose.This is what 18AWG should look like inside:

author: Simon
author_rank: Unknown
timestamp: March 26, 2013, 08:03:45 am
content: something sounds screwy

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 02:45:06 pm
content: Quote from: Wytnucls on March 26, 2013, 08:01:03 amYeah - I realized that. When I tested the GatorClips I made, I realized how they bite the wire makes a difference of 0.02ohms to 0.04 ohms. So I tested the probe sockets and made sure the contacts are good.Quote from: Wytnucls on March 26, 2013, 08:01:03 amThat inspired me to clean the tips again. I must have done a bad job before (or I misread the meter before). Now it is 0.08ohms when I touch the two leads. At 34mOhms each lead per your measurement, that is 68mOhms total. So I am in the same range you quoted.Now I am reminded: poor and/or dirty contacts can make a big difference.I don’t usually measure something that small. Learning how to use my new tool has been educational.Thanks, guys - the comments you guys posted are helpful and educational too!

author: Wytnucls
author_rank: Super Contributor
timestamp: March 26, 2013, 04:27:54 pm
content: Glad you came right eventually.As a reminder the best conductors are:Silver 6.30x10^7 Siemens/meter at 20degCCopper 5.96x10^7Gold 4.10x10^7Aluminum 3.50x10^7Iron 1.00x10^7Carbon Steel 6.99x10^6Stainless Steel 1.45x10^6Notice how bad stainless steel is, as a conductor, compared to copper.

author: robrenz
author_rank: Super Contributor
timestamp: March 26, 2013, 05:17:41 pm
content: Measured the following with calibrated micro ohm meter with kelvin connections and offset compensated ohms.The following were measured using a pomona shouded probe adapter to enable using myKelvin banana jack adaptor.The adaptor resistance including the contact resistance variations is 1.5 to 0.9 mOhm itself.Edit: these are individual lead resistance values not the pair.52" Fluke 87 leads 30.09 mOhm54" Fluke leads that came with my 8846A 34.25 mOhm54" Fluke 2X4 kelvin probe leads 58.65 mOhm36" stackable banana leads 21.74 mOhm connected with the male banana part 19.26 mOhm connected with the female part34" circuit specialists CSI2010 meter leads (free meter with purchase listed at $29) 262.00 mOhm

author: robrenz
author_rank: Super Contributor
timestamp: March 26, 2013, 05:25:04 pm
content: Brass (nickel plated) is commonly used for connector and probe construction and has 2.67 times higher resistance than copper

author: Matje
author_rank: Regular Contributor
timestamp: March 26, 2013, 11:42:27 pm
content: Quote from: Rick Law on March 26, 2013, 04:38:34 amThey don't. Well, good ones don't...Quote from: Rick Law on March 26, 2013, 04:38:34 amThey have such a high resistance because they are crap.A special place is reserved in hell for the makers of the UT61E test leads. Mine failed, in a very nasty way - resistance would randomply jump between some dozen mohms and some dozen ohms. Drove the continuity tester nuts...So i retired them, cut off the banana plug and then cut that POS open. Turns out the wire had a jacket (is that the right english word?) crimped onto it (that is good), and that went into a hole in the metal body of the banana plug.Now the problem was this: all that was securing the connection between the jacketed wire and the plug body was a round indent (1 to 1.5 mm) pressed into the banana plug body over the hole, and the molded plastic around it (see image). That means the jacketed wire end can wiggle slightly in the somewhat larger then the jacket hole - piece of crap, going to go bad sooner rather than later. If they had compressed the plug metal along the whole length of the hole, from both sides, it might actually have worked reliably.The resistance between the probe tip and the other end of the wire was constant and good, but I don't trust that anymore.So I got myself some Fluke leads and hope these are properly made.

author: Rick Law
author_rank: Super Contributor
timestamp: March 27, 2013, 01:07:03 am
content: Ah...Thanks for letting us know. We (I) can now be on the look out for it. While the UT's stock leads may not be the best, but for now, it best suits my budget. If I am in your shoes, after uncovering the problem - out comes the Crazy Glue. I will glue the darn thing back together and keep using them.I'll say, each time I look at those old HP3478/3457 and the like, or a nice dual/quad channel scope, I have a hard time controlling myself. I really have no need for it. I would even have a hard time (patience) to let it warm up and stabilize. but I sure like to have something more accurate.Rick

author: amspire
author_rank: Super Contributor
timestamp: March 27, 2013, 01:41:15 am
content: Quote from: Wytnucls on March 26, 2013, 04:27:54 pmGraphene can theoretically achieve about 35% better conductivity then silver. Graphene has far less available mobile electrons then silver, but they move much faster giving an overall gain.Impurities though in a lot of the current graphene samples mean that the resistance is still more then silver at the moment.

author: Rerouter
author_rank: Super Contributor
timestamp: March 27, 2013, 08:02:57 am
content: yes but how easily can you cheaply produce graphene wire?

author: Wytnucls
author_rank: Super Contributor
timestamp: March 27, 2013, 05:51:53 pm
content: Thanks Amspire. I wasn't aware of the electrical properties of graphene.It is really an amazing substance.The Wikipedia article makes for an interesting read, if anybody is ready for an in-depth look:http://en.wikipedia.org/wiki/Graphene