How would I test DMM leads to see if they are crap or not? - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: September 27, 2020, 05:09:26 am
content: I got a cheap meter lately that I tossed into the trash, but I kept the leads. I tested them for resistance and they had about .04 Ohms. I'm going to use them for 10 amps of current. I just want to now if I should toss them too? What else should I test them for? If they are decent, I don't want to waste a good pair of leads. They are insulated like they should be.

author: krzysssztof
author_rank: Contributor
timestamp: September 27, 2020, 07:36:26 am
content: I would have done a simple test:Connect one lead to the constant current source, as much Amperes as you can get out from the source (ideally 10A, because this is what you want to measure later).After a while measure voltage drop on the lead including connectors (this will allow you to calculate resistance quite precisely) and check if the lead did not heat too much - this will give you an idea if the copper wire diameter is suitable for such currents.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 27, 2020, 06:23:37 pm
content: Quote from: krzysssztof on September 27, 2020, 07:36:26 amYeah good idea. I don't have any 10A supplies. I really need to get a bench power supply. Is there on on Amazon you would recommend that isn't too expensive?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 27, 2020, 06:50:27 pm
content: You can easily make a current source / sink from a LM317 and a 1.25Ohm resistor, (and some capacitors for stability)With this you have a pretty easy / stable / reliable fixed 1A device that you can use for measuring very low resistances. Just put this 1A through your test probe, and every mV measured is a mOhm. With this simple device you also automatically have a 4-wire measurement, which is pretty crucial for accurate measurements. You can also probe the cable in multiple locations, measure contact resistances of intermediate connectors etc.If you want more current, just put more of these devices parallel.It's of course also not so difficult to make an adustable source / sink with an opamp, power MOSfet, sunt resistor potentiometer and voltage reference. But the fixed 1A is easier for measurements and does not get out of calibration easily.

author: fordem
author_rank: Regular Contributor
timestamp: September 27, 2020, 08:07:04 pm
content: First - a cheap meter will have cheap leads - if you have any doubts about the leads just toss them.Next - if the meter had a 10A scale, the leads are probably good for 10A - note - the meter's 10A scale may have a duty cycle, 10A for X seconds - assume the leads to have the same duty cycle. What makes the leads crap for me is longevity, how long do they last before the copper starts to separate, before the insulation starts to crack, etc.Are you using the leads around mains voltage? Do they have a CAT rating moulded into them? If you have any doubts just toss them - your life may depend on them.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 27, 2020, 09:56:12 pm
content: Quote from: fordem on September 27, 2020, 08:07:04 pmNo mains and just using them for low voltage DC stuff, mainly to measure continuity, resistance, and current usually 5Amps or less.

author: floobydust
author_rank: Super Contributor
timestamp: September 27, 2020, 10:12:59 pm
content: Most reviewers just seem to do a visual of test leads and speculate.The safety standard for test leads and probes is 61010-031. There's a whole slew of tests. Some of them include:5.3 Durability of Markings Test6.3.1 Voltage, Current, and Capacitance Test6.6 Voltage Test (humidity preconditioning, tin foil wrap, 1min. hi-pot 600-1,000V gets 4,400VAC reinforced ins.)6.7.4.1 Pull Test (double 10N/mm^2 pull)6.7.4.2 Flexing/Pull Test (5,000 times 10N/mm^2 pull)6.7.4.3 Rotational Flexing Test (500 times)6.7.5 Insulation of a Probe Cable8.1 Rigidity Test (3 times, 20N 12mm hard rod poke)8.2 Drop Test (3 times, 1m drop)8.3 Impact Swing Test9.2 Temperature Test (run at rated current)10.1 Maximum Ambient Temperature TestI would look for the cheapness - skinny copper wire, short length, and crapola plating at the connectors/probe ends. Cheap plastic insulation and bad seams.The rated-current test with checking for hot spots or melting plastic is very revealing. Hobby use, it would still be nice to know if the probe and wire insulation is decent.Fluke testleads meet the standard,Brymen silicone test leads BL21S2-T4SCdon't mention any compliance. It would be important if you are an electrician working in Cat. III or Cat. IV environments.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 28, 2020, 01:01:06 am
content: Quote from: floobydust on September 27, 2020, 10:12:59 pmThose reviews are stupid. "Oh, I like the color, it's neat. Does everything I need it to do, and so high quality too!" F me. They wouldn't know copper wire vs fishing string if they were holding it.I'm gonna cut the ends off and attach alligator clips. I can have a look and see what type of wire they were using, at least. If I don't like it, I'll buy some lead ends and make my own.I still want a bench PSU.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2020, 12:00:07 pm
content: Sound likeQuote from: DW1961 on September 27, 2020, 05:09:26 amQuoteWith all your posts on CAT ratings and safety I assumed that you were working in that area, not low voltage DC stuff. As mentioned, the leads may not handle continuous 10A, or even 5A. The meters will not normally handle that. If you have a need, I would just use them.I saved all the leads from the meters I ran and placed a constant current through them, ramping the current until they failed. I plot the resistance, current and power the leads dissipated during the cycle. There was a set from Mastech that did better than many of them.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 28, 2020, 05:35:54 pm
content: Quote from: joeqsmith on September 28, 2020, 12:00:07 pmJoe, you duh man! Test 'em 'til l they roast!One question: If leads are rated, for example, "10 Amps for 30 seconds every 15 minutes," why test they for longer than that? Wouldn't you expect a failure?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2020, 05:53:50 pm
content: Quote from: DW1961 on September 28, 2020, 05:35:54 pmWhen comparing the products, what ever test method I come up with, I typically test them to failure. So, yes, I expect them to fail. No matter if we are talking about meters or their leads, some handle the tests better than others.

author: MosherIV
author_rank: Super Contributor
timestamp: September 28, 2020, 05:59:05 pm
content: Try running 10A through the leads and carefully check how hot the leads get.Good leads should not even get warm.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 28, 2020, 06:48:15 pm
content: Quote from: joeqsmith on September 28, 2020, 05:53:50 pmOK, thanks Joe. Just a couple more questions:When the meter gives a time limit for current, I assume that doesn't apply to the leads, just the meter?In your video, all of the leads passed resistance given their voltage and current ratings?

author: wizard69
author_rank: Super Contributor
timestamp: September 28, 2020, 07:09:35 pm
content: The gauge of the wire is your primary concern. If the meter was real cheap the quality of the insulation will also be a factor if this is high voltage. Since you are asking the question I'm assuming you have not looked into these issues thus would have to say the wire is not up to the task.By the way don't assume the insulation is actually rated for the voltage you intend to run at. Cheap multimeters often are of questionable safety for high energy circuits.Quote from: DW1961 on September 27, 2020, 05:09:26 am

author: floobydust
author_rank: Super Contributor
timestamp: September 28, 2020, 07:24:52 pm
content: The cheapola test leads each was around 50mohm and thus dissipating almost 20W at 20Awhich is a lot for a short run of wire. Things will melt, especially if it's a connection that haas the high resistance. Because the resistance goes up with temperature, the heat increases more and more for a given constant current.The wire insulation test uses a metal chain curtain up against the wire, with HV applied. As the wire sample is pulled through, any cracks or breaks in the insulation will arc to the chain.

author: wizard69
author_rank: Super Contributor
timestamp: September 28, 2020, 07:31:08 pm
content: Quote from: DW1961 on September 28, 2020, 06:48:15 pmIt depends upon the leads. If the meter has an internal shunt that will heat up thus defining one time limit. Your leads however can be just about anything so you need to know the ampacity of the wire.QuoteI'm really thinking that you need to relate what is happening to OHMS law! All conventional test leads (actually all wire) are in fact resistors and thus have a associated voltage drop. What that resistance is depends upon length and the diameter of the wire. Depending upon what the wire is being used for you may have to take that resistance into account. There are various ways to address this, for example a power supply might have remote voltage sensing.I'm really perplexed by your responses here. First off you really need to be able to determine if any test setup is safe before moving forward. That includes making a determination that your wiring is safe. Frankly if you have to ask if a cheap pair of test leads are safe they are by definition not safe.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2020, 07:32:12 pm
content: Quote from: DW1961 on September 28, 2020, 06:48:15 pmI have no idea. You would be best off asking the manufacture. I would not be surprised if you discover the leads are made by a different company.QuoteI'm not sure what you are asking. I don't collect any data for the leads and have no pass / fail criteria for them. Similar, I have a somewhat arbitrary cutoff for what I consider an electrically robust meter. In the end, I just present the data. The viewers can see how they compare and make their own determination.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 28, 2020, 08:06:43 pm
content: Quote from: wizard69 on September 28, 2020, 07:31:08 pmYeah, I understand that. I was just trying to understand if the leads would give a bad reading simply becasue they are "that bad." I should have been more specific. I'm just using them for DC voltage, resistance, and current stuff. I thought I could do that be testing resistance and or as otehrs have stated, running current through it and checking temp.I think I'll just do what you suggested and cut the ends off (which I planned to do anyway and solder clamps back on) and see what gauge the wire is, and if it is really copper core. That's the easiest way.Anyway, I think maybe I'll NOT do that and just get some meter side insulator plugs and solder my own wire onto them.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 28, 2020, 08:32:35 pm
content: I just cut the probe off of one of the leads, and stripped it. It's AWG 20 (of close) and after holding a lighter up to the wires, they all disintegrated into thin air. It's not even copper wire. Just to test that I was seeing what I was seeing, I did the same test with some 22 AWG pure copper wire and it held up perfectly under the flame.I just tossed the leads and that horrid POS meter into the trash. The only thing I kept was the box in came in so I can recycle it.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2020, 12:13:59 am
content: Quote from: floobydust on September 28, 2020, 07:24:52 pmYes, 50 mohms @ 20W seems about right for the majority of the probes I looked at. As we can see, there are some much better than others.I had ran some transient tests on them as well, with the leads bundled up. I don't remember any of the breaking down.