Cheap test leads, why not?

Posted by on July 6, 2011

Because! They suck.

I ordered these (link) knowing they were not the best test leads around, and you know what gave it away? — the price.

However I wondered how bad could they be?, can they be that bad?, well yes — they can.

 

To prove this

I made a 45CM replica test lead with the same cable and clips I’ve used for my previous ones (those are much shorter, so it’s not a fair test).

Then proceeded to measure it’s series resistance through a constant current source of 100mA, the result was averaged out from multiple samples and it ended up giving me about 10 mOhms. Not bad.

Now, what about the Chinese test leads?, cover your kid’s eyes! — About 308 mOhms!

Yes, that’s 0.3 Ohms of WTF.

 

Just to make sure…

I decided to link all of the test leads in series and measure the complete series resistance using my LCR meter, results were:

3.2 ohms with all 10 of them in series, unmodified linked front to front and with the DMM’s leads REL’ed out.

Well, that’s actually 31.83Ohms, if you wanted the whole number.

If we divide it by 10, we get the 0.31 ohms we found previously with our current source setup on a single test lead. So far we’re happy with our measurments, but unhappy with the test leads we just bought.

 

Reworking them

Upon inspection it’s clear these were just barely crimped and not soldered at all. The cable is also low quality, it has about 10 fine strands and no markings whatsoever… But let’s not panic, undoing the crimp and twisting up the copper, tinning it and then soldering both together on both ends gives us an improvement of around 300% (10.6mOhm) – Not bad!

 

An observation

The insulation on these test leads does not like heat, so do not crimp before soldering, crimp after.

I didn’t have to clip off the ends because the exposed copper still looked clean enough and flux does wonders removing the thin layers of oxides, so I didn’t bother with that, plus it would’ve made the leads a bit shorter and I’m sure someone would’ve argued “hey that’s unfair!” — There’s always that person.

 

The verdict:

Well, I thought I could save some time so I bought the test leads already made… turns out they sell these almost everywhere; same crappy quality whether you like it or not… I guess the only solution is to make them yourself, really.

However for the price and considering the fact that a quick stab with the iron made them behave properly, I guess it’s not that bad after all!

Remember, do not trust any wire/cable, specially if it comes from China. Whether it’s a scope probe, test leads for your DMM, an audio cable, or even just plain mains hook-up wire: always MEASURE to avoid headaches.

Cheers.

 

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