#0019: Creating custom cables for test equipment

Preamble

I wanted to write a little on test cables in general, more specifically about the actual quality of the cables available at the lower end of the consumer market. Highlighting the prevalence of these types of cables, including the products that they tend to accompany. I also intend to instruct briefly how the end user can test these cables and how they can create their own superior home-made versions.

Low-end retail cables for test equipment

In my opinion low quality cables are especially evident in instances where the cables come bundled in with many lower-end chinese consumer test devices, as opposed to purchased separately. This is especially true in cases where the product is unbranded. Think cheap test equipment such as: £10 multimeters, or £50 bench power-supplies. For example the probe cables that come with the DT-830 multimeter.

To clarify I am not talking about genuinely dangerous or illegal (according to british safety law) cables, just low quality ones. Things such as chinese power cables that do not comply with the british safety standards – which are genuinely hazardous to the user and not fit for purpose; are outside the scope of this article. Genuinely dangerous cables like that run the risk of being confiscated at customs. I wrote an article on such a cable; it was a non-compliant BS1363. Link below. No, I am specifically talking about cables such as the ones on multimeter probes, and on bench top power-supplies. Inoffensive dreck.

article hyperlink: #0004-dangerous-non-compliant-bs1363-plug

Example of low quality cables bundled with multimeter

What factors dictate a cables quality?

Many times low quality wires will have a noticeable resistance value across them. This is often due to the cost cutting measures of the manufacturers. These include constructing items with lax specifications (quality control), or by saving on materials used. For example by minimising the number of actual strands of the copper conductors present within the cable, or by opting for a cheaper material substitute such as aluminium.

These cables being built to such a restrictive price-point: is what makes them very flimsy, basically disposable in many cases. Many of these types of cables are in my opinion, are merely designed to just tick the “comes with accessories” box in a products’ marketing materials.

This unfortunately makes the cable virtually without genuine use outside of the specific one that it was manufactured for. Often not even that use-case for long, due to their general fragility. So if you have some of these low quality cables in use. It may benefit you to replace them as soon as convenient; as they may be hampering your devices’ ability to perform.

For example, cheaper multimeters such as the previously mentioned DT-830 or the XL830L, are generally rather accurate. Often staying within a less than 2% error deviation from each other. However any variable resistances across their probes’ cables may affect their readings. They may for example: cause a notable voltage drop when in use, or affect the threshold for a continuity test.

A good example of a low quality cable would be: a power-supply cable that I got with my chinese bench-top power-supply. An unbranded QW-MS305D. By the by, it was actually the same unit that came with the non-compliant BS1363 plug. Never-the-less, the power-supply’s cable in question was used to power devices under test with DC voltages; as such they contain a male banana jack on one end and a crocodile clip on the other.

Although it is a relatively cheap power-supply, it could still reliably output it’s rated 30 volts at 5 amps. The only issue with the out-of-box setup (minus the bad plug), was that the output cable heated up and often got soft when exposed to the higher amperages that the power-supply could output. Although it took sometime for this to happen, i.e. it needed continuous output over several minutes. I still judge the cable as unsuitable for purpose. One thing I found humorous after the fact is that the online seller that I bought from had this same accessory packaged with the 10 amp version of this power-supply as well (QW-MS3010D).

Example of low quality cables that came with the power-supply

Using salvaged materials to create custom cables

As a response to the performance of the bought cable, I retired it. I removed the cable ends and attached them to a salvaged mains cable; and it has worked fine since. I really like using salvaged mains cables for these types of applications. Especially ones from UK safety certified devices; and especially ones from heating units. Such as electric room heaters, or toasters, electric grills, kettles, etcetera. Basically anything that uses electricity to generate resistive heat. This is because their cables are specced to allow large amounts of current to pass through them without heating up themselves.

For example: a typical mains oil heater, is rated for 1500 watts. This means that it’s cables need to safely pass that amount of power across them concurrently while the device is in use. When converted to volts and amperes; this means that these cables are able to handle 240 volts AC at 6.25 amps. This leaves me confident that it can safely handle the maximum 5 amps, 30 volts DC of my power-supply over long periods of time.

They do this by having very little resistance across the cable length. This is accomplished by actually putting copper in your copper cables. Although after looking at the example photos that I have, it appears that this particular cable is actually using aluminium strands as conductors.;) However the point stands; there are sufficient conductors within the cable, that the current can pass across it unhindered. I.e. The cable is of a big enough gauge, not to bottleneck the higher currents.

One thing to pay attention to when creating your own cables beyond the quality of the cable itself; is how you connect the various plugs you wish to use to it. This is because a poor connection here can impede the passage of electricity; and add resistance to the line. I suggest firm connections with as much conductive surface area touching as practical. It is also good form to do a resistance test across the entire thing once completed.

Another good source for quality salvaged cabling, is old or damaged ethernet cables. The twisted pairs within work very well in lower voltage DC applications; including carrying signal voltages (like binary data). The pair windings are configured to minimising interference for their carried digital signals after-all.

I like to use them to make home-made breadboard jumper cables. This can be done by just unwinding a length of cabling, cutting to size, then tinning the ends so that they can interface with the breadboard. Will it pretty? probably not. Will it be functional? 100%.

You could also use ethernet cables for replacing a cable between an external DC power-supply (or power-brick) and it’s paired device (e.g. Laptop). I tend to opt to wind together a pair of cables, if I wish to reliably carry current at higher DC voltages (e.g. 20VDC @ 3A for a laptop). However any higher than that and you’d be better served by using thicker gauge wires.

Home-made custom cable for power supply

Example of resistance test used to determine cable quality

Closing thoughts

That’s all really. I just wished to highlight that some low end cables are not good, and to encourage you to create your own superior cables using parts from common household devices.

Thank you for reading.