I needed some custom test leads recently and after experimenting with a few types of banana plugs and cables I found a nice combination and set about assembling them. This short blog post contains a possible recipe, i.e. what worked for me. It was also an opportunity for me to use a ferrule crimp tool. The end result was good – it appears to be a reliable set of test leads. (For innovative ideas on test probes, see here: Shop Tips - Intrusive Meter Probe ).
In the end I went for stackable banana plugs for my particular need, and they also have a retractable sheath to prevent accidental shorts. Internally there is ample room to fit the 4mm overall diameter cable that I wanted to use. The entire parts list is further below.
Check out the video for more detail:
Stripping the cable
The chosen cable is really nice. The insulation is thermoplastic elastomer and 525 strands of copper wire making it very flexible. It is designed for hand-held test leads according to the manufacturer Multi-Contact (PDF datasheet). It is usable at temperatures down to -30 degrees C, and can withstand up to 105 degrees C.
Ordinarily I’d use a wire stripper for cable stripping but this 4mm cable wasn’t something I normally use; it has hundreds of very thin copper strands and a very thick and soft plastic insulation for extra flexibility and therefore I was worried about the risk of nicking copper strands with the strippers I had (electrician’s wire strippers which are designed for solid core wire of certain sizes).
Since I wasn’t making many lots of leads it was very easy to gently slice at the insulation with a scalpel, bend the wire slightly to confirm that it hadn’t reached the copper core, and then dig a fingernail into the insulation and tear apart the insulation. After that the wire was twisted back into its original lay. These would possibly make the job a lot easier; this style of wire stripper does not rely on the strength of the insulation.
Note that although other cables could be used (such as a speaker cable if the banana plugs are for an audio system), it is fairly important that the overall diameter is around 4mm because the strain relief on the chosen banana plug shell is designed for 4mm diameter cables.
Applying the Ferrule
The wire has more than 500 strands; I wanted all strands to be captured and attached to the banana plug. I therefore made use of the Knipex Crimping Pliers reviewed here and a sleeve-less ferrule. The result was perfect, every strand captured. For this type of extreme multi-stranded cable the ferrule is essential otherwise the strands will just bunch on either side when the screw is tightened.
Attaching the Banana Plug
The user instructions for the banana plug specify that 6mm of wire needs to be bared; the ferrule exposes more than that so it was measured with a ruler and the excess was cut off with wire-cutters.
The banana plug shell is in two pieces and the cable was threaded through the rear piece.
Next the cable was inserted into the screw terminal and held in a centred position until the screw was tightly fastened. To keep it from working loose over time, thread-locking adhesive can be applied before the screw is tightened. I tightened very firmly since there was no chance of the strands breaking up under the crush of the screw, due to the use of the ferrule.
Fully Assembling the Banana Plug
The banana plug has a retractable clear plastic shield with a spring action. The shield and spring are inserted first. The rear plastic piece of the shell clicks into place.
Testing the Assembled Lead
Apart from a simple tug test during assembly, no other mechanical tests were done tot he test lead. Although the cable will be difficult to detach by pulling, and the banana plug has strain relief to prevent some flexing at the end of the cable, there is nothing preventing the cable being rotated. I doubt this will be a problem, and most banana probes would have the same behaviour. However if I was assembling it again I probably would use a short length of adhesive-lined heat-shrink tubing to secure the cable to the strain relief on the banana plug.
As an electrical check the resistance was measured. Of course, for many measurements the precise resistance (or having very low resistance) does not matter, but for want of measuring something, the resistance was measured. The two banana plugs at either end, plus 140cm of cable (which alone has a resistance of around 7mOhm) totalled 17mOhm (checked with a 2-wire and a 4-wire measurement; the 2-wire measurement had the banana plugs inserted into the multimeter, i.e. how the test lead would ordinarily be used. For the 4-wire measurement a dedicated was used). It was concluded that the resistance was as good as many typical test leads, some in fact have a much higher resistance.
It is possible to assemble custom test leads that should perform at least as well as off-the-shelf ones with hopefully similar or better levels of reliability, with little effort and no soldering involved. The particular cable and plugs used are designed for 30A.
I hope the information here was useful; I wanted to examine if ferrules would be useful for terminating wires into connectors such as these, and from the result (reasonable strength from experience trying to force apart the ferrule, and the reasonably low resistance) I'm impressed that it solves the problem of how to connect heavily stranded cables to screw terminals.
These were the parts that were used:
|Parts (Click for more information)|