Sunday, 18 October 2009

Cheap and Easy 4mm Connectors

An interim post; I felt it was worth sharing. I have used red bullet crimp connectors for some years as a cheap alternative to 4mm 'banana' plugs, which are used for test and measurement connections. Only the red ones work; the yellow and blue crimps are too big. Earlier today, I found it was easy to use the female crimps to make workmanlike sockets for use in test equipment.

All you need is a red female bullet crimp, and a pair of M8 nuts. It's simple. Use a nut to cut a shallow thread in the body of the crimp, and then use a pair of nuts to mount the crimp in the front panel of your new home-brewed meter, power supply, generator, whatever. I found it easier if the wire was already crimped into the terminal; the flattened, crimped section can be held with pliers while the nut is wound up the barrel of the crimp terminal.

For a really smart finish, you need a powerful soldering iron. A 100-Watt soldering iron has enough power to solder an M8 nut to a piece of FR4 copperclad. Drill an 8mm hole in the panel, lightly assemble a nut to the rear of the panel with a short bolt in the hole, and heat the nut. When it's hot enough, flow a little solder around the nut. Just add enough to give a 3mm 'fillet' joint. Wait until the nut has cooled before winding the crimp barrel into it; if you are tempted to do this in a hot nut, the crimp will shrink with the heat and drop out!

These crimp terminals are sold in motor factors, DIY stores and good, old-fashioned hardware shops. You'll find the M8 nuts in the same places. The connectors made in this way are fully compatible with 4mm banana plugs and sockets, and the tooling is cheap, too. Buy the simple pressed-steel cheap one you see in the photos; you can pay a lot more, but these work well enough for our purposes. Besides, you get a wire stripper, screw cutter and wire cutter in the tool as well.

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