Choosing a Connector: What Has to Come Apart, and What Must Not

A RING terminal goes over a stud and the nut goes on top. It physically cannot come off while the nut is there, whatever happens to the wire. A SPADE terminal is a fork, so it slides out sideways once the screw is slackened — which means the screw only has to be loosened, not removed, for service. That convenience is exactly the same property as being able to vibrate free. So: rings for anything that vibrates, anything carrying real current, and anything where coming loose is dangerous. Spades where a panel gets unbolted regularly and a dropped screw would be a nuisance. This is a genuine trade, not a quality ranking.

A screw terminal block is cheap, takes any wire, and needs no special tool. It is the right answer for a great many low-current connections. Its weakness is that it depends on a clamping force that decays. Thermal cycling, vibration and the slow creep of the metal all loosen it, and a loose screw terminal heats up, which loosens it further. Screw terminals in anything that runs hot or vibrates want checking periodically, and almost never get it. Put a bootlace ferrule on stranded wire before it goes in, as the stripping rung covers. And tighten properly: a terminal screw that merely touches the wire is the single commonest cause of a connection that works, then warms, then fails.

If a connector can be plugged in backwards, eventually it will be. A label saying which way round it goes is a hope, not a design. Use a connector with a KEY — an asymmetric shell, a notch, a polarising rib — so the wrong orientation physically will not mate. This is what JST, Molex and almost every purpose-made connector do, and it is most of what you are paying for. Where only a symmetrical connector is available, add the key yourself: remove one pin and block the matching hole, or offset the mounting so the shell only fits one way. For anything where reversed polarity would destroy the circuit, this is worth real effort.

A connector's current rating is per CONTACT, and it assumes a good crimp. Doubling up two contacts for one conductor is a legitimate way to carry more current, and it is how many board connectors handle their power pins. MATING CYCLES matter more than people expect. A cheap header is rated for tens of insertions, a good connector for hundreds, a USB socket for thousands. A prototype that gets unplugged twenty times a day will wear out a connector chosen on price alone. Dupont jumpers are prototyping wires and nothing else: they have no latch, a poor crimp, and they fall out. They are ideal on a breadboard and wrong in anything that leaves the bench — which is exactly the transition the breadboarding rung describes from the other side.