Monday, September 21, 2015

SPST, SPDT, DPDT and Beyond!

I had a switch go bad in the oven hood. I don't know the source of the failure, but the solution is simple: replace the switch.It's a simple repair, cut two wires, solder two connections. Anyone who knows their way around a soldering iron can do it. But not every switch has two connections... For the purposes of this explanation lets exclude relays and focus on electromechanical switches. 

First let's establish that a switch must have a minimum of two terminals. A switch interrupts current. So one terminal is the input and the other terminal is the output, like an on/off switch. The power goes in one side and comes out the other. But a switch can have more than one pole, and also more than one "throw."  The number of poles determines exactly how many separate circuits the switch can control. A guitar toggle switch for example can route current to one or more of many pick-ups in the guitar.  The word "circuit" in this case just refers to a route current can take through a system. It doesn't require a PC board. The throw-count defines the number of separate circuits within a pole,  i.e. if a switch has two throws, each pole in the switch can be connected to one of two terminals.The number of poles and throws a switch has is how they are classified.Let's go through the common types:
A single-pole, single-throw (SPST) switch is the basic switch. The two terminals are either connected or disconnected. like a light switch. The switch will either be closed or open like a make/break circuit.

These have three terminals: one common pin and two pins which vie for connection to the common. They are used for switching between swapping inputs. SPDT switches usually have three terminals.

These are basically two SPDT switches, toggled by single actuator. DPDTs will have six terminals. I've never actually needed one of these but I think I have one in a milk crate somewhere. Once we get past one or two poles or throws, we just start sticking numbers in the abbreviation ex. 6PDT would mean six pole, double throw.

It's also worth mentioning the actuator. I've referred to that above as a button or confusingly as a lever or switch but the technical terms are paddle, pushbutton, rocker, slides, rotaries, dip rotaries and toggles.