Switches - What are Poles and Throws?
Switches are often described by the number of poles and throws they have, such as:
- SPST (single pole, single throw)
- SPDT (single pole, double throw)
- DPDT (double pole, double throw)
- 3PDT (3/triple pole, double throw)
- 4PDT (4/quadruple pole, double throw)
In switch terminology, the number of poles is the number of separate connection circuits the switch controls. The number of throws is the number of possible output connections that can be made. To demonstrate, we will take a look at various switch types.
This SPST switch has one throw, because there is only one possible output connection that can be made - through the red LED and current-limiting resistor (CLR) to ground. The switch is either connected, causing the LED to light up, or disconnected, causing an open circuit and the LED to turn off. The switch is considered to have one pole because it is only controlling one connection circuit, the one which determines where +9V will be routed.
A double throw switch has two possible output connections. While a SPST switch can either be connected or disconnected (LED on or off), a SPDT can either be connected to one output circuit (red LED on) or connected to the other (green LED on). This switch is still considered to have a single pole because it is still only controlling a single connection circuit which determines where +9V will be routed. Note that SPDT switches can be used like SPST switches if nothing is connected to either pin 1 or pin 3.
Double pole switches have two separate connection circuits. A DPDT switch is analogous to two SPDT switches that are controlled by the same mechanism. In this switch, each connection circuit has two possible output paths (double throw). Two circuits are being controlled, the routing of +9V and the routing of +5V (double pole). Toggling the switch in one direction turns on the red LED and the yellow LED. Toggling the switch in the other direction turns on the green LED and the blue LED.
Again, note that DPDT switches can be used like DPST switches if nothing is connected to either pins 1 and 4 or pins 3 and 6. This holds true for any number of poles. If the throws on one side are not connected to anything, the switch behaves like a single throw switch.
A more practical use of DPDT switches is routing signals in a guitar pedal. The below schematic diagram shows how a DPDT switch can be wired for true bypass.
Note how one pole is controlling the routing of the input, while the other controls the routing of the output. The input and output jack can either be connected directly to each other (effect off, as currently pictured in the schematic), or they can be sent to the respective "In" and "Out" of the effect board (effect on). Note: there are arguably better ways of wiring a DPDT switch for true bypass, but this is the simplest.
Perhaps more often, guitar pedals use a 3PDT switch. A 3PDT switch has one more pole which is dedicated to powering the LED. The LED is on when the effect is on, and it is off when the effect is off. By leaving nothing connected to switch pin 7, the LED pole is wired to function like a single throw switch as we only need the LED connection to be on or off.
There are other switch types available beyond what has been covered here. 4PDT switches are fairly common. Switches with 5 or more poles are rare, but do exist. Many switch types cannot have more than two throws due to the nature of their construction, and though rotary switches are usually discussed in terms of "positions" rather than "throws" (e.g. 1 pole, 12 position), they are a useful solution if the functionality of 3+ throws is needed.