Circuit Analysis For Dummies

Most electronic circuits contain an on/off switch. In addition to the on/off switch, many circuits contain switches that control how the circuit works or activate different features of the circuit.

One way to classify switches is by the connections they make. If you were under the impression that switches simply turn circuits on and off, guess again. Two important factors that determine what types of connections a switch makes are

• Poles: A switch pole refers to the number of separate circuits that the switch controls. A single-pole switch controls just one circuit. A double-pole switch controls two separate circuits.

A double-pole switch is like two separate single-pole switches that are mechanically operated by the same lever, knob, or button.

• Throw: The number of throws indicates how many different output connections each switch pole can connect its input to. The two most common types are single-throw and double-throw:

• A single-throw switch is a simple on/off switch that connects or disconnects two terminals. When the switch is closed, the two terminals are connected and current flows between them. When the switch is opened, the terminals are not connected, so current does not flow.

• A double-throw switch connects an input terminal to one of two output terminals. Thus, a double-pole switch has three terminals. One of the terminals is called the common terminal. The other two terminals are often referred to as A and B.

When the switch is in one position, the common terminal is connected to the A terminal, so current flows from the common terminal to the A terminal but no current flows to the B terminal. When the switch is moved to its other position, the terminal connections are reversed: current flows from the common terminal to the B terminal, but no current flows though the A terminal.

Switches vary in both the number of poles and the number of throws. Most switches have one or two poles and one or two throws. This leads to four common combinations:

 SPST (single pole, single throw): A basic on/off switch that turns a single circuit on or off. An SPST switch has two terminals: one for the input and one for the output. SPDT (single pole, double throw): An SPDT switch routes one input circuit to one of two output circuits. This type of switch is sometimes called an A/B switch because it lets you choose between two circuits, called A and B. An SPDT switch has three terminals: one for the input and two for the A and B outputs. DPST (double pole, single throw): A DPST switch turns two circuits on or off. A DPST switch has four terminals: two inputs and two outputs. DPDT (double pole, double throw): A DPDT switch routes two separate circuits, connecting each of two inputs to one of two outputs. A DPDT switch has six terminals: two for the inputs, two for the A outputs, and two for the B outputs.

Here are a few other points to ponder concerning the arrangement of poles and throws:

• Switches with more than two poles or more than two throws are not commonplace, but they do exist. Rotary switches lend themselves especially well to having many throws. For example, the rotary switch in a multimeter typically has 16 or more throws, one for each range of measurement the meter can make.

• A common variation of a double throw switch is to have a middle position that does not connect to either output. Often called center open, this type of switch has three positions, but only two throws. For example, an SPDT center open switch can switch one input between either of two outputs, but in its center position, neither output is connected.