What is the purpose of a relay in an electric circuit?

Answer 1

A electrical relay device is usually a small electro-mechanical switch which, when energized, will close a contact-set to complete another circuit.

A relay is used regularly by people who drive motor vehicles: the key switch (ignition switch) is turned to "Start" and 12 volts (approximately) is applied to the "starter solenoid" (which is actually a big relay). The solenoid's coil is energized - drawing only one to five amps or so from the battery - and it closes its high-current-carrying contacts so that the battery voltage is delivered to the starter motor, which usually draws around 100 amps or more...

If you didn't have that starter solenoid to do the job for you, to turn on that kind of current using only your ignition key you'd have to turn a very large key to turn a very big, heavy - and ugly! - switch on your steering column...

There are variations on this theme to which the term "relay" can be applied, but the idea remains the same: a small switch of some kind controls another (usually higher voltage and/or current) circuit.

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A relay is a switch as the previous contributor described; however, it does not necessarily control a higher voltage circuit. Relays are also commonly used in low power applications to switch a signal to one or more circuits. The common terminal of a relay is typically used as the input, and the throw describes the number of switches that are connected ("thrown") at the same time.

A relay is controlled by a solenoid. When the proper voltage is applied across the solenoid, the relay energizes, which activates the throw and the common terminals are connected to the Normally Closed (N/C) terminals. When no voltage is applied across the solenoid, the relay is unenergized and the common terminals are connected to the Normally Open (N/O) terminals of the relay.

Some common relays are:

SPST: Single pole single throw - has a single common and single output. The unenergized position is N/O because the signal at the common terminal is not present at the output until the relay is energized. Once the relay is energized, the input signal at the common terminal is connected to the output at the N/C terminal. This type of relay is used to connect a single common terminal to a single output terminal.

SPDT: Single pole double throw - has a single common and two outputs. In the unenergized position, the common terminal is connected to the N/O terminal. Once the relay is energized, the signal at the common terminal is connected to the N/C terminal. This type of relay is used to connect a single common terminal to one of two output terminals.

DPST: Double pole single throw - has two common inputs and two outputs. This type of relay is electrically equivalent to two SPST relays that operate simultaneously. In the unenergized state, the two common terminals (C1 and C2) are not present at either output terminal. In the energized state, C1 and C2 are switched to their respective output terminal at the same time.

DPDT: Double pole double throw - has two common inputs and four outputs. This type of relay is electrically equivalent to two SPDT relays that operate simultaneously. In the unenergized state, the two common terminals (C1 and C2) are present at the NO terminals (NO1 and NO2) of the relay. When the relay is energized, C1 and C2 are switched to the N/C terminals (NC1 and NC2) at the same time.