Switches & Relays

Switches and relays provide a means of connecting and disconnecting power and signals to an electric circuit. A switch is operated by a lever or button and must be operated either manually or by some piece of mechanics, such as a limit stop in a machine control. A relay is simply a switch where the operating lever has been replaced by an electro-magnet.

Circuit Symbols

 

All switches and relays have contact ratings that must be observed. These specify the maximum (and sometimes minimum) operating voltage and current that the switch can handle. Some contacts have more than one set of ratings that specify the acceptable limits under differing operating conditions. A relay has an additional rating for the coil that sets out the range of voltages than will give reliable operation. Some relay coils are specially designed to be driven by an AC source at low frequency (typically 50 60 Hz).

When a switch or relay has multiple sets of contacts they are represented on a circuit diagram either by joining the contacts with a dotted line (as shown for SW4 in the above diagram) or by labelling each set of contacts with a differing suffix (SW6:A, SW6:B). There are a number of ways of describing the arrangement of contacts within any given switch or relay. Generally each individual set of contacts is referred to as a pole (which also means the common contact, or wiper in the set). Each pole can connect to a number of different positions referred to as ways or throws. Common terms are as follows:

 

Contact

Arrangement

Description

Example

SPST

Single Pole Single Throw

On-Off operation

SW3

SPDT

Single Pole Double Throw

A single circuit change over or one of two selector

 

SPCO

Single Pole Change Over

As SPDT  

DPDT

Double Pole Double Throw

A two circuit change over switch. Can be used as a simple reversing switch for a DC motor

SW4

DPCO

Double Pole Change Over

As DPDT SW4

3PCO

3 Pole Change Over

Normally only found in relays allows two way selection of up to 3 circuits

 

1P12W

1 Pole 12 Way

Usually a rotary switch allowing selection of up to 12 circuits

 

2P6W

2 Pole 6 Way

Usually a rotary switch that allows simultaneous selection of up to 6 signals each for 2 independent circuits

 

NO

Normally Open

A push to make switch such as a bell push.

For a relay, the contact connected to the common contact when the coil is de-energised.

SW1

RL1

NC

Normally Closed

A push button switch that has to be operated to break a circuit.

For a relay, the contact that only connects to the common when the coil is energised.

SW2

 

Sample Circuits


Door bell


A typical light circuit


A 2 way light circuit


A latching relay with on and off push buttons

 


Safety Note
When wiring a change over contact as a normally open or a normally closed contact always connect the incoming signal or power to the selected way and the out going signal or load to the common contact. This prevents the unused contact position from becoming live when the circuit is switched off.
 
 
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