# SWITCHES AND RELAYS

```UNIT 224: CONSTRUCTING TESTING AND FAULT FINDING ELECTRONIC CIRCUITS
HANDOUT 4: ELECTRONIC COMPONENTS – SWITCHES AND RELAYS
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LEARNING OUTCOMES
After studying this section, you should be able to:
1. State the function of a switch and a relays, and their circuit symbols.
2. Identify types of practical switches and relays and their circuit symbols.
3. Describe the construction of types of switches
4. Identify, test and troubleshoot faults in switches
5. Describe the application of types of switches
INTRODUCTION
An electrical switch is a device used to open or close an electrical circuit. Switches play an important
role in electronics – to stop or allow the flow of electric current. Every electrical and electronics
application uses at least one switch to perform ON and OFF operation of the device.
So the switches are the part of a control system and without it, control operation cannot be achieved. A
switch can perform two functions, namely fully ON (by closing its contacts) or fully OFF (by opening its
contacts).
When the contacts of a switch are closed, the switch creates the closed path for current flow and hence
load consumes the power from source. When the contacts of a switch are open, no power will be
consumed by the load as shown in below figure.
There are numerous switch applications found in wide variety fields such as home, automobiles,
industrial, military, aerospace and so on. In some applications multi way switching is employed (like
building wiring), in such cases two or more switches are interconnected to control an electrical load
from more than one location.
MECHANICAL SWITCHES
Mechanical switches can be classified into different types based on several factors such as method of
actuation (manual, limit and process switches), number of contacts (single contact and multi contact
switches), number of poles and throws (SPST, DPDT, SPDT, etc.), operation and construction (push
button, toggle, rotary, rocker, slide etc), based on state (momentary and locked switches), etc.
Based on the number of poles and throws, switches are classified into following types. The pole
represents the number of individual power circuits that can be switched. Most of the switches are
designed have one, two or three poles and are designated as single pole, double pole and triple pole.
The number of throws represents the number of states to which current can pass through the switch.
Most of the switches are designed to have either one or two throws which are designated as single
throw and double throw switches.
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(1) Single Pole Single Throw (SPST) Switch
A Single Pole Single Throw (SPST) switch is a switch that only has a single input and can connect only
to one output. This means it only has one input terminal and only one output terminal. A Single
Pole Single Throw switch serves in circuits as on-off switches. When the switch is closed, the circuit
is on. When the switch is open, the circuit is off. SPST switches are, thus, very simple in nature.
Below is an example of a circuit which utilizes a single pole single throw switch. When the SPST is
closed, the circuit is closed and light from the lamp switches on. When the SPST is then opened, the
light from the lamp goes out and the circuit is off.
Single Pole Single Throw (SPST) Switch Circuit
Summary
SPST switch is the basic ON and OFF switch consisting of one input contact and one output contact.
• It switches a single circuit and it can either make (ON) or break (OFF) the load.
• The contacts of SPST can be either normally open or normally closed configurations.
(2) Single Pole Double Throw (SPDT) switch
A Single Pole Double Throw (SPDT) switch is a switch that only has a single input and can connect to
and switch between 2 outputs. This means it has one input terminal and two output terminals.
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Below is an example of a circuit which utilizes a single pole double throw switch. The switch is wired
to operate the circuit in either one of two modes. When the switch is connected one way, the lamp
will turn on, while the LED is off. When connected the other way, the LED then turns on, and the
lamp shuts off.
Single Pole Double Throw (SPDT) Switch Circuit
Summary
This switch has three terminals, one is input contact and remaining two are output contacts.
• This means it consist two ON positions and one OFF position.
• In most of the circuits, these switches are used as changeover to connect the input between
two choices of outputs.
• The contact which is connected to the input by default is referred as normally closed contact
and contact which will be connected during ON operation is a normally open contact.
(3) Double Pole Single Throw (DPST)
A Double Pole Single Throw (DPST) switch is a switch that has 2 inputs and 2 outputs; each input has
1 corresponding output. Each of the terminals of a double pole single switch can either be in the on
position (closed) or in the off position (open).
Below is an example of a circuit which utilizes a double pole single throw switch. The DPST switch is
wired to operate the circuit in any 1 of 2 modes. When the switch is connected one way for circuit A
and circuit B, the lamp and LED will both be ON. When connected the other way, the lamp and the
LED are both OFF. So a DPST switch allows for control of 2 outputs, turning either both on or both
off together.
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Double Pole Single Throw Switch (DPST) Circuit
Summary
This switch consists of four terminals, two input contacts and two output contacts.
• It behaves like a two separate SPST configurations, operating at the same time.
• It has only one ON position, but it can actuate the two contacts simultaneously, such that each
input contact will be connected to its corresponding output contact.
• In OFF position both switches are at open state.
• This type of switches is used for controlling two different circuits at a time.
• Also, the contacts of this switch may be either normally open or normally closed configurations.
(4) Double Pole Double Throw (DPDT)
A Double Pole Double Throw (DPDT) switch is a switch that has 2 inputs and 4 outputs; each input has 2
outputs that it can connect to. Each of the terminals of a double pole double switch can either be in 1 of
2 positions. This makes the double pole double throw switch a very versatile switch. With 2 inputs, it can
connect to 4 different outputs. It can reroute a circuit into 2 different modes of operation.
Note: A Double Pole Double Throw Switch is actually two single pole double throw (SPDT) switches.
Double Pole Double Throw Switch (DPDT) Circuit
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DPDT Toggle switch
(5) Push Button Switch
This is a momentary contact switch that makes or breaks connection as long as pressure is applied
(or when the button is pushed).
Generally, this pressure is supplied by a button pressed by someone’s finger.
• This button returns its normal position, once the pressure is removed.
• The internal spring mechanism operates these two states (pressed and released) of a push
button.
• It consists of stationary and movable contacts, of which stationary contacts are connected in
series with the circuit to be switched while movable contacts are attached with a push button.
• Push buttons are majorly classified into normally open, normally closed and double acting push
buttons as shown in the above figure.
• Double acting push buttons are generally used for controlling two electrical circuits.
RELAYS
Introduction
A relay is an electrical switch that control (switch on &amp; off) a high voltage circuit using a low voltage
source. A relay completely isolates the low voltage circuit from the high voltage circuit.
Relay Circuit Symbols (coil and switch)
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Practical relay and symbol
Pin Configuration
Pin
Number
Pin Name
Description
1
Coil End 1
Used to trigger(On/Off) the Relay, Normally one end is connected to 12V
and the other end to ground
2
Coil End 2
Used to trigger(On/Off) the Relay, Normally one end is connected to 12V
and the other end to ground
3
Common
(COM)
Common is connected to one End of the Load that is to be controlled
Normally Close
(NC)
Normally Open
(NO)
The other end of the load is either connected to NO or NC. If connected to
NC the load remains connected before trigger
The other end of the load is either connected to NO or NC. If connected to
NO the load remains disconnected before trigger
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TYPES OF RELAYS BY CONSTRUCTION
General Purpose Electromechanical Relays
Mechanical relays are usually the largest and most rugged of all relays. For a typical mechanical relay, a
current sent through a coil magnet acts to pull a flexible, spring-loaded conductive plate from one
switch contact to another.
In general, mechanical relays are designed for high currents (typically 2A to 15A), but they have
relatively slow switching (typically 10ms to 100ms).
Practical Electromechanical relays
TYPES OF RELAYS BY POLES AND THROWS
Single Pole Single Throw (SPST) Relay
A Single Pole Single Throw Relay is a relay that has one input and one output terminal.
Internally, it is wired so it is connected as shown below:
Being that it only has one input and one output, they act as simple on-off switches in circuits, as they can
only take on 1 of 2 states. When the relay does not receive any power, it is off and the Normally Open
(NO) contact pin remains opens. When the relay receives sufficient power, the NO contact pin closes and
whatever load is connected to it will power on.
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Single Pole Double Throw Relay
A Single Pole Double Throw Relay is a relay that has one input and two outputs.
Internally, it is wired so it is connected as shown below:
Being that it has 2 outputs, it is more dynamic than a single throw relay. It can connect to 2 different
outputs, so it can switch a circuit in between any 1 or 2 states, such as ready mode-pause mode, etc.
Double Pole Single Throws (DPST)
This type of relay has two separate poles, that is 2 inputs and 2 outputs. When activated, both poles
connect to their respective terminals simultaneously.
Each of the inputs can connect to one ouput. A DPST relay is constructed internally as if they are 2
separate SPST relays connected together. So a DPST is really just 2 separate SPST relays.
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