What is SPDT Switch : Working & Its Applications

Switches are the most essential devices in an electric field. The main function of a switch is to open or close an electrical circuit. Generally, the electric circuits change from simple make or simply break circuits to multi make or multi-break circuits. Although the logic is very simple, so the appropriate circuit must be selected for the application. There are different types of switches available in the market like SPST, DPST, SPDT & DPDT. So this article discusses one of the types of switches like SPDT switch & its working with examples.


What is SPDT Switch?

A switch that has a single input and two dissimilar outputs is known as SPDT. The term SPDT stands for Single Pole Double Throw Switch which is used to control two dissimilar circuits through a similar single input. The operating of this switch can be done manually or included through the electromagnetic coil.

The best example of the Single Pole Double Throw Switch is ACS 550 or 800 VFD’s output terminal relays. Here the configuration of the relay switch can be done like one input with two dissimilar outputs. The schematic symbol of the SPDT switch is shown below.

SPDT Switch Symbol
SPDT Switch Symbol

SPDT Switch Structure

The basic structure of this switch is shown below. This structure includes pole, throw, open and close.

SPDT Structure
SPDT Structure

Pole

The pole within the switch is the controller. Generally, we use a single pole or double pole to explain how many separate circuits are controlled through the switch.

Throw

PCBWay

When the circuit is controlled through the pole then it is called throw. Generally, we use single or double throw to explain how many different circuits will be controlled through a single Pole.

Open

If the pole is detached by a single throw, then the condition of this throw is known as an open or open state.

Close

If the pole is attached to a single throw, then the condition of this throw is known as the close or close state.

Connections

SPDT has three connections which are normally open, normally close & common.

Normally Open (NO)

If the pole is detached from the throw circuit by default then the circuit is called a NO circuit or N/O circuit. Similarly, the switch is known as the NO switch or N/O switch.

Normally Closed (NC)

If the pole is connected to the throw by default then it is called an NC circuit or normally closed circuit. Similarly, the switch is known as NC switch or N/C switch,

The Common

The common connection allows the flow of current to make the circuit either with the normally closed (NC) or the normally open (NO) contacts.

This switch includes five terminals like two inputs and three outputs where two terminals are connected to the loads & one terminal is connected to the common terminal.

If one of the outputs of this switch doesn’t connect to any circuit then the switch works like an SPST switch. Usually, this switch is used in some special applications like switching two dissimilar power supplies in reverse conditions.

How does SPDT Switch Works?

It is a simple ON/OFF switch, used to connect or disconnect two terminals. Once the switch is closed, then two terminals will be connected, so current supplies in between them. Similarly, once the switch is opened, then two terminals will not be connected, thus the current does not supply.

Types of SPDT Switch

The single pole double throw switch can be divided into two types based on the way of operating like BBM type switch (Break Before Make) & the MBB type switch (Make Before Break).

BBM Type Switch

In BBM SPDT switch, the pole is connected by default to the normally closed throw circuit & detached from the normally open throw circuit. Once the BBM switch is activated, first it will separate from the NC circuit & after that, it will connect to the normally open circuit.

MBB Type Switch

In the MBB SPDT switch, the pole is connected to the NC throw circuit by default & detached from the normally open throw circuit. Once the MBB switch is activated, first it will connect to the normally open circuit, after that it will detach from the normally closed circuit.

SPDT Switch Circuit Diagram

In many electronic circuits, the toggle switch plays an essential role. So this switch works like a simple ON/OFF switch within a circuit. Toggle switches are very common components within different electronic circuits.

We know that an SPDT toggle switch is a three-terminal device that includes single input & two outputs. Here from two outputs, one output is obtained from COM & A whereas the second output is obtained from COM & B but at a time one output only. This switch is mainly used in the three-way circuit to control an electrical appliance from two locations.

SPDT Switch Circuit
SPDT Switch Circuit

The SPDT switch diagram is shown below. The required components to make this circuit mainly include an SPDT toggle switch, a 12V battery, a motor, and a lamp. As per the circuit diagram shown, the connections are made. In this circuit, two loads are used one is a lamp and another one is a motor. So the operation of this toggle switch is, once the toggle switch is ON, initially the lamp will be ON & motor will be turned OFF.

After that, the motor will be turned ON and the lamp will be turned OFF. So, two loads in the circuit can be controlled using a single switch.

Advantages & Disadvantages

The advantages of the SPDT switch include the following.

  • It is less cost
  • It is reliable for most the applications
  • It can work with high voltages & currents.
  • High switching currents let the device control relays, pumps & drive motors.

The disadvantages of the SPDT switch include the following.

  • We cannot control two loads at a time.
  • Increased cost.

SPDT Switch Applications/Uses

The SPDT switch uses include the following.

  • This switch is used in a three-way circuit for controlling a load from two locations like from the top and bottom of a staircase.
  • This switch is used to control two different circuits through a similar single input.
  • Control Circuit
  • ABB/Siemens VFDs
  • PLC system output switches
  • Selector switches through a single common arrangement.

How do you use an SPDT switch?

This switch is used as an ON/OFF switch based on the circuit connection.

What is relay SPDT used for?

The relay SPDT is used for switching in between two circuits.

What’s the difference between a router and a switch?

A router connects several switches whereas a switch connects several devices to form a network.

Thus, this is all about an overview of the SPDT switch that has only a single input and two output terminals. This switch can serve different functions within a circuit based on its wiring like the ON/OFF switch. For instance, an SPDT switch can connect to create a standby mode and a ready mode within a printer. Here is a question for you, what is the SPST switch?

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