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SPDT Single pole double throw

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A Single Pole Double Throw (SPDT) switch is an electronic component commonly used in electrical circuits to control the flow of current. It has three terminals: one common terminal (COM) and two other terminals, typically referred to as the Normally Open (NO) and Normally Closed (NC) terminals.

Let's break down the terminology:

  1. Pole: The pole refers to the common terminal (COM), which is the central terminal of the switch. It acts as the input/output point for the electrical current.
  2. Throw: The throw refers to the two positions the switch can be in. In the case of an SPDT switch, it has two throws, meaning it can be in two different states or positions.

Now, let's understand the two throws:

  1. Normally Open (NO): This terminal is connected to the common terminal (COM) when the switch is in one of its positions. It is called "normally open" because it is open (not connected) when the switch is at rest or in its default position.
  2. Normally Closed (NC): This terminal is connected to the common terminal (COM) when the switch is in the other position. It is called "normally closed" because it is closed (connected) when the switch is at rest or in its default position.

To better visualize the operation of an SPDT switch, consider the following scenarios:

Switch in the NO position:

  • The common terminal (COM) is connected to the Normally Open (NO) terminal.
  • Current can flow between the common terminal (COM) and the Normally Open (NO) terminal, creating a path for the current.

Switch in the NC position:

  • The common terminal (COM) is connected to the Normally Closed (NC) terminal.
  • Current can flow between the common terminal (COM) and the Normally Closed (NC) terminal, creating a path for the current.

By toggling the switch, you can change the position from NO to NC or vice versa, altering the connectivity between the common terminal (COM) and the other two terminals.

SPDT switches find applications in various electrical circuits, such as:

  1. Simple on/off control: It can be used to switch between two different circuits or devices.
  2. Changeover switches: They are often used for selecting between two power sources, like switching between the main power supply and a backup power source.
  3. Relay control: SPDT switches can be used in conjunction with relays to control the switching of larger currents or voltages.

Overall, the SPDT switch provides flexibility in controlling the flow of electrical current, allowing for the selection between two different states or paths within a circuit.