The Equivalent Circuit of a Silicon Unilateral Switch (SUS)

Introduction:
The Silicon Unilateral Switch (SUS) is a four-layered device that is commonly used in electronic circuits for switching applications. It is also known as a Silicon Controlled Rectifier (SCR) or Thyristor. The equivalent circuit of an SUS consists of a diode in anti-parallel with a Programmable Unijunction Transistor (PUT). This circuit configuration allows the SUS to function as a switch for both AC and DC applications.

Explanation:
To understand why the equivalent circuit of an SUS consists of a diode in anti-parallel with a PUT, let's first discuss the characteristics of these two components.

1. Diode:
A diode is a two-terminal device that allows current to flow in only one direction. It acts as a one-way valve for electric current. When the voltage across the diode is positive with respect to its anode, it allows current to flow through it. However, when the voltage is negative, the diode blocks the current flow.

2. Programmable Unijunction Transistor (PUT):
A Programmable Unijunction Transistor (PUT) is a three-terminal device that has a negative resistance characteristic. It consists of three layers of alternating P-type and N-type semiconductor material. The PUT behaves like a diode until a certain voltage threshold is reached, at which point it triggers a process called "conduction."

Now, let's consider the operation of an SUS:

- When a positive voltage is applied to the anode terminal of the SUS, the diode in anti-parallel allows current to flow through it, as the diode is forward-biased.

- The current flows through the diode and triggers the PUT. The PUT enters a state of conduction and allows a large current to flow through it.

- This conduction state is maintained even when the voltage across the PUT drops below the triggering level. The current continues to flow until the anode current drops below a certain level called the "holding current."

- To turn off the SUS, the current flowing through it must be reduced below the holding current. This can be achieved by reducing the anode current or applying a reverse voltage across the anode and cathode terminals.

Conclusion:
The equivalent circuit of an SUS consists of a diode in anti-parallel with a Programmable Unijunction Transistor (PUT). This configuration allows the SUS to function as a switch for both AC and DC applications. When a positive voltage is applied, the diode allows current to flow through it and triggers the PUT, which enters a state of conduction. The SUS remains in this state until the current drops below the holding current. Applying a reverse voltage or reducing the anode current can turn off the SUS.