Silicon Controlled Rectifier

The Silicon-Controlled Rectifier (SCR) can be thought of as an ordinary rectifier with a control element. The current to the control element, which is termed the gate, determines the anode-to-cathode voltage at which the device commences to conduct. The gate bias may keep the device off, or it may permit conduction to commence at any desired point in the forward half-cycle of a sinusoidal input. The SCR is widely applied as an ac power control device. Many other devices, such as the DIAC, TRIAC etc., are based on the SCR Principle. Collectively, SCR-type device are known as thyristors. This term is derived from thyratron and transistors, the thyratron being a gas-filled to be which behaves like an SCR. The operation of a Unijunction Transistor (UJT) is quite different from that of bipolar and field effect transistors, although it is a three-terminal device. The device input, called the emitter has a resistance which rapidly decreases when the input voltage reaches a certain level. This effect, termed a negative resistance characteristics, make the UJT useful in timing and oscillator circuits.

Silicon-controlled rectifiers are a part of the thyristor type devices. Thyristors are considered to be digital devices because they are either on or off. Just like a manual light switch. There is no in between. They are especially useful in circuits that an elec­tronic device is used to control a large amount of electric power. When an SCR has been installed in a circuit and is in the off position there will be a voltage which is equal to the supply voltage indicated between the anode and the cath­ode. There is no current flow through the circuit at this time. When the SCR is turned on, there should be approxi­mately one volt indicated between the anode and the cath­ode of the SCR. The resistor in the circuit limits the current flow to about 2.4 amps (240 volts t- 100 Ω = 2.4 amps). The SCR now has 2.4 amps of current flowing at one volt. Therefore it must dissipate 2.4 watts of heat (1 volt x 2.4 amps = 2.4 watts).

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• What is Silicon controlled rectifier?