SCR Application Medium

Firing Angle and Conduction Angle

• Firing angle (θf) is the phase angle at which the SCR turns on.
• Conduction angle (θc) is the angle between the start and end of conduction.
• Example: θf = 90°, θc = 180° - 90° = 90°.

Half-Wave Power Control

• A common application of SCRs is in the control of ac power for lamp dimmers, electric heaters, and electric motors.
• A half-wave, variable-resistance, phase-control circuit is used to control the power.
• By adjusting R2, the SCR can be made to trigger at any point on the positive half-cycle of the ac waveform between 0° and 90°.

Full-Wave Power Control

• Similar to half-wave power control, but either one of the SCRs are conducting in either positive or negative cycle of the input signal.
• Average Voltage: VAVE = Vm / π (1 + cosθf)
• Average Current: IAVE = VAVE / RL

SCR Applications

• On-Off Control of Current: SCR circuit that permits current to be switched to a load by the momentary closure of switch SW1 and removed from the load by the momentary closure of switch SW2.
• Half-Wave Power Control: used in lamp dimmers, electric heaters, and electric motors.
• Full-Wave Power Control: used in similar applications to half-wave power control.
• Crowbar: used to protect a load from overvoltage.

SCR Crowbar

• A power supply of VCC is applied to a protected load.
• Under normal conditions, VCC is less than the breakdown voltage of the zener diode.

Silicon-Controlled Switch (SCS)

• Similar to SCR.
• Has two gate terminals, cathode and anode gate.
• Four-terminal thyristor used to trigger the device ON and OFF.
• Faster turn-off time than SCR.
• Used in counters, registers, and timing circuits.

How to turn on SCS

• A positive pulse applied on the cathode gate or a negative pulse applied on the anode gate.

How to turn off SCS

• A positive pulse applied on the anode gate or a negative pulse applied on the cathode gate.
• Reducing the anode current below the holding current by using BJT as a switch to interrupt anode current.

DIAC

• Two-terminal four-layer thyristor which conducts in EITHER direction when properly activated.

Quiz on AC circuits, covering firing angle, conduction angle, and average current calculations. Topics include trigonometry and electrical engineering principles.