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Questions and Answers
How does the firing angle of an SCR affect the average voltage delivered to a load in a half-wave power control circuit?
In a full-wave power control circuit, how many thyristors are typically used?
What happens to the SCR when the ac input goes negative in a half-wave power control circuit?
What is the advantage of using an SCR in a half-wave power control circuit?
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How does the conduction angle of an SCR affect the average current delivered to a load in a half-wave power control circuit?
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What is the main advantage of SCS over SCR?
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What is the main characteristic of a DIAC?
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In a full-wave power control circuit, what is the range of the firing angle θf?
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What is the purpose of controlling the firing angle in a full-wave power control circuit?
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What is the significance of the extinction angle θe in a full-wave power control circuit?
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Study Notes
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.
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Description
Quiz on AC circuits, covering firing angle, conduction angle, and average current calculations. Topics include trigonometry and electrical engineering principles.
