EDC | SCRs | 2Marks

Latching current (IL):
During turn-on, the minimum value of anode current that must be attained so that the thyristor wil
l remain in conduction state even if gate signal is removed.

Holding current (IH):
When the gate is open, a certain value of anode current below which SCR will switch to OFF state is called holding current.
Methods to turn ON SCRs:
  1. Gate triggering - SCR can be triggered into conduction mode by gate pulse signal.
  2. Forward-breakover Voltage (VFBO) - When voltage across SCR (VAK) exceeds forward-breakover voltage (VFBO), SCR turns ON.
Methods to turn OFF SCRs:
  1. Reverse biasing anode-cathode terminals of SCR using external supply, called forced commutation.
  2. Interrupting or reducing the current through the SCR (Anode current, IA) below holding current
List of SCR ratings:
  1. Forward-breakover voltage, VFBO
  2. Holding current, IH
  3. Gate trigger current, IG
  4. Average forward current, IF(avg)
  5. Reverse-breakdown voltage, VRBO
List of SCR characteristics:
  1. Forward-blocking region
  2. Reverse-blocking region
  3. Forward-conduction region
Advantages of SCR as a switch over mechanical switch:
  1. Noiseless
  2. High switching speed
  3. Control over current with small gate current
  4. No mechanical movable contact, increasing life
Some applications of SCR:
  1. Speed control of motor, dimming of lights
  2. Power control
  3. Half-wave and full-wave rectifiers with current and power control 
Shockley Diode Vs SCR Vs TRIAC Vs DIAC

S. No.
Shockley Diode
SCR
TRIAC
DIAC
1.       
Uni-directional device
Uni-directional device
Bi-directional device
Bi-directional device
2.       
No gate terminal
Gate terminal is present
Gate terminal is present
No gate terminal
3.       
Basic 4 layer device
Shockley diode with gate terminal
Two SCRs in Inverse-parallel connection
Two Shockley diodes in Inverse-parallel connection
(OR)
Low current TRIAC without gate terminal
4.       
Terminals:
Anode and Cathode
Anode, Cathode and Gate
Main terminal1, Main terminal2 and Gate
Main terminal1 and Main terminal2
5.       
Break-over voltage cannot be controlled
Break-over voltage can be controlled with gate signal
Break-over voltage can be controlled with gate signal
Break-over voltage cannot be controlled
 

 

Disclaimer:
The above article doesn't give you any guarantee and the sole purpose of this article is to share my learning in the way I understood. Any comments to refine this article are welcome with great pleasure. Please report any breaking link by commenting below.

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