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