SCR IN ELECTRONICS
SCR In Electronics SCR is a three terminal (anode, cathode and gate), three junction and four layer semiconductor switching device.
Principle of operation
In the normal operating conditions, a positive voltage is applied to the anode (A) and a small positive voltage is applied to the gate (G) with respect to the cathode (K)
When the gate is kept open Vg — 0, the SCR is similar to a PNPN diode. The junctions J. and 3 operate in forward bias and the junction J, operates in reverse bias. So no current flows through the SCR. Therefore the SCR is in OFF state. The SCR now offers high resistance.When the anode voltage is gradually increased, the junction J, attains breakdown at a particular voltage.
Now the SCR conducts heavily going to ‘ON’ state. The anode voltage, at which the SCR conducts heavily (ON state) without gate voltage is called Breakover voltage.
‘When a small positive voltage is applied to the gate, the junction Jz is in forward biased and the junction J, is in reverse biased by this voltage.
Now the electrons move from N type layer (cathode) to P type layer, through the junction Jz. The electrons in junction J, are also attracted by junction J„ and gate current starts flowing.
The holes also move from P layer (G) to N‑layer (K) which in turn increases the anode current, and makes more electrons available at junction J, This cumulative process make to breakdown the junction J in a short time. Now the SCR conducts heavily.
once an SCR is turned ON the gate loses its control. Even if the gate voltage is removed the SCR does not go to OFF state.
To turn the device OFF is only done by lowering the anode voltage and makes the current less than holding current
SCR AS A SWITCH
The SCR has only two states, namely ON s. and OFF state. There is no state in between the states. When appropriate gate current is passed, the son starts conducting heavily and remains in this sta definitely, even if gate voltage is removed.
This corresponds to ON condition.wins in this state inHowever, when the anode current is reduced to the holding current, the SCR turned OFF.
This behaviour of SCR is similar to a mechanical switch an SCR is an electronic device, therfore it is also called electronic switch.
When a sufficient positive voltage is applied to its gate terminal, the SCR conducts and anode current flows to a maximum level.
Now the SCR goes in to the ON state, worked as a closed switch. Its output voltage is low.When a proper negative voltage is applied to its gate terminal, the SCR goes into OFF state, Now no current flows through the SCR, worked as an open switch. It’s output voltage is high.
2)High speed operations