Ways of triggering SCR(Silicon Controlled Rectifier)

SCR (Silicon Controlled Rectifier) :

The silicon control rectifier is the oldest and most popular thyristor. It is an extremely reliable device and can be expected to deliver billions of operations before failure. The SCR (Silicon Controlled Rectifier) is built in one PNPand one NPN transistor.The basic structure of SCR is below.

The Transistor structure of a SCR

The equivalent two transistor structure is below

SCR volt-ampere characteristics curve

The SCR (Silicon Controlled Rectifier) Switch Turned on by five ways :
By avalanche: When the anode is made much more positive then the cathode, Forward break over occurs and latch the device on.
By rate of change: If the forward bias voltage across the device increases very quickly, a current will flow to charge the collector-base capacitance of the PNP transistor. This charging current represent base for theNPN transistor and turns it on
By high temperature: Reversed-biased silicon junction show a leakage current that approximately doubles for every 8° C temperature rise.At some temperature, the leakage current will reach a level that latches theSCR on.
By transistor action: This is the normal mode of operation for all but light-sensitive thyristors. An external gate pulse or signal is used to switch on the NPN transistor and latch the SCR.
By light Energy: Light entering the junction area will release electron-hole pairs and letch the SCR on.
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