# SCR Triggering

## SCR Triggering Circuit

Tags: Methods of Triggering of SCR, Working of RTriggering, Working of RCTriggering, Working of UJTTriggering, Theory of R, RC, UJT Triggering of SCR, Triggering of SCR,
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###### Introduction

Triggering of SCR by using three methods,

• R-Triggering
• RC-Triggering
• UJT-Triggering

###### R-Triggering

It includes one fixed resistor, variable resistor, diode, SCR (Silicon Controlled Rectifier), Load resistor. The circuit diagram of an R Triggering is shown below (Figure 1).Simple resistor; diode combinations trigger and control SCRs over the full 180 electrical degree ranges, performing well at commercial temperatures. These types of circuits operate most satisfactorily when SCRs have fairly strong gate sensitivities. Since in a scheme of this type a resistor must supply all of the gate drive required to turn on the SCR, the less sensitive the gate, the lower the resistance must be, and the greater the power rating.

Figure 1 shows a very simple variable resistance half-wave circuit. It provides phase retard from essential zero (SCR full “on”) to 90 electrical degrees of the anode voltage wave (SCR half “on”).Diode D1 blocks reverse gate voltage on the negative half-cycle of anode supply voltage.

Figure 1: Simple Half-wave variable Resistor Phase Control – Circuit Schematic

It is necessary to rate blocking to at least the peak value of the AC supply voltage and the trigger voltage producing the gate current to fire IGF are in phase. When EAC = Em, at the peak of the AC supply voltage, the SCR can still trigger with the maximum value of resistance between anode and gate.

Used for the phase control application such as converter circuits (half & full controlled).RC-Triggering

###### RC-Triggering

It includes variable resistor, two diodes, SCR (Silicon Controlled Rectifier), Capacitor, Load resistor.

The circuit diagram of an RC Triggering is shown below (Figure 2). Figure 2 shows an R-C-Diode circuit giving full half-cycle control (180 electrical degrees).

On the positive half-cycle of SCR anode voltage the capacitor charges to the trigger point of the SCR in a time determined by the RC time constant and the rising anode voltage. The top plate of the capacitor charges to the peak of the negative voltage cycle through diode D2 on the negative half-cycle, resetting it for the next charging cycle.

Figure 2: Simple Half-wave RC Phase Control – Circuit Schematic

###### Working of Capacitor

During negative half cycle capacitor charges in reverse direction when the supply voltage increases towards positive side the capacitor voltage also recharges in opposite direction. When this capacitor voltage reaches threshold voltage SCR will turn on and capacitor discharges through diode D2 and its voltage become very small positive voltage.

Firing angle can be varied from 0 to 180 degree.

###### UJT-Triggering

It includes UJT triggering, SCR (Silicon Controlled Rectifier), Load resistor.

A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance.

###### Working of Capacitor

Initially the capacitor charges through R whose voltage is applied to the emitter of UJT.

When the capacitor voltage reaches peak point voltage of UJT. The UJT will switch to on condition. Now the capacitor discharges through the output resistance. Thus the pulse is generated in the circuit.