R Triggering Circuit

R Triggering Circuit

Tags: R Triggering Circuit, Circuit for R Triggering, Experiment for R Triggering, Resistor Based triggering for SCR, R Triggering Output using Lamp Load, Half wave Phase Control,
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R Triggering Circuit

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 range, 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: Simple Half-wave variable Resistor Phase Control – Circuit Schematic

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. 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).

Lab Experiment


To Study and verify R Triggering circuit of SCR.

Apparatus required

  • R Triggering circuit kit
  • Patch cards
  • Power card
  • CRO
  • 60W Bulb

Circuit Diagram


CRO Settings

  • Time (X) Axis : 5ms/div
  • Voltage (Y) Axis : 20V/div
  • Probe : 1:10 CRO Probe is suitable one.

Initial Settings

  • Ensure the 230V Power Supply is in proper with Tester.
  • Ensure the CRO is working properly with probe checking and proper ground line axis.

Mimic Diagram for R Triggering

Experiment Steps

  • The connections are made as per the circuit diagram given above.
  • The R Load must be 60W bulb.
  • Turn ON SW1 and observe the waveform across the R load.
  • Measure the X-axis time interval of ON time of Load Voltage and OFF Time of Load Voltage.
  • Adjust the value of Firing angle and note down the load voltage
  • Repeat the step 4.
  • Repeat the experiment for different values of α and note down Vo.




Off time (ms)

Alpha(α in Degree)






















Model Graph

Total Time Period

The Average Output Voltage


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