Conduction angle and Firing delay angle of SCRs Figure 14 Figure 14 shows the

# Conduction angle and firing delay angle of scrs

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Conduction angle and Firing delay angle of SCRs Figure 14 Figure 14 shows the load waveforms and SCR voltage waveforms, which is set so that the SCR fires when the input when the input signal reaches 45°.
Conduction angle and Firing delay angle of SCRs Figure 15 Figure 15 shows the load waveforms and SCR voltage waveforms, which is set so that the SCR fires when the input signal is near 0°.
Conduction angle and Firing delay angle of SCRs Figure 16 Shows the Typical Gate Control Circuit Lower value of R 2 Higher value of R 2 I GT amount of gate current needed to fire a particular SCR (0.1 50 mA) ig instantaneous value of gate current R 1 protect gate from overcurrents. R 2 used to adjust firing delay angle. 0 O < Firing Delay Angle < 90 O
Sample Problems 1. For the circuit shown, what voltage is required at point X to fire the SCR? The gate current needed to fire a 2N3669 is 20mA under normal conditions. 2. For a typical gate control circuit, assume the supply is 115Vrms, IGT= 15mA, and R1= 3k. The firing delay is desired to be 90O. To what value should R2be adjusted? 3. If the resistance of the load is 40 and the supply is 115 Vrms, how much average power is burned in the SCR when the firing delay angle is 90 O ? Assume that the forward voltage across the SCR is constant at1.5V when it is turned ON and that reverse leakage current through the SCR is so small as to be negligible. Figure 17
Conduction angle and Firing delay angle of SCRs Capacitors Used to Delay Firing The purpose of the capacitor is to delay firing past 90 O . When ac supply is negative, the reverse voltage across the SCR is applied to the RC triggering circuit, charging the capacitor negative on top of the plate. When the supply enters its positive half cycle, the forward voltage across the SCR tends to charge C in the opposite direction, however, voltage buildup in the new direction is delayed until the negative charge is removed from the capacitor. Figure 18
Conduction angle and Firing delay angle of SCRs Double RC Network for Gate Control Fixed resistors R 1 and R 3 sets the minimum firing delay angle. R 2 is used to control firing delay angle. For 60 Hz supply, the time constant of RC circuit must fall in the range of 1 30mS. 1mS < (R 1 +R 2 )C < 30mS 1mS < (R 1 +R 2 )C 1 and R 3 C 2 < 30mS Figure 19
Conduction angle and Firing delay angle of SCRs Double RC Network for Gate Control Example: Suppose that it has been decided to use C 1 = 0.068 F and C 2 = 0.033 F in the double RC gate control circuit, approximate the sizes of R 1 , R 2 , and R 3 to give a wide range of firing adjustment. Answer: R 1 = 2.94 Kohms, R 2 = 340 Kohms, and R 3 = 150 Kohms Figure 20
Sample Problems Example: Suppose that it has been decided to use C 1 = 0.068 F and C 2 = 0.033 F in the double RC gate control circuit, approximate the sizes of R 1 , R 2 , and R 3 to give a wide range of firing adjustment.

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• Fall '15
• Engr.Vergara
• Cathode, SCR, SCRs

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