Test: Firing Circuits - Electrical Engineering (EE) MCQ

A single phase semi-converter has only two SCRs & two diodes. Hence, only two pulses are required in each cycle, one in each half.

60° x 6(devices) = 360°.

 The driver circuit consists of a pulse amplifier to increase the magnitude of the gate pulse to a sufficient value. The pulse transformer then provides pulses to individual SCRs.

(P_gm) = (P_gav)/Duty Cycle.

Higher the temperature lesser will be the gate current required as the temperature must have already excited some of the atoms.

(1+10 Ig).Ig = 5 Watts
Ig = 0.59 A.

Es = Rs.Ig + Vg
Vg = 1+10 Ig
Therefore Rs = (15-1)/0.659.

(P_gm) = (P_gav)/Duty Cycle
Duty Cycle = f x T = (P_gav)/(P_gm)
Duty Cycle = 0.3/5
T = 20 μsec.
0.3/5 = f x T
f = (0.3)/(5 x 20 x 10^-6) = 3000 Hz.

The duty cycle is defined as the ratio of pulse-on period to periodic time of pulse.
The pulse on period is T, and the periodic time is 1/f.
It is to be noted that T = pulse width whereas f = (1/T1) = frequency of firing or pulse repetition rate.

Isolation of the two circuit is done by the transformer, as the transformer is a magnetically coupled device and any mishap at the load side will not damage the other side of the circuitry.

The R firing circuits cannot be used for alpha greater than 90 degrees.

R = 100/2 = 50 Ohm.

The diode is placed between the resistances and gate which ensures that the current flows in one direction only.

Unlike the R firing circuit, the RC firing circuits can be used to obtain firing angle greater than 180. Although practically 0 and 180 degree is improbable.

As R2 is the variable, R1 makes sure that the current does not exceed the maximum value when R2 is kept at zero position.

For turning on the device, the peak of gate voltage must be equal to the gate triggering voltage.

For the values of Vgp great than the gate triggering voltage the firing angle is less than 90°. And for V_gp = V_gt the firing angle is equal to 90°. Α cannot go beyond 90° in case of a R firing circuit.

The given circuit is a RC half-wave firing circuit.

The current flows through Vs+ – C – D2 – Load – Vs.

R = (Vs-Vgt-Vd)/Igt.

 The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.

Circuit is that of a half-wave RC firing circuit where natural commutation takes place at π due to the sinusoidal voltage source.

 The SCR is triggered at α therefore voltage appears across the load from ωt = α to π only as in case of a half wave firing circuit the SCR is naturally commuted at π.

 The SCR is not triggered(turned-on) until α, hence no current flows until α.

 RC = 157/ω, Where ω is the angular frequency.

R & RC produce prolonged pulses which increases the gate power dissipation. There is no RLC firing circuit.

The terminals are Emitter, Base1 & Base 2.

When the UJT is fired, C starts to discharge.

When the C starts to discharge through the resistance with time constant RC the UJT turns on & hence pulses are generated.

 T = 1/f = 1/2 kHz
R = T/C ln(1/1-η).