Q1) a) Explain with neat circuit diagram operation of R-C triggering circuit of Thyristor [5 M]

**Ans. **

- The circuit diagram for R-C full wave triggering is shown below. :-

- It consists of four diodes D
_{1}, D_{2}, D_{3}, D_{4}which convert Ac into DC.

- This pulsating DC is supplied to SCR.

- When SCR is off, the capacitor start charging from 0v. When it is charged to V
_{gt,}SCR triggers and rectified voltage Vs appears across the load as Vo.

- The load impedance can have various values , therefore for paticular value of R, the instant of firing is determied by load impedance.

- For fixed load impedance , low values of R causes quick charging and early firing whereas high value of R causes delayed charging and delayed firing .

The waveforms are shown below.

- The merits are –

i) It is very simple and economical.

ii) The firing angle is controlled in the wide range from 0^{o} to 180^{o}.

- The demerits are –

i) Control circuit is directly connected to high voltage power circuit.

ii) The performance is different for different SCRs.

The value of RC is

RC=\frac{157}{W}- At the instant of triggering , if capacitor voltage is constant at V
_{gt}and supplying a current to gate , the triggering gate current I_{gt}has to be supplied by Vs through R.

V_{s} > R. I_{gt} + V_{gt }

Q.1. b) Write short note on Class E Chopper. [05 M]

**Ans. **

Four-quadrant chopper is nothing but class-E chopper, which can be formed by combining two class C choppers.

**First quadrant Operation**

When chopper S_{1} and S_{4} are ON, input dc source voltage Vs appears across load and dc source takes load current i_{L}. Thus both V_{L} and i_{L} are positive . So chopper is said to be operated as rectifier and power flows from dc source to load. When S1is turned off, the load inductor voltage reverses itself and drives i_{L} in same direction through S_{4} and D_{2}. The load voltage V_{L} becomes zero.

**Second quadrant operation**

When D_{1} and D_{4} are on, load inductance voltage VL drives current through D_{1}, V_{s }and D_{4}. Now the load voltage VL is positive which is equal to Vs but the load current i_{L} is negative. So the power is fed back by load inductor to the source . Therefore the chopper acts as an inverter.

When S_{2} is fired , D_{1} is turned off and negative load current i_{L} is maintained through S_{2}, D_{4 }and load . Now VL becomes zero.

**Third Quadrant operation**

When S_{2} , S_{3} are on, V_{L}=-Vs and negative load current flows, the power is applied by the source to the load so it is operated as rectifier. If S_{3} is off, the load current flows through S_{2}, D_{4} and load. Now V_{L} becomes zero.

- Fourth Quadrant operation

When diodes D_{2} and D_{3} are on, the energy stored in load inductor is given out and positive load current i_{L} is maintai9ned. The load voltage V_{L}=-Vs. The power is fed back to source by load inductor hence it is operated as inverter.

When S_{4} is fired , D_{3} is oFF and iL starts flowing through S_{4}, D_{4} and load. The load voltage becomes zero.

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