--- title: "Pulse train generation for thyristor firing" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:pulse-train-gate-firing" source: https://www.bctengineer.com/q/semester_7/power_electronics/pulse-train-gate-firing site: BCT Engineering --- # Pulse train generation for thyristor firing Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2072 Kartik, 8 marks - Video: [Thyristor Firing Pulse](https://www.youtube.com/watch?v=nEvul-QAbKA) — Sitangshu Sekhar Biswas - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/pulse-train-gate-firing - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the operation of pulse train generation for gate firing circuit for thyristor showing all the necessary components. ## Short answer A zero-crossing detector and ramp generator produce a synchronised gating window from α to 180° compared with a control voltage. A 555 astable oscillator generates 5–10 kHz pulses, ANDed with the window. The result drives a pulse transformer via a transistor, providing isolated gate pulses. This gives reliable firing with inductive loads, low gate dissipation, and allows a small transformer. ## Answer **Pulse train firing** applies a burst of high-frequency, short pulses to the thyristor gate for the whole interval in which it should conduct (from $\alpha$ to about $180^\circ$), instead of a single long pulse. It gives reliable firing with inductive loads and needs only a small pulse transformer. #### Circuit (block form) ```text +-----------+ +------------+ | zero-cross|-->| ramp & |-- gating signal | detector | | comparator | (alpha to 180 deg) +-----------+ +------------+ | ^ Vc (control) v +-----------+ +------+ | 555 timer |--- 5-10 kHz pulses->| AND | | astable | | gate | +-----------+ +--+---+ | +Vcc v | pulse transformer +-----------+ PT primary <------------| driver Q | | secondary --D--R--> G | (transistor) | --> K +-----------+ ``` #### Operation 1. **Synchronisation:** a step-down transformer and zero-crossing detector produce a signal at the start of each half cycle; it resets a ramp (sawtooth) generator. 2. **Firing angle:** a comparator compares the ramp with a d.c. control voltage $V_c$. When the ramp exceeds $V_c$ the comparator output goes high, at angle $\alpha$, and stays high until the end of the half cycle. Changing $V_c$ changes $\alpha$. 3. **High-frequency carrier:** a 555 astable oscillator produces a pulse train at about 5 to 10 kHz. 4. **AND gate:** combines the two, so carrier pulses appear only from $\alpha$ to $180^\circ$. 5. **Driver and pulse transformer:** a transistor amplifies the pulses and drives the pulse transformer. The secondary supplies isolated gate current through a diode (blocks negative pulses) and a current-limiting resistor. A freewheeling diode across the primary resets the transformer core. #### Waveforms ```text vs /\ /\ / \ / \ -----/----\----/----\----- gate ______ ______ window | | | ---------+ +---+ pulse |||||| |||||| train ---++++++-----++++++-- ^ alpha ``` #### Advantages - Reliable turn-on even if the anode current rises slowly (inductive loads) or the SCR is momentarily reverse biased. - Low gate dissipation because each pulse is short. - A small pulse transformer suffices, since it never saturates. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a train of short high-frequency pulses applied to the gate instead of one long pulse? A. It gives reliable firing with inductive loads and needs only a small pulse transformer B. It makes the SCR turn off faster C. It increases the gate dissipation so the SCR fires harder D. It removes the need for synchronisation with the supply Answer: A. Pulses last for the whole conduction window so the SCR latches even when anode current rises slowly, and the transformer never saturates so it can be small. (Key point: Why a pulse train) 2. How does the circuit change the firing angle alpha? A. By changing the 555 oscillator frequency B. By changing the d.c. control voltage Vc compared with the ramp C. By changing the turns ratio of the pulse transformer D. By changing the resistor in series with the gate Answer: B. The comparator output goes high when the ramp exceeds Vc, so varying Vc moves the instant alpha. (Key point: Setting the firing angle) 3. What is the function of the AND gate in the circuit? A. It resets the ramp generator at each zero crossing B. It amplifies the pulses to drive the transformer C. It lets the 5-10 kHz carrier pulses through only from alpha to 180 degrees D. It blocks negative pulses to the gate Answer: C. The AND gate combines the gating window from the comparator with the 555 pulse train so pulses appear only between alpha and 180 degrees. (Key point: Role of AND gate) 4. What is the purpose of the zero-crossing detector and step-down transformer? A. To generate the high-frequency carrier B. To isolate the gate from the cathode C. To set the pulse amplitude D. To mark the start of each half cycle and reset the ramp generator Answer: D. The zero-crossing signal synchronises firing to the supply by resetting the ramp every half cycle. 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