--- title: "Thyristor gate triggering circuit" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-gate-triggering-circuit" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-gate-triggering-circuit site: BCT Engineering --- # Thyristor gate triggering circuit Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2080 Bhadra, 8 marks - Video: [Resistor trigger circuit of SCR (Basics, Circuit, Working, Waveforms & Parameters) Explained](https://www.youtube.com/watch?v=RIPdA-QKB54) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-gate-triggering-circuit - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain gate triggering circuit of a Thyristor. ## Short answer Gate triggering circuits supply synchronized pulses to turn on the thyristor at the desired angle. Resistance triggering has limited range; RC triggering uses a capacitor charged through a variable resistor to extend the range near 180°. UJT relaxation oscillators provide isolated, sharp pulses via a pulse transformer for precise control. ## Answer A **gate triggering (firing) circuit** produces the gate pulses that turn ON a thyristor at the required instant (firing angle $\alpha$). It must give enough gate current and voltage, a suitable pulse width, synchronism with the supply, and isolation between the low-voltage control and the high-voltage power circuit. #### Requirements - Gate current and voltage above $I_{gt}$, $V_{gt}$ but within the gate power limit. - Pulse width long enough for the anode current to reach the latching current. - Firing angle adjustable over the needed range and synchronised with the a.c. supply. - No reverse gate voltage and no false pulses. #### 1. Resistance (R) triggering ```text ~ ----+---- load ----+ | | A R1 (fixed) SCR | | K R2 (var) --D-- G | | ~ ----+--------------+ ``` The gate gets a fraction of the supply voltage. When it reaches $V_{gt}$ the SCR fires. Firing angle range is only $0^\circ$ to $90^\circ$; simple but sensitive to temperature and device spread. #### 2. RC triggering **Resistance-capacitance (RC) firing circuit (half wave):** ```text +------- load -------+ | | ~ Vs A | | SCR | R D2 |K +---/\/\/\-----|>|-- G | (variable) | | D1 C | +---|<|---+---||-----+ ``` 1. In the negative half cycle the capacitor $C$ charges through $D_1$ to $-V_m$ (upper plate negative). 2. In the positive half cycle $C$ charges positively through the variable resistor $R$. 3. When the capacitor voltage reaches the gate trigger voltage $V_{gt}$ plus the drop of $D_2$, gate current flows and the SCR fires. 4. Increasing $R$ slows the charging, so firing is delayed. The firing angle can be varied from about $0^\circ$ to $180^\circ$ (an R-only circuit gives only $0^\circ$ to $90^\circ$). $D_2$ blocks reverse gate voltage. #### 3. UJT triggering **UJT relaxation oscillator firing circuit:** ```text +Vbb ----+-----------+ | | R R2 | | +------- E B2 | UJT C B1 | | GND -----+ R1 ---> pulse to gate | (via pulse GND -----------------+ transformer) ``` 1. The capacitor $C$ charges through $R$ from $V_{BB}$ with time constant $RC$. 2. When the capacitor voltage reaches the UJT peak point voltage $V_P = \eta V_{BB} + V_D$ ($\eta$ = intrinsic stand-off ratio), the emitter-$B_1$ path becomes conducting and $C$ discharges quickly through $R_1$. 3. The discharge produces a sharp voltage pulse across $R_1$, which is passed to the SCR gate through a pulse transformer. 4. When the capacitor voltage falls to the valley voltage, the UJT turns OFF and the cycle repeats. The pulse period is $T = RC \ln\frac{1}{1-\eta}$. 5. For a.c. circuits, $V_{BB}$ is obtained from the same supply through a rectifier and a Zener clamp, so the first pulse of every half cycle is synchronised with the supply; changing $R$ changes the firing angle. The output pulses are given to the gate through a **pulse transformer** or **opto-coupler**, which provides isolation; a diode blocks negative gate voltage and a resistor across gate-cathode prevents noise triggering. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must the gate pulse be wide enough in a thyristor firing circuit? A. So the anode current can reach the latching current before the pulse ends B. So the gate voltage exceeds the breakover voltage C. So the SCR turns OFF at the end of the pulse D. So the gate power limit is exceeded for reliable firing Answer: A. If the pulse ends before the anode current reaches the latching current, the SCR returns to the OFF state. (Key point: Gate pulse width) 2. What is the firing angle range of a simple resistance (R) triggering circuit, and how does RC triggering compare? A. R: 0 to 180 degrees; RC: 0 to 90 degrees B. R: 0 to 90 degrees; RC: about 0 to 180 degrees C. Both give 0 to 90 degrees D. R: 90 to 180 degrees; RC: 0 to 90 degrees Answer: B. R triggering gives only 0 to 90 degrees; the capacitor phase lag in RC triggering extends the range to about 0 to 180 degrees. (Key point: R triggering range) 3. In the RC half-wave firing circuit, what happens when the variable resistor R is increased? A. The capacitor charges faster and the firing angle decreases B. The capacitor charges slower, so firing is delayed and the firing angle increases C. D2 conducts in the reverse direction D. The SCR fires only in the negative half cycle Answer: B. A larger R slows the charging of C, so the gate trigger voltage is reached later in the cycle and firing is delayed. (Key point: RC triggering delay) 4. In the UJT relaxation oscillator, when does the UJT emitter-B1 path start conducting and produce the gate pulse? A. When the capacitor voltage falls to the valley voltage B. When the capacitor voltage reaches the peak point voltage VP = eta*VBB + VD C. When the supply voltage crosses zero D. When the pulse transformer saturates Answer: B. C charges through R until it reaches VP; the UJT then conducts, C discharges through R1 and a sharp pulse appears across R1. (Key point: UJT firing pulse) ## Related in Power Electronics - [Thyristor V-I, latching/holding, gate signal circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-gate-circuit) — Power Electronics, chapter 1 - [di/dt, dv/dt protection and gate circuit design](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aprotection-gate-design-factors) — Power Electronics, chapter 1 - [Thyristor V-I and firing circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-firing-circuit) — Power Electronics, chapter 1, asked 2× - [Microcontroller-based thyristor firing circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Amicrocontroller-firing-opto) — Power Electronics, chapter 1 - [Thyristor V-I and opto-coupler isolation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-opto-isolation) — Power Electronics, chapter 1, asked 2× - [Thyristor V-I, application and opto-isolation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-application-opto) — Power Electronics, chapter 1, asked 2×