--- title: "Thyristor V-I and firing circuit" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-vi-firing-circuit" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-firing-circuit site: BCT Engineering --- # Thyristor V-I and firing circuit Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2075 Chaitra, 8 marks; 2074 Chaitra, 8 marks (asked 2 times) - Video: [R Triggering of SCR | Power Electronics | Lecture 17](https://www.youtube.com/watch?v=XmsCi3YgiMA) — Electronics Maddy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-firing-circuit - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the V-I characteristic of a thyristor. Also explain a thyristor firing circuit. ## Short answer The thyristor V-I curve shows reverse blocking, forward blocking, and forward conduction modes; it turns on by gate pulse below breakover voltage. An RC firing circuit charges a capacitor through a variable resistor; when capacitor voltage reaches gate trigger voltage, the SCR fires, and increasing R delays firing up to about 180 degrees. ## Answer A **thyristor (SCR)** is a four-layer P-N-P-N, three-junction ($J_1, J_2, J_3$) semiconductor switch with three terminals: anode (A), cathode (K) and gate (G). It can be turned ON by a small gate pulse when forward biased, but once ON the gate loses control; it turns OFF only when its anode current falls below the holding current. #### V-I characteristic ```text IA ^ | |<- ON state (VT = 1-2 V) | | IL ---|--| IH ---|--+. | `. Ig2 > Ig1 > Ig=0 | `-.___ ___ -VBR | `----' | -----+-------+---------------------+--> VAK | | forward blocking VBO | | (leakage current) | reverse blocking | (leakage current) v reverse avalanche ``` 1. **Reverse blocking mode:** cathode is positive with respect to anode. Junctions $J_1$ and $J_3$ are reverse biased and $J_2$ is forward biased, so only a small reverse leakage current flows. If the reverse voltage reaches the **reverse breakdown voltage** $V_{BR}$, avalanche breakdown occurs at $J_1$ and $J_3$; the current rises sharply and the device is usually destroyed. 2. **Forward blocking mode:** anode is positive and gate is open. $J_1$ and $J_3$ are forward biased but $J_2$ is reverse biased, so only a small forward leakage current flows. The SCR is OFF and blocks the forward voltage. 3. **Forward conduction (ON) mode:** if the forward voltage is raised to the **forward breakover voltage** $V_{BO}$ (with $I_g = 0$), junction $J_2$ breaks down by avalanche and the SCR switches suddenly to the ON state (the negative-resistance jump). The voltage across it drops to about 1 to 2 V and the current is limited only by the load. With gate current, $J_2$ breaks down at a lower voltage: the larger $I_g$, the smaller the breakover voltage ($I_{g2} > I_{g1}$). In practice the SCR is always turned ON by a gate pulse at a voltage well below $V_{BO}$. Two important currents on the curve are the **latching current** $I_L$ (minimum current just after turn-on to stay ON) and the **holding current** $I_H$ (minimum current to remain ON); $I_L > I_H$. #### Thyristor firing circuit A firing (triggering) circuit supplies a gate pulse of correct magnitude, width and timing (firing angle $\alpha$) and is synchronised with the supply. **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. A UJT relaxation oscillator or a digital (microcontroller) circuit with a pulse transformer or opto-coupler is used when sharp pulses, wider range and isolation are needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the forward blocking mode of an SCR, which junction blocks the voltage? A. J1 B. J2 C. J3 D. All three equally Answer: B. With the anode positive and gate open, J1 and J3 are forward biased but J2 is reverse biased and blocks. (Key point: Forward blocking) 2. How does increasing the gate current Ig affect the forward breakover voltage? A. It increases VBO B. It has no effect on VBO C. It makes the SCR block in reverse D. It decreases VBO Answer: D. With gate current J2 breaks down at a lower voltage; the larger Ig, the smaller VBO. In practice the SCR is fired by a gate pulse well below VBO. (Key point: Effect of gate current on breakover) 3. Once an SCR is ON, how does it turn OFF? A. By removing the gate pulse B. By applying negative gate current only C. By raising the forward voltage D. When anode current falls below the holding current Answer: D. After turn-on the gate loses control; the SCR turns OFF only when its anode current falls below IH. (Key point: Turn-off condition) 4. In the RC half-wave firing circuit, what happens when the variable resistor R is increased? A. Capacitor charges slower, so firing is delayed (larger firing angle) B. Capacitor charges faster, so firing is advanced C. The SCR is turned off permanently D. The gate trigger voltage Vgt increases Answer: A. A larger R slows charging of C, so the capacitor takes longer to reach Vgt plus the D2 drop and the firing angle increases. (Key point: RC firing control) ## Related in Power Electronics - [Microcontroller-based thyristor firing circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Amicrocontroller-firing-opto) — Power Electronics, chapter 1 - [Pulse train generation for thyristor firing](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apulse-train-gate-firing) — Power Electronics, chapter 1 - [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 - [Thyristor gate triggering circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-gate-triggering-circuit) — Power Electronics, chapter 1 - [Full-wave thyristor rectifier, 220 V, 50 Ω, 45°](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Afull-wave-thyristor-220v-50ohm-45) — Power Electronics, chapter 2 - [Thyristor V-I; latching vs holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-vs-holding) — Power Electronics, chapter 1 ## Related in other subjects - [3-phase thyristor bridge: no-load speed and firing angle](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afull-bridge-motor-no-load-speed) — Utilization of Electrical Energy, chapter 3, asked 3× - [TRIAC firing angles (lamp circuit)](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atriac-firing-20v-lamp) — Advanced Electronics, chapter 7 - [3-phase half controlled bridge: no-load speed, firing angle](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ahalf-bridge-motor-no-load-speed) — Utilization of Electrical Energy, chapter 3, asked 3× - [SCR two-transistor analogy; TRIAC firing angles](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ascr-analogy-triac-firing-25v) — Advanced Electronics, chapter 7