--- title: "Thyristor V-I, latching/holding, gate signal circuit" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:thyristor-vi-latching-gate-circuit" source: https://www.bctengineer.com/q/semester_7/power_electronics/thyristor-vi-latching-gate-circuit site: BCT Engineering --- # Thyristor V-I, latching/holding, gate signal circuit Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2073 Chaitra, 8 marks - Video: [Latching and holding current of a thyristor](https://www.youtube.com/watch?v=_24_ImQjXao) — •Learning Space• - 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-latching-gate-circuit - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain V-I characteristics of a Thyristor and explain the meaning of latching current and holding current. Describe a gate signal generating circuit for firing a Thyristor. ## Short answer The characteristic has reverse blocking to V_BR, forward blocking to V_BO, then ON state at 1-2 V; gate current lowers V_BO. Latching current is minimum anode current at turn-on to remain ON after gate removal; holding current is minimum to keep conducting, with IL > IH. A UJT oscillator fires the gate at Vp = ηVBB + VD. ## 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 $V_{BO}$. #### Latching and holding current - **Latching current ($I_L$):** the minimum anode current that must flow through the SCR immediately after it is triggered, while the gate pulse is still present, so that it stays ON after the gate pulse is removed. If the anode current has not reached $I_L$ when the gate pulse ends, the SCR turns OFF again. - **Holding current ($I_H$):** the minimum anode current that must keep flowing to hold an already conducting SCR in the ON state (gate open). If the anode current falls below $I_H$, the SCR turns OFF and returns to forward blocking. - $I_L$ is greater than $I_H$, usually $I_L \approx 2$ to $3\,I_H$. Example: for an SCR with $I_L = 40$ mA and $I_H = 15$ mA, the gate pulse must last until the anode current reaches 40 mA, but once ON it stays ON until the current drops below 15 mA. #### Gate signal generating circuit **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the latching current IL? A. Minimum anode current needed just after triggering so the SCR stays ON after the gate pulse is removed B. Minimum current to keep an ON SCR conducting with the gate open C. Maximum gate current D. The leakage current in forward blocking Answer: A. If anode current has not reached IL when the gate pulse ends, the SCR turns OFF again. (Key point: Latching current) 2. What happens if the anode current of a conducting SCR falls below the holding current IH? A. The SCR stays ON because of the gate B. The SCR turns OFF and returns to forward blocking C. The SCR breaks down by avalanche D. The SCR enters reverse conduction Answer: B. IH is the minimum current to hold the SCR ON; below it the SCR turns OFF. (Key point: Holding current) 3. For an SCR with IL = 40 mA and IH = 15 mA, how long must the gate pulse last? A. Until anode current falls to 15 mA B. Until anode current reaches 40 mA C. Only a few ns regardless of current D. Until the voltage reaches VBO Answer: B. IL > IH. The pulse must last until the anode current reaches IL, then the SCR stays ON until current drops below IH. (Key point: IL versus IH) 4. In the UJT firing circuit, what sets the interval between pulses? A. Charging of C through R, with time constant RC and period T = RC ln(1/(1-eta)) B. The pulse transformer ratio only C. The valley voltage alone D. The SCR holding current Answer: A. C charges through R to the peak point voltage; the period is T = RC ln(1/(1-eta)), so changing R changes the firing angle. (Key point: UJT pulse timing) ## Related in Power Electronics - [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 - [Thyristor V-I, holding/latching; vertical BJT](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-vertical-bjt) — Power Electronics, chapter 1 - [Latching and holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Alatching-holding-current) — 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 gate triggering circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-gate-triggering-circuit) — 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