The silicon controlled rectifier (SCR), or thyristor, is the workhorse of power electronics. It switches thousands of amperes and kilovolts in controlled rectifiers, DC and AC motor drives, soft starters, static VAR compensators and HVDC converter valves. Questions on its structure, V-I characteristics, latching and holding current, turn-on methods and the firing-angle formula appear regularly in PGCB, BPDB, DPDC, BREB, BCS (EEE) and university exams. This post covers them with an animated V-I curve, an animated controlled rectifier, solved problems and MCQs.
- SCR = 4-layer PNPN device with 3 junctions (J1, J2, J3) and 3 terminals: anode (A), cathode (K) and gate (G).
- Three modes: reverse blocking, forward blocking (OFF) and forward conduction (ON).
- A small gate pulse turns it ON; once ON, the gate loses control. It turns OFF only when the anode current falls below the holding current IH.
- Latching current IL > holding current IH (typically IL ≈ 2–3 × IH).
- Half-wave controlled rectifier (R load): Vdc = (Vm/2π)(1 + cos α); full-wave: Vdc = (Vm/π)(1 + cos α).
1. Structure and Symbol
An SCR has four alternate layers, P–N–P–N, forming three junctions: J1 (P1–N1), J2 (N1–P2) and J3 (P2–N2). The outer P layer is the anode, the outer N layer is the cathode, and the gate is connected to the P layer next to the cathode (P2). The N1 layer is wide and lightly doped so that it can block high voltage. The circuit symbol is a diode with a gate lead on the cathode side.
- It is a unidirectional device: current flows only from anode to cathode.
- It is a latching switch: it stays ON after the gate pulse is removed.
- On-state voltage drop ≈ 1–2 V. Ratings range from a few amperes to several kA and up to about 8–10 kV per device.
2. V-I Characteristics
| Mode | Bias | Junction state | Behaviour |
|---|---|---|---|
| Reverse blocking | Cathode positive with respect to anode | J1, J3 reverse-biased; J2 forward | Only a small leakage current; reverse breakdown if V exceeds VRBD |
| Forward blocking (OFF) | Anode positive, no gate signal | J1, J3 forward; J2 reverse | Small leakage current until VBO |
| Forward conduction (ON) | Anode positive, triggered | All junctions forward | Large current, drop ≈ 1–2 V, limited by the external load |
Latching current (IL): the minimum anode current that must flow immediately after triggering for the SCR to stay ON once the gate pulse is removed. Holding current (IH): the minimum anode current that keeps an already-conducting SCR ON; below IH it turns off. Since IL > IH, the gate pulse must last long enough for the anode current to rise above IL, which matters especially with inductive loads.
3. Two-Transistor Model
The PNPN structure can be split into a PNP transistor (Q1) and an NPN transistor (Q2). The collector of each drives the base of the other, forming a positive-feedback (regenerative) loop. With common-base current gains α1 and α2:
IA = [α2 IG + ICBO1 + ICBO2] / [1 − (α1 + α2)]In the OFF state, α1 + α2 is small. A gate current (or higher voltage or temperature) increases the currents, the α values rise, and when α1 + α2 → 1 the denominator approaches zero. The anode current then grows regeneratively and is limited only by the external circuit. This explains why the gate cannot turn a conventional SCR off.
4. Turn-On (Triggering) Methods
- Gate triggering (most common and controllable): a positive gate pulse with respect to the cathode.
- Forward-voltage triggering: raising VAK above VBO. This is avoided because it can damage the device.
- dv/dt triggering: a fast-rising anode voltage charges the J2 junction capacitance; the current i = Cj dv/dt can turn the SCR on unintentionally. RC snubbers prevent this.
- Temperature (thermal) triggering: leakage current rises with temperature and can trigger the device (undesirable).
- Light triggering: light-activated SCRs (LASCR) are fired by light through an optical fibre. They are used in HVDC valves because of their electrical isolation.
Gate pulses are usually produced by UJT relaxation oscillators, dedicated firing ICs or microcontrollers, and are isolated through pulse transformers or opto-couplers.
5. Turn-Off (Commutation)
To turn an SCR off, its anode current must be reduced below IH, and a reverse voltage must be held for at least the turn-off time tq (≈ 10–100 µs for converter-grade SCRs) so that the stored charge recombines.
- Natural (line) commutation: in AC circuits the current naturally passes through zero every half-cycle. This is used in phase-controlled rectifiers and AC voltage controllers.
- Forced commutation: in DC circuits (choppers, inverters) an auxiliary circuit with L, C and another SCR forces the current to zero. Classes A–F are listed in textbooks. Today, GTOs, IGBTs and MOSFETs are preferred for such DC switching.
6. Controlled Rectifier and Firing Angle
In a phase-controlled rectifier the SCR is fired at an angle α (the firing or delay angle) after the supply voltage crosses zero. The output DC voltage falls as α increases, which gives smooth control of DC motors, battery chargers, heaters and electroplating supplies.
| Converter (R load) | Average output voltage |
|---|---|
| Single-phase half-wave | Vdc = (Vm/2π)(1 + cos α) |
| Single-phase full-wave (bridge or centre-tap) | Vdc = (Vm/π)(1 + cos α) |
| Single-phase full converter, highly inductive load (continuous current) | Vdc = (2Vm/π) cos α; becomes negative (inversion) for α > 90° |
| Three-phase full converter, continuous current | Vdc = (3√3 Vm/π) cos α = 1.35 VL cos α (Vm = peak phase voltage, VL = RMS line voltage) |
Example 1. A 230 V, 50 Hz supply feeds a half-wave controlled rectifier with a resistive load, α = 60°. Vm = √2 × 230 = 325.3 V.
Vdc = (325.3/2π)(1 + cos 60°) = 51.77 × 1.5 ≈ 77.7 VExample 2. The same supply with a full-wave controlled rectifier, α = 60°: Vdc = (325.3/π)(1.5) ≈ 155.3 V. With R = 10 Ω, the average load current is 15.5 A.
Example 3. A three-phase full converter on 400 V (line), α = 30°, continuous current: Vdc = 1.35 × 400 × cos 30° ≈ 467.7 V.
7. Ratings and Protection
| Stress | Risk | Protection |
|---|---|---|
| High di/dt at turn-on | Local hot spots near the gate | Small series inductor |
| High dv/dt | False triggering | RC snubber across the SCR |
| Over-voltage (switching, lightning) | Breakdown | MOV / TVS, snubber |
| Over-current / short circuit | Thermal damage | Fast-acting semiconductor fuse (I²t rating), circuit breaker |
| Heat | Junction temperature > ≈ 125 °C | Heat sink, forced-air or water cooling |
| Gate noise | Spurious firing | Gate-cathode resistor or capacitor, shielded leads |
8. Thyristor Family and Applications
| Device | Key feature | Typical use |
|---|---|---|
| SCR | Unidirectional, gate turn-on only | Controlled rectifiers, HVDC, soft starters |
| TRIAC | Bidirectional (two SCRs in anti-parallel, one gate) | Fan regulators, light dimmers, AC switches |
| DIAC | Bidirectional, no gate, breaks over at ≈ 30 V | Triggering TRIACs |
| GTO | Can be turned OFF by a negative gate pulse | High-power inverters, traction (older designs) |
| LASCR | Light-triggered | HVDC valves, isolated switching |
Applications include DC motor speed control, induction-motor soft starters, battery chargers, UPS rectifiers, induction heating, static VAR compensators (thyristor-controlled reactors) and HVDC converter stations.
9. Practice MCQs
1. An SCR has: (a) 3 layers, 2 junctions (b) 4 layers, 3 junctions (c) 4 layers, 4 junctions (d) 2 layers, 1 junction
2. In forward blocking mode, the reverse-biased junction is: (a) J1 (b) J2 (c) J3 (d) None
3. Which is TRUE? (a) IH > IL (b) IL > IH (c) IL = IH (d) Both are zero
4. Once an SCR is ON, the gate: (a) Controls the anode current (b) Loses control (c) Must stay energized (d) Turns it OFF with a negative pulse
5. Increasing gate current makes the forward breakover voltage: (a) Increase (b) Decrease (c) Stay the same (d) Infinite
6. An RC snubber across an SCR protects against: (a) High di/dt (b) High dv/dt (c) Over-current (d) Low gate current
7. A half-wave controlled rectifier on 230 V with α = 90° gives Vdc ≈: (a) 103.5 V (b) 51.8 V (c) 0 V (d) 207 V
8. The SCR is turned off by: (a) Removing gate current (b) Reducing anode current below IH (c) Increasing gate voltage (d) Increasing load
9. In the two-transistor model, the SCR turns on when: (a) α1 + α2 → 0 (b) α1 + α2 → 1 (c) α1 = α2 = 0.5 always (d) β → 0
10. A bidirectional thyristor used in fan regulators is the: (a) SCR (b) TRIAC (c) GTO (d) UJT
10. বাংলায় দ্রুত রিভিশন
- SCR: চার স্তর (PNPN), তিনটি জাংশন (J1, J2, J3), তিনটি টার্মিনাল (অ্যানোড, ক্যাথোড, গেট)।
- ফরোয়ার্ড ব্লকিংয়ে J2 রিভার্স বায়াসড; গেট পালস দিলে অন হয়, অন হওয়ার পর গেটের নিয়ন্ত্রণ থাকে না।
- ল্যাচিং কারেন্ট IL > হোল্ডিং কারেন্ট IH; অ্যানোড কারেন্ট IH-এর নিচে নামলে অফ হয়।
- টার্ন-অন পদ্ধতি: গেট, ফরোয়ার্ড ভোল্টেজ, dv/dt, তাপমাত্রা, আলো (LASCR)।
- হাফ-ওয়েভ: Vdc = (Vm/2π)(1 + cos α); ফুল-ওয়েভ: Vdc = (Vm/π)(1 + cos α)।
- সুরক্ষা: di/dt-এর জন্য সিরিজ ইন্ডাক্টর, dv/dt-এর জন্য RC স্নাবার, ওভারকারেন্টে দ্রুত ফিউজ।
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References: M. H. Rashid, Power Electronics: Circuits, Devices and Applications; P. S. Bimbhra, Power Electronics; Ned Mohan et al., Power Electronics. Images and animations: original work by Totalecer.
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