Thyristors and SCRs
A thyristor is a latching semiconductor switch. The silicon controlled rectifier, or SCR, is the most common thyristor. It turns on when gate current is applied while the anode is positive, then remains on until current falls below the holding current.
Learning Objectives
By the end of this lesson, you should be able to explain SCR latching, distinguish trigger current from holding current, describe phase control on AC, select voltage and current ratings, and identify when a MOSFET or relay is a better switch.
SCR Terminals and Latching
An SCR has three terminals:
- Anode (A): positive main terminal during forward conduction.
- Cathode (K): return main terminal.
- Gate (G): trigger input referenced to cathode.
Once triggered, the gate loses control. The SCR turns off only when anode current drops below holding current (I_H), usually at an AC zero crossing or by forced commutation.
Key Ratings
Important datasheet values include:
- (V_{DRM}): repetitive off-state voltage.
- (I_T(RMS)): RMS on-state current.
- (I_{GT}): gate trigger current.
- (V_{GT}): gate trigger voltage.
- (I_L): latching current, needed immediately after triggering.
- (I_H): holding current, below which the SCR turns off.
- (dV/dt): maximum off-state voltage rise rate.
- (dI/dt): maximum current rise rate after turn-on.
AC Phase Control
In AC control, delaying the gate pulse after each zero crossing changes how much of the sine wave reaches the load. This is used in lamp dimmers, heater controls, and some motor controls. It is not a universal solution: phase control creates harmonics and electromagnetic interference.
title "Illustrative SCR phase control"
time start=0 end=20 unit=ms divisions=10
VAC: sine label="AC source" amplitude=1 cycles=1 unit=pu color=#2563eb
GATE: pulse label="Gate pulse" low=0 high=1 at=5 width=1 unit=pu color=#16a34a
LOAD: pulse label="Load conducts" low=0 high=1 at=5 width=5 unit=pu color=#dc2626
marker ZC at=0 label="zero cross" color=#475569
marker FIRE at=5 label="trigger" color=#475569
The waveform is conceptual. Real load current depends on source, load type, gate circuit, and holding current.
Protection and Snubbers
SCR circuits often need:
- fuse or breaker sized for fault energy;
- RC snubber to limit false triggering from high (dV/dt);
- gate resistor and isolation where the control circuit is low voltage;
- heat sink for RMS conduction loss;
- MOV or TVS for mains transients when appropriate.
Mains-connected SCR circuits are hazardous and must follow isolation, creepage, clearance, enclosure, fuse, and local electrical-code requirements.
SCR vs TRIAC vs MOSFET
An SCR conducts in one direction. A TRIAC is related but can control AC in both directions. MOSFETs are preferred for low-voltage DC switching when controllable turn-off, PWM, and high efficiency are required.
| Device | Best fit | Main limitation |
|---|---|---|
| SCR | High power rectified AC, crowbar protection | Cannot gate-turn-off normally |
| TRIAC | Low-cost AC load control | Sensitive loads and EMI |
| MOSFET | DC switching and PWM | Needs gate drive and voltage margin |
| Relay | Isolation and simple switching | Mechanical wear and slow speed |
Common Mistakes
- Expecting the gate to turn the SCR off.
- Ignoring holding current with light loads.
- Missing (dV/dt) false-trigger limits.
- Using phase control on loads that require clean sine power.
- Designing mains circuits without isolation and safety review.
Summary
SCRs are rugged latching switches for controlled rectification, AC power control, and crowbar protection. Their latching behavior is useful only when the design accounts for holding current, commutation, heat, transient limits, and safety.
Further Reading
- Littelfuse, "Thyristor Theory and Design Considerations."
- STMicroelectronics, "SCR and TRIAC Application Notes."
- IEC 60747 semiconductor device terminology.