Thyristor / SCR Symbol
Definition: The Thyristor / SCR symbol represents a silicon controlled rectifier — a four-layer (PNPN) power semiconductor device that acts as a bistable switch conducting from anode to cathode only when a gate trigger pulse is applied and the anode-to-cathode voltage is positive, depicted as a standard diode triangle-bar symbol with an additional Gate lead connected to the junction of the cathode bar, conforming to IEC 60617-05 and ANSI/IEEE 315, with three terminals: Anode, Cathode, and Gate.
Also known as: SCR, silicon controlled rectifier, thyristor, PNPN switch, controlled rectifier.
What the Thyristor / SCR symbol means
The Thyristor / SCR symbol in a circuit diagram represents a power switching device that combines rectification with electronic control. Unlike a standard diode that conducts whenever the anode is positive, the SCR remains blocking (off) even with a positive anode voltage until a brief current pulse is applied to the Gate terminal. Once triggered, the SCR latches into conduction and continues to conduct as long as the anode current stays above the holding current — even after the gate pulse is removed. The SCR can only be turned off by reducing the anode current below the holding current (in AC circuits, this occurs naturally at the voltage zero crossing).
In power electronics schematics, the SCR symbol indicates a controlled rectification or power-switching element. It appears in phase-controlled rectifiers, AC motor speed controllers, soft starters, DC motor drives, battery chargers, and high-power switching circuits where gate-controlled turn-on with natural commutation turn-off is required.
How to identify the Thyristor / SCR symbol
The SCR symbol is drawn as a standard diode symbol — a triangle (anode, left) pointing to a vertical bar (cathode, right) — with an additional line extending from the junction between the triangle apex and the cathode bar, bending downward to form the Gate terminal. The Gate lead exits at an angle (typically 45°) from the cathode bar junction. Anode is on the left, Cathode on the right, and Gate is the downward angled terminal at the cathode side. The symbol is distinguished from a standard diode by the Gate lead, and from a TRIAC by having only one gate lead and unidirectional conduction.
Function in a circuit
An SCR operates as a latching unidirectional switch. With a positive anode-to-cathode voltage (VAK > 0) but no gate signal, the SCR blocks current (forward blocking state). A positive gate current pulse (IGT, typically 10–200 mA for a brief microsecond-duration pulse) triggers the SCR into conduction: the four-layer PNPN structure switches on, forward voltage drops to approximately 1–2 V (VT), and the SCR conducts the full load current. The gate loses control once the SCR is latched on; conduction continues until the anode current falls below the holding current IH (typically 5–100 mA). In AC circuits, the SCR turns off naturally at each current zero-crossing. In DC circuits, a commutation circuit (auxiliary thyristor or LC network) must force the current below IH.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 defines the SCR (thyristor) symbol as a p-gate thyristor: the standard diode triangle-bar symbol with a gate lead connecting to the p-region side (cathode bar junction for a standard SCR). IEC 60747-6 covers the thyristor device standard. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 (and IEEE 315A-1986) defines the SCR symbol as a triangle pointing to a bar with a gate lead exiting at an angle from the cathode bar junction. The ANSI and IEC symbols are functionally identical. |
| Key difference | IEC and ANSI SCR symbols are essentially identical: both use a diode triangle-bar with a gate lead at the cathode junction. Minor stylistic differences exist in the gate lead angle (IEC sometimes shows it perpendicular to the cathode bar; ANSI typically at 45°), but both are unambiguous. |
Terminals / pins
| Pin | Name |
|---|---|
| anode | Anode |
| cathode | Cathode |
| gate | Gate |
Typical values
Forward blocking voltage (VDRM): 50 V to 10,000 V. Average on-state current (IT(AV)): 1 A to 5000 A. Gate trigger current (IGT): 10–200 mA (pulse). On-state voltage (VT): 1–2 V at rated current. Holding current (IH): 5–100 mA. Turn-on time (tgt): 1–3 µs. Turn-off time (tq): 20–500 µs (depends on type). Operating temperature: −40 °C to +125 °C (junction).
Where the Thyristor / SCR symbol is used
- Phase-controlled AC rectifiers — controlling DC output voltage by varying the firing angle of SCRs in a three-phase bridge rectifier for industrial DC motor drives
- AC motor soft starters — ramping motor voltage from zero to full by progressively advancing the SCR firing angle over several seconds to limit inrush current
- Battery chargers — high-current SCR bridge rectifiers converting AC mains to controlled DC charging current for industrial battery banks
- AC power controllers and dimmers — phase-angle control of resistive heating loads (ovens, furnaces) using back-to-back SCR pairs (or a TRIAC equivalent)
- Overvoltage crowbar protection circuits — an SCR triggered by a voltage comparator short-circuits the supply rail when overvoltage is detected, blowing a series fuse
- High-power DC switch applications — SCRs as static circuit breakers in HVDC transmission and UPS bypass switching
Example
In a three-phase fully controlled bridge rectifier for a DC motor drive, six SCR symbols are arranged in a bridge configuration — three in the positive half and three in the negative half — with Gate terminals driven by a firing circuit that delays gate pulses by a phase angle α (0°–180°) relative to the AC supply; at α = 0° the output DC voltage equals the maximum rectified value (VDC = 2.34 × VL-L,rms); increasing α reduces VDC linearly, controlling motor speed.
Key facts
- An SCR (thyristor) is a PNPN four-layer device that latches ON when triggered by a gate pulse and a positive anode-to-cathode voltage; once latched, the gate has no further control until the anode current falls below the holding current (IH).
- The SCR conducts in one direction only (anode to cathode) and must receive a gate trigger pulse each AC half-cycle to maintain controlled conduction in phase-controlled AC circuits.
- Phase angle control: delaying the gate trigger by firing angle α (degrees after voltage zero-crossing) varies the average output DC voltage from maximum (α = 0°) to zero (α = 180°), enabling smooth power control.
- SCR turn-off in AC circuits relies on natural commutation — the anode current falls to zero at the AC voltage zero-crossing, extinguishing the thyristor. In DC circuits, forced commutation circuitry is required.
- Gate trigger specifications: minimum gate trigger voltage (VGT ≈ 0.7–2 V) and current (IGT ≈ 10–200 mA); exceeding these thresholds with a brief pulse (1–10 µs) reliably triggers conduction.
- SCRs and thyristors are the same device: 'thyristor' is the generic term (IEC/international); 'SCR' (silicon controlled rectifier) is the equivalent North American term popularised by General Electric.
- Reference designator for SCRs and thyristors in schematics is Q (semiconductor switch) or SCR followed by a number (e.g., Q1, SCR1); no single IEC 60617 designator is universally standardised, though Q is most common in North America.
Frequently asked questions
What does the thyristor / SCR symbol mean in a circuit diagram?
The SCR symbol represents a gate-triggered, latching, unidirectional power switch. It conducts from anode to cathode only after a gate pulse is applied while the anode-to-cathode voltage is positive, and continues conducting until the anode current falls below the holding current — used for phase-controlled power conversion and high-current switching.
What does the SCR symbol look like?
The SCR symbol is a diode symbol (triangle pointing to a bar) with an additional gate lead exiting at an angle from the cathode bar junction. The Anode is on the left (triangle base), the Cathode on the right (bar), and the Gate is the angled terminal at the cathode side. The Gate lead distinguishes the SCR from a standard diode or Schottky diode.
What is the difference between a thyristor and an SCR?
Thyristor and SCR are the same device: 'thyristor' is the IEC/international generic term for the PNPN four-layer controlled rectifier, while 'SCR' (silicon controlled rectifier) is the equivalent North American industry name introduced by General Electric in the 1950s. Both terms refer to the same device and use the same circuit symbol.
How do you turn off an SCR?
An SCR can only be turned off by reducing the anode-to-cathode current below the holding current (IH), typically 5–100 mA. In AC circuits, this occurs naturally at each voltage zero-crossing (natural commutation). In DC circuits, a forced commutation circuit — an auxiliary thyristor and LC network — must briefly reverse or interrupt the main current to extinguish the SCR.
What is the firing angle of a thyristor / SCR?
The firing angle α is the delay in electrical degrees (0°–180°) between the AC supply voltage zero-crossing and the gate trigger pulse. α = 0° gives maximum DC output; α = 90° gives 50% of maximum output; α approaching 180° gives near-zero output. Varying α controls the average power delivered to the load in phase-controlled rectifiers.
What is the difference between an SCR and a TRIAC?
An SCR conducts in one direction only (anode to cathode) and requires a gate pulse each AC half-cycle for symmetrical phase control. A TRIAC conducts in both directions (bidirectional) from a single device and is triggered by a gate pulse in either half-cycle, making it simpler for AC load control. SCRs are used for higher power and DC applications; TRIACs are used for AC loads up to a few hundred amperes.
What standard defines the SCR / thyristor symbol?
IEC 60617-05 (semiconductor graphic symbols) defines the thyristor (SCR) symbol as a diode triangle-bar with a gate lead at the cathode junction. ANSI/IEEE 315-1975 defines the same symbol. The device standard is IEC 60747-6 (thyristors). Both IEC and ANSI symbols are identical.
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