What's the difference between Triacs and Thyristors(SCRs)?
Triacs and Thyristors(also known as SCRs) both belong to the thyristor family, and their core function is that of a controllable power switch. However, they differ fundamentally in their conduction direction, triggering method, and application scenarios.
The following is a detailed comparison:
| Item | Thyristor (SCR,unidirectional controlled rectifier) | triac (Bidirectional Thyristor) |
| Conduction direction | Unidirectional conduction: current can only flow from the anode (A) to the cathode (K). | It allows current flow in both directions; current can flow from main electrode MT1 to MT2, or in the reverse direction. |
| Electrode structure | Three electrodes: anode (A), cathode (K), and gate (G) | Three electrodes: main electrode MT1, main electrode MT2, and gate (G). |
| Trigger conditions | A positive trigger voltage is applied to the gate (G) relative to the cathode (K), and the anode (A) voltage is higher than the cathode (K) voltage. | The gate (G) can be triggered into conduction by applying a positive or negative trigger voltage relative to MT1, regardless of the voltage polarity between MT1 and MT2. |
| Shutdown conditions | The following conditions must be met: 1. The anode current drops below the holding current. 2. A reverse voltage is applied between the anode and cathode. | One of the following conditions must be met: 1. The main electrode current drops below the holding current. 2. The voltage between the main electrodes crosses zero (natural turn-off in AC circuits). |
| Power supply | Mainly used in direct current (DC) or unidirectional pulsating circuits. | Mainly used in alternating current (AC) circuits. |
| Core Features | Once triggered into conduction, the gate loses its control function, and external conditions are required to turn it off. | After being triggered, the gate also loses control, and the device needs to be turned off by current zero-crossing or current interruption. |
| Typical applications | DC motor speed control, rectifier circuits, battery charging, DC switches | AC dimming (lighting, electric stoves), AC motor speed control, solid-state relays (AC-ssr), AC switches |
Key Supplementary Information
Essential Relationship:
A Triac can be considered as two anti-parallel SCRs integrated onto the same chip, sharing a common gate. This allows for bidirectional conduction, simplifying the control design of AC circuits.
Trigger Sensitivity Differences:
The trigger current and voltage parameters of a Thyristor are relatively stable; however, the trigger sensitivity of a Triac is affected by the polarity of the MT1/MT2 voltage, and some models require a specific triggering circuit (such as a Diac bidirectional trigger diode).
Voltage and Current Capabilities:
Under the same manufacturing process, Thyristors have higher voltage and current handling capabilities (reaching thousands of volts and thousands of amperes), making them suitable for high-voltage, high-power DC applications; Triacs are more suitable for medium- and low-voltage AC applications (typically with voltage ratings ≤1200V and current ratings ≤ several hundred amperes).
Core Selection Principles:
If controlling a DC load or unidirectional current → choose a Thyristor (SCR)
If controlling an AC load and needing to simplify the circuit and avoid anti-parallel SCRs → choose a Triac.