What's the difference between Triacs and Thyristors (SCRs)?

Triacs [https://www.cnkane.com/bta24-series-25a-triacs-product/]and Thyristors [https://www.cnkane.com/100-8-series-1a-sensitive-scrs-product/](also known as SCRs) both belong to the thyristor [https://www.cnkane.com/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 [https://www.cnkane.com/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 less than or equal to 1200V and current ratings less than or equal to several hundred amperes).

Core Selection Principles:

If controlling a DC load or unidirectional current right choose a Thyristor (SCR)

If controlling an AC load and needing to simplify the circuit and avoid anti-parallel SCRs right choose a Triac.

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