DIAC Symbol

Definition: The DIAC symbol represents a bidirectional two-terminal thyristor that conducts current in either direction once the applied voltage exceeds its breakover voltage (V_BO), used primarily as a trigger device for TRIACs and SCRs in AC phase-control circuits; it is defined in IEC 60747-6 and depicted in IEC 60617-05 as two opposed diode triangles sharing a common cathode bar, indicating symmetrical bidirectional switching.

Also known as: DIAC, bidirectional trigger diode, trigger diac, symmetrical trigger thyristor, DB3 (common DIAC IC).

What the DIAC symbol means

The DIAC symbol denotes a symmetrical two-terminal semiconductor device with no gate or control electrode. The DIAC remains in a blocking state (high impedance) until the voltage across its terminals reaches the breakover voltage V_BO (typically 28–36 V for the common DB3). Once V_BO is exceeded in either direction, the DIAC switches rapidly to a low-impedance conducting state, discharging a trigger capacitor into a TRIAC or SCR gate to fire the controlled rectifier.

In circuit diagrams the DIAC symbol always appears in conjunction with a TRIAC in AC phase-control applications. The DIAC produces a sharp, well-defined trigger pulse with fast rise time because of its negative resistance breakover characteristic, ensuring reliable, repeatable firing of the TRIAC at a predictable phase angle every half-cycle.

How to identify the DIAC symbol

The DIAC symbol consists of two diode triangles arranged pointing in opposite directions (one pointing up, one pointing down) with their tips meeting at a common node, and a horizontal bar crossing both tips. This bidirectional opposed-triangle arrangement is the key visual identifier, distinguishing the DIAC from a standard rectifier diode (single triangle + bar) or a Zener diode (bent bar). The two external terminals are labelled A (Anode 1, or MT1) and B (Anode 2, or MT2); both terminals are functionally equivalent because of the device's symmetry.

Function and usage boundary

A DIAC blocks current in both directions until the voltage across its two terminals (A–B or B–A) reaches V_BO. At breakover, the DIAC switches to a conducting state with a holding current I_H (typically a few milliamps). In a TRIAC phase-control circuit, a resistor-capacitor (RC) network charges a capacitor during each AC half-cycle; when the capacitor voltage reaches V_BO, the DIAC fires and discharges the capacitor into the TRIAC gate, triggering the TRIAC at a phase angle controlled by the RC time constant.

Standards: IEC vs ANSI

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Logical terminals

Where the DIAC symbol is used

• TRIAC gate trigger circuits in light dimmer switches controlling incandescent and halogen lamps.
• AC motor speed control circuits using TRIAC phase-angle control for universal motors.
• Fan speed controllers in HVAC equipment using phase-control dimmer topology.
• Heating element power control in electric ovens and soldering stations using TRIAC phase-angle firing.
• Soft-start circuits for AC motors where gradual TRIAC firing angle ramp reduces inrush current.
• Power tool speed control (drills, routers) using universal motor TRIAC controllers.

Key facts

• A DIAC is a bidirectional two-terminal thyristor that switches from blocking to conducting state when the applied voltage (in either direction) exceeds its breakover voltage V_BO, typically 28–36 V.
• The DIAC symbol is two diode triangles pointing in opposite directions with a common crossbar at the tips, visually indicating symmetric bidirectional switching—distinguishing it from a single-direction diode.
• The two terminals are labelled A and B (or MT1 and MT2); both are electrically equivalent due to the device's symmetry—there is no anode or cathode polarity.
• IEC 60617-05 and IEEE 315 / ANSI Y32.2 both represent the DIAC with back-to-back opposed diode triangles; the two standards use essentially identical glyphs.
• The DB3 is the most common commercial DIAC, with V_BO ≈ 32 V, widely used with BT136 or BT138 TRIACs in dimmer and motor-speed-control circuits.
• The DIAC's negative resistance characteristic (voltage drops sharply at breakover) produces a fast-rising trigger pulse that reliably fires a TRIAC gate, even at the end of the AC half-cycle when drive voltage is low.
• Unlike a TRIAC or SCR, the DIAC has no gate electrode and cannot be triggered externally—it fires automatically when V_BO is exceeded.

Frequently asked questions

What does the DIAC symbol look like?

The DIAC symbol shows two diode triangles pointing in opposite directions (one upward, one downward) with their tips meeting and a horizontal bar crossing both tips. This back-to-back arrangement conveys the device's ability to switch bidirectionally. The two external terminals extend from the top and bottom of the symbol.

What does a DIAC do in a circuit?

A DIAC remains in a high-impedance blocking state until the voltage across it exceeds its breakover voltage (typically 28–36 V). At breakover, it abruptly switches to a low-impedance state, discharging the trigger capacitor into a TRIAC or SCR gate. After the current falls below the holding current, the DIAC returns to the blocking state.

What is the difference between a DIAC and a TRIAC?

A DIAC is a two-terminal trigger device with no gate; it fires automatically when V_BO is exceeded. A TRIAC is a three-terminal (gate, MT1, MT2) bidirectional power switching device that is turned on by a gate pulse (typically from a DIAC) and remains on until the holding current falls below I_H. The DIAC triggers the TRIAC; the TRIAC controls the load.

What are the terminals of a DIAC?

A DIAC has two terminals, commonly labelled A and B, or MT1 and MT2 (Main Terminal 1 and 2). Both terminals are functionally identical because the DIAC is symmetrical; there is no anode–cathode polarity convention as with a rectifier diode.

What is the breakover voltage of a typical DIAC?

The most common DIAC, the DB3, has a breakover voltage of approximately 28–36 V (typically specified as 32 V). This value is chosen to be well above the TRIAC gate trigger voltage but below the AC mains peak voltage, ensuring reliable triggering during phase-control dimmer operation.

What standard defines the DIAC symbol?

The DIAC is defined in IEC 60747-6 (thyristor devices). The schematic symbol—two opposed diode triangles with a common bar—is specified in IEC 60617-05 and IEEE 315 / ANSI Y32.2. Both standards use essentially identical back-to-back diode symbols to represent the device.

Can a DIAC be replaced with two Zener diodes?

Two Zener diodes connected back-to-back (in series, opposing polarity) can approximate a DIAC's bidirectional breakover behaviour, with the combined breakover voltage equal to V_Z + V_forward ≈ V_Z + 0.7 V per device. However, Zener diodes have less sharp breakover transitions and higher capacitance than a dedicated DIAC, resulting in less reliable TRIAC triggering, especially at high firing angles near the end of the AC half-cycle.

Related symbols

• AND Gate symbol
• Logic Level Shifter symbol
• NPN Transistor symbol
• Op-Amp symbol
• OR Gate symbol
• TRIAC symbol

Physical breadboard, PCB, and panel mapping

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