DIAC Symbol, Pinout, and Schematic Reference Guide

The DIAC (Diode for Alternating Current) is a bidirectional trigger diode used primarily to fire TRIACs in AC phase-control circuits like light dimmers and motor speed controllers. Unlike a TRIAC, the DIAC symbol lacks a gate terminal, reflecting its symmetrical, bidirectional breakover behavior. Below is the immediate reference for schematic symbols, regional standards, and electrical specifications.

DIAC Schematic Symbols and Regional Standards

When reading a schematic, the DIAC is visually distinguished by its symmetry. It conducts current in both directions once the voltage across it exceeds its breakover voltage ($V_{BO}$). The exact symbol you encounter depends on the standard your region or organization follows. According to Electronics Tutorials, the IEC standard is the most globally recognized, but US-based legacy schematics often use the IEEE/ANSI variant.

Table 1: DIAC Schematic Symbol Variants by Standard

Standard	Visual Description	Regional Usage	Key Differentiator
IEC 60617	Two opposing triangles (anode/cathode symbols) pointing at a central vertical line, enclosed in a circle.	Global / EU / Modern US	The enclosing circle is mandatory in strict IEC compliance, though often omitted in informal schematics.
IEEE/ANSI 315	Two opposing triangles sharing a central vertical bar, typically without an enclosing circle.	North America / Legacy	Lacks the circle; the triangles often have inward-pointing arrows or distinct line weights to denote bidirectional flow.
Legacy / DIN	Two standard diode symbols drawn in parallel but facing opposite directions, sometimes with a connecting box.	Obsolete / Old EU Manuals	Looks like two discrete Zener diodes in anti-series. Rarely used in modern CAD libraries.

What the symbol means in practice: The dual-triangle geometry explicitly tells the designer that the component has no polarity. You can solder it in either direction. The absence of a third line (the gate) differentiates it from a TRIAC. If you see a third line extending from the central junction, you are looking at a TRIAC, not a DIAC.

Specification Table: Breakover Voltages and Package Data

Selecting the right DIAC requires matching the breakover voltage ($V_{BO}$) to your TRIAC's gate trigger requirements. The STMicroelectronics DB3 datasheet remains the industry benchmark for these components. Below is a data-dense reference table of the most common DIACs found in commercial and DIY power electronics.

Table 2: Common DIAC Specifications and Package Types

Part Number	Package Type	Breakover Voltage ($V_{BO}$)	Breakover Current ($I_{BO}$)	Max Power Dissipation
DB3	DO-15 (Through-hole)	28V to 36V (Nominal 32V)	50 µA	150 mW
ST2	DO-15 (Through-hole)	26V to 34V (Nominal 30V)	50 µA	150 mW
HT32	DO-15 (Through-hole)	28V to 36V (Nominal 32V)	50 µA	500 mW (High Temp)
SMDB3	SOIC / DO-214AA (SMD)	28V to 42V (Nominal 35V)	50 µA	300 mW
ER9024	DO-35 (Glass, Small)	30V to 40V (Nominal 35V)	100 µA	100 mW

Rows and Symbols People Get Wrong

• Assuming SMD DIACs are transistors: The SMDB3 and similar SMD DIACs sometimes use 3-pin SOT-23 footprints for automated pick-and-place compatibility, even though the third pin is internally disconnected (No Connect / NC). Always check the datasheet pinout; do not assume a 3-pin SMD component is a BJT or MOSFET.
• Misinterpreting the $V_{BO}$ range: Notice the DB3 has a spread of 28V to 36V. Designers often get wrong by assuming a hard 32.0V trigger. Your RC timing circuit must account for this ±4V tolerance, or your AC dimmer will exhibit hysteresis and flicker at low settings.
• Confusing DIAC with Zener diodes: While two Zeners in anti-series can mimic a DIAC, a true DIAC exhibits negative resistance after breakover. It snaps to a low voltage drop (usually ~5V) and stays there until current falls below the holding threshold. Zeners do not exhibit this snap-back negative resistance.

Pinout Rules, Faded Markings, and Bench Testing

Because a DIAC is a purely symmetrical, bidirectional device, it does not have an anode and cathode in the traditional DC sense. In schematic netlists and advanced datasheets, the terminals are designated as A1 and A2 or MT1 and MT2 (Main Terminal 1 and 2), mirroring TRIAC terminology. On the physical component, the leads are identical in length and thickness, and the body lacks any polarity band or stripe.

Safe interpretation when markings are faded or missing:
If you are salvaging components or repairing a board where the DB3 or ST2 silkscreen has burned off, you cannot rely on visual identification. Furthermore, standard semiconductor theory dictates that a typical digital multimeter (DMM) will fail to test a DIAC. A standard DMM diode-test mode outputs only 2V to 3V. Since the DIAC requires ~30V to break over, the meter will read "OL" (Open Loop) in both directions, leading novices to incorrectly throw the component in the trash, assuming it is dead.

To properly identify and test an unmarked or faded DIAC, you must build a simple high-voltage test jig to measure its $V_{BO}$:

1. Power Source: Use an isolated bench power supply or a 50V DC boost converter. Never use a non-isolated mains-derived supply for this bench test.
2. Current Limiting: Place a 10 kΩ, 1W resistor in series with the positive output to limit the current to a safe 5 mA upon breakover.
3. Measurement: Connect the DIAC in series with the resistor. Place your DMM (set to DC Volts) directly across the DIAC terminals.
4. Observation: Slowly ramp the supply voltage from 0V upward. The meter will track the supply voltage until it hits the $V_{BO}$ (e.g., 32V). At that exact threshold, the voltage across the DIAC will suddenly snap back to roughly 5V to 10V. This snap-back confirms it is a DIAC and reveals its specific breakover voltage.

By understanding the distinct dual-triangle symbol, accounting for the manufacturing tolerance in $V_{BO}$, and using the correct high-voltage testing methodology, you can reliably design, troubleshoot, and salvage DIACs in any power electronics project.