In electronics, most diodes—like rectifiers or LEDs—have clear anode and cathode terminals. But the DIAC (Diode for Alternating Current) is different: it has no polarity at all. This symmetric, two-terminal device is designed to conduct equally in both directions once a threshold voltage is reached.
What Is a DIAC?
A DIAC is a bidirectional trigger diode, typically built with a 3- or 5-layer semiconductor structure (e.g., NPN or PNPN). Its I-V curve is symmetrical and “S”-shaped. Below its breakover voltage (VBO, usually 20–40V), it remains in a high-resistance “off” state. Once |V| exceeds VBO—regardless of polarity—it switches rapidly to a low-resistance “on” state.
Because of this perfect symmetry, a DIAC has no anode or cathode. Its two leads are electrically identical and interchangeable—a stark contrast to standard diodes, which only conduct in one direction.
Primary Application: Triggering TRIACs
The most common use of a DIAC is to trigger a TRIAC in AC power control circuits, such as light dimmers, fan speed controllers, and heater thermostats. In these setups, the DIAC sits between the TRIAC’s gate and one main terminal. As the AC waveform rises, the DIAC fires at ±VBO, sending a sharp current pulse to the TRIAC gate to turn it on. Since AC alternates polarity every half-cycle, the trigger device must work identically in both directions—hence the need for a non-polar DIAC.
How to Identify and Use It?
Physically, DIACs (e.g., DB3, DB4) come in small glass or plastic packages with two identical leads—no markings;
Schematically, the DIAC symbol shows two opposing arrows or a symmetrical “Z,” indicating bidirectional operation;
During PCB assembly or breadboarding, pin orientation doesn’t matter.
Common Misconceptions:
Trying to test a DIAC with a multimeter’s diode mode—most meters supply <3V, far below VBO, so the DIAC appears “open”;
Assuming all diodes are polarized—note that bidirectional TVS diodes and varistors are also non-polar.
Testing Tip:
To verify functionality, slowly apply increasing DC voltage (e.g., via an RC charging circuit) until conduction occurs near the rated VBO. Use caution to avoid destructive overvoltage.
In Summary:
The DIAC’s lack of polarity is not a flaw—it’s a feature essential for AC phase-control applications. Recognizing that it has no positive or negative terminal prevents wiring errors and ensures reliable operation in thyristor-based power systems.
