DIAC
A DIAC remains in a high-impedance state in either polarity until its breakover voltage is reached. It then conducts a pulse, making it useful for triggering a TRIAC from an RC timing network. Breakover voltage and symmetry depend on the chosen DIAC.
Littelfuse describes the DIAC as similar to an open-base NPN structure. Its characteristic is bidirectional: after breakover it enters a negative-resistance region, with voltage falling as current increases. In an RC trigger circuit, this lets the capacitor discharge through the trigger path. It is not simply two ordinary avalanche diodes, and a SIDAC's on-state voltage must not be copied into a DIAC specification.
In plain terms
A pressure-relief valve that won't open until the water pressure crosses a threshold, then suddenly snaps open — and the same valve works whether water flows north or south.
Why designers use it
- Creating a trigger pulse at a specified breakover voltage.
- Providing a bidirectional trigger element for suitable TRIAC phase-control circuits.
- Building a simple RC threshold circuit without a separate gate-control terminal.
Best for
- Lamp dimmers
- TRIAC triggering
- AC threshold detect
Key specifications
- Breakover voltage: Specified VBO range, not one universal DIAC voltage
- Symmetry: Difference between positive and negative breakover
- Pulse capability: Check current, duration and repetition limits with the trigger circuit
When not to use it
- When precision timing exceeds what the DIAC, capacitor, supply and temperature tolerances can provide.
- When the actual lamp or motor is incompatible with the chosen phase-control circuit.
- As a substitute for the load-carrying TRIAC or for mains isolation.
Common mistakes
- Selecting a trigger network that cannot meet the TRIAC's gate and latching requirements.
- Assuming positive and negative breakover are perfectly identical rather than checking the symmetry specification.
- Confusing DIAC and SIDAC conduction characteristics.
- Treating a simple demonstration circuit as a complete, safe mains-powered product.
Where you will find it
- In a suitable phase-control circuit, a capacitor charges through resistance. When the DIAC breaks over, the resulting pulse triggers a TRIAC. Changing the resistance changes the charging time and therefore the intended firing point.
- A designer checks both half-cycles and the actual load because trigger symmetry does not guarantee identical load current or flicker-free operation.
A short history
A DIAC provides bidirectional breakover behaviour without a gate terminal. It can produce a trigger pulse when an RC network reaches its breakover voltage, which makes it useful in TRIAC phase-control circuits. It is a triggering device, not a replacement for the load-carrying TRIAC.