- DIAC Definition: A DIAC is defined as a diode that begins to conduct electricity only after its breakover voltage is exceeded, crucial in electrical circuits for controlling current flow.
- Activation Mechanism: The DIAC can be activated by increasing the voltage beyond its avalanche breakdown voltage and deactivated by reducing it below this level, allowing for controlled electrical conductivity.
- Applications: DIACs are commonly used in circuits like lamp dimmers, heat controllers, and motor speed controls, showcasing their versatility in various electronic applications.
- Construction Features: DIACs have symmetrical switching characteristics for either voltage polarity due to their unique construction of no base terminal and balanced doping levels.
- Operating Benefits: Unlike some other thyristors, DIACs maintain a low on-state voltage drop until the current falls below the holding current, making them efficient for sustained use.
What is a DIAC?
A DIAC is a two-terminal, bidirectional trigger diode. It remains in a high-impedance state until the magnitude of the applied voltage reaches its breakover voltage, VBO, in either polarity. DIAC commonly expands to diode for Alternating Current. The device belongs to the broader thyristor trigger-device family and is widely used in the triggering of thyristors. Its symmetrical symbol indicates bidirectional operation.
A DIAC has no gate electrode. Instead, terminal voltage causes breakover. This differs from a TRIAC, which has a gate and can be triggered by a suitable gate-current pulse.
When the magnitude of the voltage reaches VBO, the DIAC enters a negative-resistance region and switches to a lower-voltage conducting state. Although this transition is sometimes compared with avalanche breakdown, circuit design should use the breakover and current specifications in the selected device’s data sheet. The DIAC returns to its blocking state after current falls below its sustaining level.
A simplified DIAC structure is sometimes described as a transistor without a base terminal. Its main feature is near-symmetrical breakover for positive and negative applied voltages.
Application of DIAC
The main application of a DIAC is in a TRIAC phase-control trigger circuit. An RC network charges during each AC half-cycle. When the capacitor voltage reaches the DIAC breakover voltage, the DIAC conducts and discharges part of the capacitor’s energy into the TRIAC gate. The TRIAC then conducts for the rest of that half-cycle and normally turns off when load current falls below its holding current near the current zero crossing.
Common phase-control applications include:
- Lamp dimmer circuits
- Electric heat-control circuits
- Universal-motor speed controls
In a typical trigger network, one DIAC terminal connects to the TRIAC gate and the other connects to the timing capacitor. The DIAC blocks until the capacitor reaches its breakover voltage, then produces a short gate-current pulse.
The DIAC stops conducting after its current falls below its sustaining level. The connected TRIAC has separate latching and holding-current behaviour, so DIAC turn-off does not itself force an already conducting TRIAC off.
Construction of DIAC
A DIAC has two main terminals and a bidirectional semiconductor structure. Manufacturers may use different internal geometries, so the selected data sheet is the source of truth for its construction and ratings. Common design features are:
- No base or gate terminal
- A structure designed for similar breakover behaviour in both directions
- Specified breakover-voltage symmetry for the two voltage polarities
DIAC Characteristics
The figure above uses a multi-region representation with two p-type regions and three n-type regions. It has no gate terminal. Other simplified diagrams show an open-base transistor-like structure, but both representations describe the same two-terminal bidirectional switching function.
The DIAC can reach breakover in either voltage polarity. In the illustrated model, when A2 is positive relative to A1, the active path is represented as P2-N2-P1-N1. When A1 is positive relative to A2, the corresponding current path is represented as P1-N2-P2-N3.
The paired paths give the device its bidirectional diode-like trigger behaviour.
Below breakover in either polarity, only a small leakage current flows and the DIAC remains in its high-impedance blocking state. The maximum leakage current is specified at a stated fraction of VBO and temperature.
When the applied voltage reaches breakover in either polarity and the circuit can supply enough current, DIAC current rises sharply while device voltage falls into the negative-resistance region.
The V-I curve is approximately symmetrical about the origin. Below breakover, the DIAC has high impedance rather than being a perfect open circuit. After breakover, it remains conductive until current falls below the sustaining level and the device returns to the blocking state.
Conclusion
The DIAC is a simple bidirectional voltage-triggered device used mainly to deliver repeatable trigger pulses.
Its main operating features are:
- Near-symmetrical breakover in positive and negative voltage directions.
- A lower on-state voltage drop after breakover, followed by a return to blocking when current falls below the sustaining level.
- A negative-resistance transition that produces a sharp current pulse for triggering another device.
