DIAC (Diode for Alternating Current)
A DIAC is a bidirectional semiconductor switching device that conducts current only after its breakover voltage is reached, regardless of current direction.
It is commonly used to trigger a TRIAC in AC power control circuits.
Symbol and Structure
A DIAC has:
- Two terminals
- No gate terminal
- Symmetrical operation in both directions
It behaves like:
- An open circuit at low voltage
- A closed switch after breakover voltage
Working Principle
When the applied voltage is below the breakover voltage:
The DIAC remains OFF.
When voltage exceeds the breakover voltage:
The DIAC suddenly turns ON and conducts heavily.
- Voltage across DIAC drops sharply
- Current increases rapidly
It turns OFF again when current falls below the holding current.
V-I Characteristics
The DIAC has:
- Symmetrical characteristics
- Negative resistance region after breakover
It conducts in:
- Positive half cycle
- Negative half cycle
Typical breakover voltage: 20V – 40V
Applications of DIAC
1. TRIAC Triggering
Most common application.
Used in:
- Lamp dimmers
- Fan regulators
- Heater control
The DIAC provides sharp triggering pulses to the TRIAC.
2. Phase Control Circuits
Used for controlling AC power by varying firing angle.
3. Pulse Generation Circuits
Used where sudden switching action is required.
Advantages
- Bidirectional operation
- Simple construction
- Reliable triggering
- Improved TRIAC symmetry
Disadvantages
- Limited power handling
- No amplification capability
- Mainly useful with TRIAC circuits
Difference Between DIAC and TRIAC
| Feature | DIAC | TRIAC |
|---|---|---|
| Terminals | 2 | 3 |
| Control Terminal | No gate | Has gate |
| Direction | Bidirectional | Bidirectional |
| Main Use | Triggering | Power control |
| Power Handling | Low | High |
Example: Fan Regulator
- Capacitor charges through resistor.
-
When capacitor voltage reaches DIAC breakover voltage:
- DIAC conducts suddenly.
- This pulse triggers the TRIAC.
- TRIAC controls AC power to the fan.
Thus, the DIAC helps provide stable and symmetrical firing of the TRIAC.