In power electronics, controlling AC voltage and current efficiently is a fundamental requirement for various industrial and domestic applications. One device that plays a crucial role in such control circuits is the DIAC (Diode for Alternating Current). The DIAC is a bidirectional trigger device that is widely used for switching and triggering TRIACs in AC circuits, particularly in applications like light dimmers, motor speed controllers, and heater control circuits.
In this article, we will explore the structure, working principle, V-I characteristics, and applications of the DIAC in detail.
1. What is a DIAC?
A DIAC (Diode for Alternating Current) is a bidirectional semiconductor switching device that can conduct current in both directions once its breakover voltage is exceeded. Unlike regular diodes, which allow current flow in one direction, DIACs are designed to handle AC signals symmetrically.
The DIAC does not have a gate terminal like a TRIAC; instead, it switches on automatically when the voltage across it exceeds a certain threshold in either polarity. This makes it an ideal device for triggering TRIACs in phase control applications.
Key Features of DIAC:
- Bidirectional conduction after breakover voltage
- No gate terminal
- Symmetrical switching characteristics
- Low on-state voltage drop
2. Structure of DIAC
The DIAC is constructed using a three-layer, two-junction structure similar to a pair of Shockley diodes connected in inverse parallel. It typically has two main terminals: MT1 and MT2.
Internally, the DIAC has:
- PNPN or NPNP structure
- Symmetrical doping to ensure equal breakover voltage in both directions
- Encapsulation in a small package for low to medium power applications
Because of its symmetrical design, the DIAC’s electrical characteristics are nearly identical in both forward and reverse directions.
3. Symbol of DIAC
The circuit symbol of a DIAC resembles two diodes connected in opposite directions without a gate terminal. It represents the bidirectional conduction capability of the device.
4. Working Principle of DIAC
The DIAC remains in a non-conductive state until the voltage across it exceeds a certain threshold called the breakover voltage (V_BO). This breakover voltage is typically between 30V and 50V, depending on the type of DIAC.
Here’s how it works step-by-step:
- Below Breakover Voltage:
The DIAC acts as an open circuit, allowing negligible leakage current. - At Breakover Voltage:
Once the voltage reaches the breakover point (positive or negative), the device switches ON rapidly. - After Switching ON:
The DIAC allows current to flow with a very small voltage drop (around 1–2V), until the current drops below the holding current level. - Switching OFF:
When the current falls below the holding current, the DIAC turns OFF and returns to its high-impedance state.
5. V-I Characteristics of DIAC
The V-I characteristics of a DIAC are symmetrical in both the first and third quadrants of the graph.
Important points in the characteristics:
- Breakover Voltage (V_BO): Voltage at which conduction starts.
- Holding Current (I_H): Minimum current required to keep the DIAC conducting.
- Symmetry: The characteristics are almost identical for positive and negative voltages.
This symmetrical switching behavior ensures that triggering in AC circuits is consistent during both half cycles.
6. Difference Between DIAC and TRIAC
While both DIAC and TRIAC are used in AC switching, they serve different roles:
| Parameter | DIAC | TRIAC |
|---|---|---|
| Terminals | 2 (MT1, MT2) | 3 (MT1, MT2, Gate) |
| Conduction | Bidirectional after breakover voltage | Bidirectional when triggered by gate or DIAC |
| Gate Terminal | No | Yes |
| Usage | Triggering TRIACs | Switching AC loads |
7. Applications of DIAC
The DIAC is widely used in low to medium power AC control circuits. Common applications include:
7.1 Triggering TRIACs
The most popular application of DIAC is to trigger a TRIAC in phase control circuits for dimming lights, controlling fan speeds, and regulating heater power.
7.2 Light Dimmers
DIACs are used to provide symmetrical triggering for TRIAC-based dimmer circuits, resulting in smooth brightness control.
7.3 Motor Speed Controllers
By controlling the firing angle of the TRIAC, DIACs help regulate the speed of AC motors in devices like fans and drills.
7.4 Temperature Control Systems
DIAC-TRIAC combinations are used in thermostats and other heating equipment to maintain desired temperature levels.
7.5 Overvoltage Protection
DIACs can be used as voltage-triggered switches for protecting circuits against voltage spikes.
8. Advantages of DIAC
- Simple and reliable triggering device
- Symmetrical switching ensures minimal distortion
- Low cost and compact design
- No need for an external gate control signal
9. Limitations of DIAC
- Limited power handling capability
- Fixed breakover voltage (not adjustable)
- Requires combination with TRIAC or other devices for load control
10. Conclusion
The DIAC (Diode for Alternating Current) is an essential device in AC power control circuits, especially for triggering TRIACs in a symmetrical manner. Its bidirectional and symmetrical characteristics make it ideal for light dimmers, fan regulators, and heater controllers. Though simple in design, it plays a vital role in ensuring smooth and efficient power control in both industrial and domestic applications.
By understanding the working principle, characteristics, and applications of the DIAC, engineers and electronics enthusiasts can design more efficient and reliable AC control circuits.
