A DIAC is a two-terminal electronic device used in AC circuits to control current voltage. It stays off at low voltage and turns on suddenly at a fixed breakover level. It works the same in both directions, making switching balanced and predictable. This article provides detailed information on its structure, operation, characteristics, applications, and limitations.
DIAC Overview
A DIAC (Diode for Alternating Current) is a two-terminal electronic component that controls current voltage. It stays in the OFF state when the applied voltage is low. When the voltage reaches a fixed level called the breakover voltage, the DIAC suddenly turns ON and allows current to flow.
The DIAC works the same in both directions, so it can handle positive and negative voltages equally. Unlike a normal diode, it does not guide current in one direction or conduct at small voltages. This makes its switching action predictable and balanced in AC circuits.
DIAC Construction
A symmetrical stack of P and N semiconductor layers forms a bidirectional switching path between MT1 and MT2. The internal regions are arranged so that no current flows at low voltages, even though a potential difference exists across the terminals. This structure keeps the device in a non-conductive state during normal conditions.
With MT1 positive relative to MT2, the upper and lower junctions experience different bias conditions. As the applied voltage rises to the breakover level, the internal junctions abruptly switch into conduction, allowing current to flow from MT1 to MT2 through the layered structure.
When the polarity is reversed, the same process occurs in the opposite direction. Once the breakover voltage is reached, current flows from MT2 to MT1. This equal response to both polarities explains the DIAC’s role as a reliable trigger in AC control circuits.
Symbol of DIAC .
Two opposing triangles placed tip-to-tip represent the bidirectional nature of a DIAC. This symbol indicates that the device does not have a preferred current direction and can respond equally to both positive and negative voltages.
MT1 and MT2 are shown as the two main terminals, sometimes labeled as Anode 1 and Anode 2. Either terminal can become positive or negative during operation, depending on the AC waveform. The absence of a gate or control lead highlights that conduction begins only when the applied voltage reaches the breakover level.
Basic Operation of a DIAC
DIAC operation depends on which terminal is positive. When MT1 is positive with respect to MT2, the P1 layer near MT1 becomes active. Current starts flowing through the internal layers in the sequence P1–N2–P2–N3. In this condition, the P1–N2 and P2–N3 junctions are forward biased, while the N2–P2 junction remains reverse biased until the breakover level is reached and conduction begins.
When MT2 is positive with respect to MT1, the P2 layer near MT2 becomes active instead. Current then flows in the opposite direction through the layers P2–N2–P1–N1. Here, the P2–N2 and P1–N1 junctions are forward biased, while the N2–P1 junction is reverse biased until switching occurs. Because the same process happens for both polarities, current conduction is possible in either direction once the required voltage level is reached.
Current–Voltage Characteristics of a DIAC
The V–I characteristic of a DIAC has a Z-shaped form and appears in the first and third quadrants of the graph. This shape shows that the DIAC can conduct current in both directions. The first quadrant represents the positive half cycle, where current flows from MT1 to MT2. The third quadrant represents the negative half cycle, where current flows from MT2 to MT1.
Initially, the DIAC exhibits very high resistance due to some internal junctions being reverse-biased. Only a minimal leakage current flows during this stage, which is known as the blocking state. When the applied voltage reaches the breakdown voltage, the DIAC suddenly switches on. Its resistance drops sharply, the voltage across it falls, and the current increases quickly. This region is called the conduction state. Most DIACs have a breakdown voltage of about 30 V, though the exact value depends on the device type. Once turned on, the DIAC remains conducting until the current falls below a minimum level called the holding current, which is the lowest current needed to keep the DIAC in the ON state.
Electrical Specifications of a DIAC
| Parameter | Typical Value |
|---|---|
| Breakover Voltage (VBO | 28–36 V |
| Holding Current (IH) | 5–50 mA |
| On-state Voltage Drop | 2–3 V |
| Peak Current | Low (trigger-level only) |
| Power Dissipation | ~300 mW |
Common Applications of DIACs
Light Dimmers
DIACs provide a stable and symmetrical trigger for TRIACs in light dimmer circuits. This helps control the conduction angle evenly in both AC half cycles, allowing smooth brightness adjustment.
Fan Speed Controllers
In fan speed control circuits, DIACs support balanced triggering during positive and negative cycles. This helps keep the fan speed steady without uneven switching.
Motor Speed Regulators
DIACs assist in controlling the switching point in AC motor speed regulators. Their fixed breakover behavior allows controlled and gradual speed changes.
Heater and Temperature Control Circuits
DIACs help regulate power supplied to heating elements. Their bidirectional switching supports consistent operation across both halves of the AC waveform.
TRIAC Gate Triggering Networks
DIACs are placed between the control circuit and the TRIAC gate to ensure triggering occurs only after a set voltage level is reached. This improves switching stability and repeatability.
DIAC Selection Tips
• Match the DIAC breakover voltage with the RC timing range to ensure proper switching.
• Check that the power dissipation rating is high enough for the expected current and heat.
• Prefer symmetrical DIACs to maintain balanced conduction in both AC directions.
• Avoid operating the DIAC close to its maximum voltage rating to keep operation stable.
DIAC Operating Limitations
• Not suitable for handling high current levels
• Triggering point is fixed and cannot be externally adjusted
• Limited to low-power signal and triggering functions
• Sensitive to rapid voltage changes, which can cause false triggering\
DIAC Compared with TRIAC and SCR
| Feature | DIAC | TRIAC | SCR |
|---|---|---|---|
| Terminals | 2 | 3 | 3 |
| Direction of Operation | Bidirectional | Bidirectional | Unidirectional |
| Gate Control | No gate control | Gate-controlled | Gate-controlled |
| Primary Role | Provides a trigger signal | Switches the AC power | Controls rectified power |
| Typical Function | Initiates TRIAC conduction | Regulates AC load current | Manages controlled rectification |
Conclusion
The DIAC functions as a voltage-triggered switching device with equal response to positive and negative voltages. Its sharp breakover behavior, simple structure, and bidirectional operation make it suitable for triggering and controlling roles in AC circuits. Its fixed triggering point and low current capacity limit it to specific low power switching and support functions.
Frequently Asked Questions [FAQ]
Can a DIAC be used in DC circuits?
A DIAC is mainly designed for AC circuits. In DC circuits, it can turn on only once when the breakover voltage is reached, but it will not switch off easily because the current does not naturally drop to zero.
What happens if a DIAC overheats during operation?
If a DIAC overheats, its electrical characteristics can change, leading to unstable triggering or permanent damage. Excess heat can reduce reliability and shorten the device’s operating life.
Are all DIACs identical in size and package type?
No, DIACs come in different package types and sizes. The choice depends on power dissipation needs, mounting method, and available circuit space.
Does temperature affect the breakover voltage of a DIAC?
Yes, temperature can slightly affect the breakover voltage. Higher temperatures usually lower the breakover point, which may cause earlier switching.
Can multiple DIACs be connected in parallel or series?
Using DIACs in parallel or series is uncommon because voltage sharing may become uneven. Small differences between devices can cause unstable operation.
How fast does a DIAC switch ON after reaching breakover voltage?
A DIAC switches on very quickly, typically within microseconds. This fast response supports accurate and repeatable triggering in AC control circuits.