DIAC (Diode for Alternating Current) is a device which is always used in conjunction with other Thyristor devices. This article will discuss in detail about what is DIAC, how it works, its I-V Characteristics, construction, its applications, advantages and disadvantages.
What is DIAC (Diode for Alternating Current)
DIAC is a bidirectional trigger Diode which has two electrodes. It is used to trigger Thyristor and can be turned on/off for both the polarity of Voltages. It is a two terminal device and typically has three-layer structure that conducts current only after its break over voltage.
Fig. 1 – Introduction to DIAC (Diode for Alternating Current)
When this device is applied with the voltage less than its break over voltage to either of its polarity, it remains in high resistance state with only a small leakage current. Basically, they have symmetric switching voltages. Asymmetric DIACs are also available. Its Power dissipation ranges from 0.5 to 1 watt.
Fig. 2, shows the circuit symbol and equivalent circuit of ‘DIACs (Diodes for Alternating Current)’. As they are bidirectional in nature, the terminals of DIAC are not labelled as Cathode or Anode. Instead they are denoted as MT1 and MT2 where MT stands for Main Terminal.
Fig. 2 – (a) Symbol of DIAC (Diode for Alternating Current) (b) Equivalent Circuit
Fig. 3 (below) shows the structure of DIAC. It can be fabricated into 3-layer, 4-layer and 5-layer structure. The three regions in a device are identical and doping concentration of all the layers are same to achieve symmetrical switching characteristics.
Fig. 3 – Structure of DIAC (Diode for Alternating Current)
In a 3-layer structure two P-type layers are sandwiched between N-type layer or two N-type layers are sandwiched between P-type layer. In 4-layer and 5-layer structure two break over Diodes are connected back to back. The I-V characteristics curve of 4-layer and 5-layer is similar to 3-layer curve.
3-layer structure is used in most of the applications.
I-V Characteristics of DIAC (Diode for Alternating Current)
A DIAC is said to be in conducting state when the applied voltage on either of the polarity exceeds the breakdown voltage and its current rises. It exhibits negative resistance characteristics. The Conduction Current rises quickly and hence Resistance is connected in series with a DIAC for a safe operating level of the circuit. It’s Voltage to Current (IV) Characteristics is as shown in the Fig below.
Fig. 4 – (a) PNP Structure (b) I-V Characteristic Curve
Let us consider PNP structure where terminals MT1 and MT2 are connected to P1 and P2 outer layers respectively and are separated by N layer. When the terminal MT1 is more positive with respect to terminal Mt2, junction P1- N is Forward biased and P2-N is Reverse biased. When it reaches break over voltage, the device is said to be in conduction mode and current flows from MT1 to MT2. Similarly the current flows from MT2 to MT1 if the terminal MT2 is more positive than MT1.
The device (Diode for Alternating Current) does not conduct when the applied voltage is very small. Due to less voltage, small amount of current flows known as leakage current which is insufficient to produce Avalanche breakdown.
How does DIAC Work
It is mainly used as a triggering device. To understand the working principle of DIAC, let us consider Heater Control Circuit as shown in the Fig. 5. When the input voltage is applied to the circuit, the capacitors C1 and C2 gets charged as determined by Resistance R2.
Fig. 5 – Heater Control Circuit
It is triggered when the voltage across C3 exceeds the breakdown voltage and conducts. DIAC current rises and Capacitor C3 discharges through the conducting device into the gate of TRIAC. Rise of the input voltage to the heater is controlled by adjusting resistance R2 and R4.
Applications of DIAC
The applications include:
- Diode for Alternating Current is mainly used in lamp dimmer circuit.
- It is also used in Heat Control Circuit.
- It is used as a triggering device for TRIAC’s.
- Speed of Universal Motor can be controlled using them.
Advantages of DIAC
The advantages are:
- Smooth control of Power and Current is achieved using DIACs.
- The device can be turned on or off by decreasing the voltage level.
- Harmonics is reduced to a great extent as the device equalizes the switching characteristics.
- Reliable for low power applications.
Disadvantages of DIAC
The main disadvantage of DIAC is that it needs a minimum of 30V to conduct.
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