DIAC- Diode for Alternating Current

DIAC- Diode for Alternating Current

DIAC- DIode for Alternating Current
The name DIAC comes from the word DIode for Alternating Current.The DIAC is a thyristor that acts like two back-to-back diodes. 
  It can conduct current in either direction, hence DIAC is a bidirectional device. It is normally used in AC circuits.
     It is two terminal device. DIAC have no gate terminal, unlike some other thyristors they are commonly used to trigger, such as SCR, TRIAC.
     Current flow in both directions hence,
  their terminals are labeled as A1 and A2 or MT1 ("Main Terminal" 1) and MT2 ("Main Terminal" 2).

· When the breakdown voltage is reached ,Diac start conducting. At that point, the DIAC goes into avalanche conduction, which creates current pulse sufficient to trigger another thyristor (an SCR or TRIAC). 
The DIAC will remains in conduction as long as the current is above the holding current (Ih).
When this occurs, ON state resistance of the diode abruptly decreases, due to this  sharp decrease in the voltage drop across the diode occurs and hence current flow through the diode  increases sharply. 
V-I Characteristics of  DIAC
DIAC is similar to two PN-Junction diode connected in anti parallel.
Therefore, it is expected that the V-I characteristics of DIAC in the 1st and 3rd quadrant of the V-I plane will be similar to the forward characteristics of a PN-Junction diode. 
Forward biased characteristics:-When MT1 connected to positive terminal of the supply with respect to MT2, Diac is  in forward biased.
Reverse biased characteristics:-When MT2 connected to positive terminal of supply with respect to MT1, Diac is in reverse biased.
The diode will remains in conduction until the current flow through it drops below the holding current. Below this value, the diode switches back to OFF state.
 Generally the breakdown voltage of DIAC is around 30 V.
The main drawback of the diac is that it cannot be triggered at just any point in the AC power cycle.
It triggers at its preset breakover voltage only.

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