Difference Between DIAC And TRIAC

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What Is DIAC (Diode For Alternating Current)?

DIAC is a bidirectional semiconductor switch that can be turned ON in both forward and reverse direction. The device is a member of the Thyristor family and it is mostly used in triggering TRIAC and other Thyristor based circuits. The DIAC starts conducting electric current if the applied voltage goes beyond its break-over voltage.

Most DIACs have a three-layer structure with break-over voltage of approximately 30V. Also DIACs have no gate electrode, unlike some other thyristors that are commonly used to trigger TRIACs. DIAC breakover voltage tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds.

What You Need To Know About DIAC

  • It is a two terminal switching device made of semiconductor material.
  • DIAC is nothing but the combination of two SCRs connected back to back.
  • The DIAC construction uses two P-type materials and three N-type materials without the gate terminal.
  • DIAC can conduct in both directions; it can thus be described as a bidirectional device.
  • DIAC can be switched from its off state to ON state for either polarity of applied voltage.
  • DIAC can be turned on by either the positive and negative half cycle of the AC supply voltage.
  • DIAC can be made either in PNP or NPN structure form.
  • All regions in DIAC are identical in size.
  • Most DIACs have a three-layer structure with breakover voltage of approximately 30V.
  • DIAC terminals are not labeled as anode and cathode, but as A1 and A2 or main terminal MT1 and MT2.
  • DIACs are primarily used as trigger devices in phase-triggering and variable power control applications because a DIAC helps provide a sharper and more instant trigger pulse (as opposed to a steadily rising ramp voltage) which is used to turn ‘’ON’’ the main switching device.
  • The DIAC only starts to conduct the current when the breakdown voltage is reached.
  • The DIAC is designed specifically to trigger TRIAC or an SCR.
  • DIACs are also referred to as ‘’symmetrical trigger diodes’’ due to the symmetry of their characteristic curve.
  • DIAC breakover voltage tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds.

TRIAC (Triode For Alternating Current)?

The TRIAC is a three-terminal semiconductor switching device that is used for controlling current flow in a circuit. It is one of the most important members of the thyristor family, it is a bidirectional device that can pass the current in both forward and reverse direction, which means that they can conduct in both the conditions of the gate signal, positive and negative.

Most TRIACs can be triggered by applying either a positive or negative voltage to the gate (an SCR requires a positive voltage). Once triggered, TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level referred to as the holding current.

What You Need To Know About TRIAC

  • The TRIAC is a bi-directional thyristor, similar to two SCRs connected in reverse parallel but using a common gate connection.
  • TRIAC is a three terminal device.
  • It does not have a gate terminal.
  • TRIAC can conduct in both directions; it can thus be described as a bidirectional device.
  • TRIAC function by either positive or negative gate control voltage.
  • TRIAC can operate in four different modes.
  • It has four layers of semiconductor.
  • TRIAC can control both positive and negative half cycles of AC signal input.
  • The TRIAC control DC as well as AC power.
  • It is less reliable.
  • It needs only one heat sink.
  • Typically, most TRIACs are available in ratings less than 40 Amp and at voltages up to 600 Volt.
  • The forward and reverse characteristics of TRIAC are similar to the forward characteristics of SCR device
  • TRIAC finds its application in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors etc.
  • TRIACs are notorious for not firing symmetrically. This means these usually won’t trigger at the exact same gate voltage level for one polarity as for the other.

Also Read: Difference Between SCR And TRIAC

Difference Between DIAC And TRIAC In Tabular Form

BASIS OF COMPARISON DIAC TRIAC
Acronym DIAC stands for Diode For Alternating Current.   TRIAC stands for, Triode For Alternating Current.
Description A DIAC is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. A TRIAC is a three terminal electronic that conducts current in either direction when triggered.
Description In Relation To SCRs DIAC is nothing but the combination of two SCRs connected back to back.   The TRIAC is a bi-directional thyristor, similar to two SCRs connected in reverse parallel but using a common gate connection.  
Number Of Terminals DIAC is a two-terminal device made of semiconductor material. TRIAC is a three-terminal semiconductor switching device that is used for controlling current flow in a circuit.
Number Of Layers DIAC has three layers of semiconductor material and two junctions. TRIAC has four-layers of semiconductor material.  
Power Handling Capacity The power handling capacity of DIAC is low. The power handling capacity of TRIAC is high.
Trigger DIACs can be triggered through both directions regardless of the supply’s polarity. Most TRIACS can be triggered by applying either a positive or negative voltage to the gate.
Gate Terminal DIACS have no gate electrode and are commonly used to trigger TRIACs. It does not have a gate terminal.  
Nature DIAC breakover voltage tend to be much more symmetrical TRIACs are notorious for not firing symmetrically.
Application DIAC finds its application in AC switches, heat control circuits and speed control of universal motors. TRIAC finds its application in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors etc.