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What is DIAC and why is it used?
The DIAC (diode for alternating current) is a diode that conducts electrical current only after its breakover voltage, VBO, has been reached momentarily. DIACs have no gate or trigger electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs.
What is used for triggering TRIAC?
Most TRIACs can be triggered by applying either a positive or negative voltage to the gate (an SCR requires a positive voltage). Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current.
What is a DIAC used for?
The DIAC is an electronics component that is widely used to assist even triggering of a TRIAC when used in AC switches and as a result they are often found in light dimmers such as those used in domestic lighting. These electronic components are also widely used in starter circuits for fluorescent lamps.
How is the DIAC connected to the TRIAC?
The DIAC is connected in series with the gate of a TRIAC. The DIAC do not allow any gate current until the triggering voltage has reached to a certain repeatable level in both directions.
How is a diac triggered in an AC circuit?
The AC mains waveform is effectively delayed or phase shifted by the RC circuit so that the diac is triggered by a discharge of current from the capacitor C1 into the triac gate. The triac then conducts for the remainder of the mains half cycle, and when the mains voltage passes through zero it turns off.
What causes a positive trigger pulse at the TRIAC gate?
At this point capacitor C will discharge current into the diac, causing a positive trigger pulse at the triac gate.
How does a TRIAC work in an AC controller?
The supply voltage of the triac to switch ON depends upon the gate current. This allows utilizing a triac to regulate AC power in a load from zero to full power in a smooth and permanent manner with no loss in the device control.