Static Switch Using Triac

Static Switch Using Triac

Circuit Diagram of Static Switch Using Triac :

Instead of using inverse parallel SCRs, we can use the Triac, as a static switch as shown in Figure A. Being a bidirectional device it can be used for connecting the load current in both the half cycles of ac supply.

Figure A

Waveforms for the static switch using triac are as follows :

Figure B

Operation of Static Switch Using Triac :

• The triac acts as a bidirectional static switch.
• The switch SW is used to provide the gate current required to trigger the triac at the desired instant of switching.
• As long as SW is open, I_G = 0 and the triac is off. So it acts as an open switch. The power delivered to the load is zero.
• As soon as SW is closed, the gate current is supplied through R₁, and the triac is turned on. It then acts as a closed switch to provide full power across the load.
• The Triac can conduct in both the half cycles acting as a switch to apply the full supply voltage across the load.
• By varying R₁, we can change the firing angle of Triac and control the load power.
• As soon as SW is open circuited, the triac stops getting the gate current. So it will turn off as soon as the supply voltages passes through zero.