triac firing angle control circuit

triac Firing angle control circuit is designed to control flow of AC power from input supply to load by changing the average voltage appearing across load. In this project triac firing angle is controlled to control amount of power flow to load. Firing angle control circuit is designed using analog electronics components like operational amplifier, resistor and capacitor. zero crossing detection, step down transformer , rectifier , comparator and ramp generator are the basic components of firing angle control circuit for triac. Triac have many applications in power electronics especially in AC voltage controller circuits.

Applications :

AC voltage controller have many applications in power electronics  devices. some of them are mentioned below :

• induction heating
• Fan dimmer
• Speed control of induction motor
• industrial heating

Basic components of Firing angle control circuit for triac is shown below in circuit diagram.

Zero crossing detection circuit :

It is used to detect zero crossing of sine wave, when AC sine wave cross zero reference voltage. Zero reference voltage is used to determine firing angle time duration for positive and negative cycle of sine wave. For zero reference detection circuit, step transformer of rating 220-12 volt is used to step down voltage and rectifier bridge is fully rectify this step down voltage. As shown in figure after full bridge, comparator is used. Full bridge output is connected with inverting input of comparator and diode is connected with non-inverting input of comparator. So when voltage at non inverting pin become grater than inverting output of comparator becomes high and pulse is produce at output of comparator for every zero crossing detection. Comparator output is shown below:

Ramp generator :

Output of comparator is connected with transistor and capacitor keep charging through 12 volt source. Transistor turn on only when zero crossing occur, because at zero crossing comparator produce pulse which turn on transistor and when transistor is on when across capacitor become immediately zero.In this way ramp is produce, when transistor is off capacitor keep charging and when transistor is on, voltage of capacitor become zero as shown in figure below :

Ramp voltage is again compared with a output voltage from variable resistor. If ramp voltage is greater than variable resistor output voltage, greater power will appear across load. Hence this variable resistor is used to control firing angle of triac. Optocoupler MOC3021 is used for isolation between high voltage side and low voltage control signals side.complete circuit diagram of firing angle control circuit for triac is shown below :

Diagram below shows output of voltage appearing across lamp. I have used lamp in Proteus for simulation purpose. Simulation results of  firing angle control circuit for triac is shown below :

When output of second compartor is on, triac is also on and voltage is also appearing across load. When output of second comparator is off triac is also off and zero voltage is appearing across load. Variable resistor can be used to adjust turn on or off time and in return voltage magnitude across load.

To understand working of  firing angle control circuit for triac watch following video :

 Top view :

Front view :

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I hope that you have gained some useful knowledge by reading this article and watching above video. If you have found it useful.kindly don’t forget to share it with others.More you share, more you gain. 🙂