Switching high voltage load using TRIAC

TRIAC

The TRIAC (triode for alternating current) is an ideal power electronics switch to use for switching applications because it can control the flow of current over both positive as well as negative half cycle of alternating waveform. Its also having advantage of low cost over back to back thyristor circuit, To control current upto 4A, voltage upto 600V and low inrush I recommended Triac, above that back to back thyristor can work fine.

Controlling high voltage devices using optically isolated power electronics device give advantages of voltage control. This simple TRAIC circuit BT136 and Opto-coupler MOC3021 can able to control high voltage devices from microcontroller. For example, arduino to control 230/220v bulb or any device which runs on high voltage. This circuit can also work for dimming and speed controlling application using PWM signal from arduino.

As the TRIAC having bidirectional valve this circuit used for AC as well as DC applications.

Circuit Diagram 

 

Triac BT136 as a Switch

Bill of Materials

S.No

Reference

Value

Part

Quantity

1

R1 330 Ohms Resistor ¼ watt

1

2

R2 360 Ohms Resistor 1/2 watt

1

3

R3 470 Ohms Resistor 1/2 watt 1
4 R4 39 Ohms Resistor 1/2 watt

1

5

C1 50nf/0.05uf,250/400V Polyester capacitor 1

6

C2 10nf/0.01uf, 250/400V Polyester capacitor

1

7 U1 MOC3021/3031/3051 Optocoupler IC

1

8

U2 6 PIN IC Socket - 1
9 TRIAC BT136 Triac

1

10

LAMP 100W 230V lamp Incandescent lamp

1

11

PCB 4 cm×10 cm 1

12

Flexible Wire 1
13 Soldering Iron

1

14

Soldering metal wire

1

15

J1 PBT Connector 2 Pin 1
16 J2 Male Berg Strip Straight 2 Pin header

1

17

Lamp Holder 1

Working of TRIAC CIRCUIT

During ON Condition :
When 5v/3.3V  is applied from microcontroller to the optocoupler MOC3021 devices consist of gallium arsenide
infrared emitting diodes at pin 1 & 2. This diode emit infrared light and trigger the optically coupled light activated silicon bilateral switch at pin pin 6 & 4 that allows current to flow between them. This supply provides GATE current to the TRIAC gate (pin 3 of TRIAC) and TRIAC starts conducting the main current between pin MT1 and MT2.

During OFF Condition :
When 0V is applied to between pin 1 & 2 of optocoupler, pin 6 & 4 act as opened switched and doesn't allow any flow of current between them, as their is no GATE current to TRIAC, it stops conducting.

Application of TRIAC SWITCH

  • Solenoid/valve controls
  • Static power switches
  • Temperature controls
  • AC motor starters
  • Lighting controls
  •  AC motor drives
  • E.M. contactors
  • Solid state relays

Disadvantages of TRIAC

In normal failure mode of electromechanical relay act as open switch while all SSR(power electronics Switches) acts as short. Which may lead to continuous supply to load at failure condition.

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