Tag Archives: Triac

Basic Types of Thyristors and Applications

In this article we will discuss about the various types of thyristors. Thyristors are 2 pin to 4 pin semiconductor devices that act like switches. For example a 2 pin thyristor only conducts when the voltage across its pins exceeds the breakdown voltage of the device. For a 3 pin thyristor the current path is controlled by the third pin and when a voltage or current is applied to this pin the thyristor conducts. In contrast to trasistors, thyristors only work on ON and OFF states and there is no partial conduction state between these two states. Basic types of thyristors are: SCR, SCS, Triac, Four-layer diode and Diac.

Silicon Controlled Rectifier (SCR)


Silicon controlled rectifier is normally in OFF state but when a small current enters its gate G it goes to ON state. If the gate current is removed the SCR remains in ON state and to turn it of the anode to cathode current must be removed or the anode must be set to a negative voltage in relation to cathode. The current only flows in one direction from anode to cathode. SCRs are used in switching circuits, phase control circuits, inverting circuits etc.

Silicon Controlled Switch (SCS)


Working of SCS is similar to SCR but also it can be turned off by applying a positive pulse on the anode gate. The SCS can also turned ON by applying a negative pulse on anode gate. The current flows only from anode to cathode. SCS are used in counters, lamp drivers, logic circuits etc.



Triac is similar to SCR but it conducts in both directions, means that it can switch AC and DC currents. The triac remain in ON state only when there is current in gate G and switched OFF when this current is removed. Current is flowing in both directions between MT1 and MT2.

Four layer diode


Four layer diode has 2 pins and works like a voltage-sensitive switch. When the voltage between the two pins exceeds the breakdown voltage it turns ON, otherwise it’s OFF. Current flows from anode to cathode.



Diac is similar to four-layer diode but it can conduct in both directions meaning it can contact both AC and DC currents.

Basic SCR Applications

Basic Latching Circuit


In this circuit a SCR is used to form a basic latching circuit. S1 is a normally open switch and S2 is a normally close switch. When S1 is pushed momentary a small current goes into the gate of SCR and turning it ON, thus powering the load. To turn it off we have to push the S2 push-button so the current through SCR stops. Resistor RG is used to set the gate voltage of SCR.

Power Control Circuit


In this circuit a SCR is used to modify a sinusoidal signal so that the load receives less power than of what would receive if source voltage was applied directly. The sinusoidal signal is applied to the gate of SCR via R1. When the voltage on the gate exceeds the trigger voltage of SCR, it goes to ON state and Vs is applied to the load. During the negative portion of the sine wave the SCR is in OFF state. Increasing R1 has the effect of decreasing the voltage applied to the gate of SCR and thus creating a lag in the conduction time. In this was the load is receiving power for less time and thus the average power to load is lower.

DC motor Speed Controller


This is a variable speed DC motor controller using a UJT, a SCR and few passive components. UJT along with resistors and capacitor form an oscillator that supplies AC voltage to the gate of SCR. When the gate voltage exceeds the triggering voltage of SCR, the SCR turns ON and motor is running. By adjusting the potentiometer the output frequency of oscillator is changing and thus the times the SCR triggered is changing, which in turn changes the speed of the motor. In this way the motor is receiving a series of pulses that average over time and the speed is adjusted.

Basic TRIAC Applications

AC Light Dimmer


This is an AC light dimmer formed by a diac, a triac and some passive components. The capacitor is charging through the two resistors and when the voltage on one end of the diac exceeds the breakdown voltage it goes ON and sends a current to the gate of triac putting the triac to ON state and thus powering the lamp. After the capacitor is discharged to a voltage below the breakdown voltage of diac, the diac, triac and lamp turn off. Then the capacitor is charged again and so on. So the lamp is only powered for a fraction of time during the full sinewave. This happens very quickly and the lamp seems dimmed. Brightness is adjusted using the potentiometer.

4 Channel Triac Board


Opto-Isolated 4-Channel Triac Board (R) is used for fast switching applications. Ideal for switching mains circuits, lights, appliances and small load. Each circuit is effectively isolated with opto-isolators on each channel, TTL inputs control each channel.

  • Trigger input 2-12 VDC
  • Mains supply input 230 VAC or 110 VAC
  • Snubber circuit for inductive load
  • Jumper for common cathode ground
  • Box Header connector for connection of control signal
  • Screw terminal connector for Load
  • Four mounting holes of 3.2 mm each
  • PCB dimensions 139 mm x 73 mm

4 Channel Triac Board – [Link]

Triacs – How to calculate power and predict Tjmax


An application note from NXP on how to calculate the power dissipated by the triac:

This Application Note describes how to calculate the power dissipation for triacs and Silicon Controlled Rectifiers. Thermal calculations are also included to help the circuit designer to predict the maximum junction temperature or calculate the required heatsink thermal resistance. Four worked examples ensure that all the power and thermal questions that arise during the design process are covered.

Triacs – How to calculate power and predict Tjmax – [Link]

WIFI Mains Power Dimmer / Switch


MAINS Power Dimmer / Switch Module project by Tracker J:

Now it’s about time to talk also about what many of you has asked for: DIMMING!
It’s a total different story, little bit complicated but, as you will see, not so hard to understand.

General considerations:
There are several types of dimmers generally available. These are used for resistive, and inductive loads, such as incandescent,cold cathode and low voltage (inductive) lamp sources. Note that not all electronic transformers used for low voltage lamps are suitable for dimming by Triac or Thyristors dimmers. In case of Thyristors you need 2 of them as Thyristor is a Unidirectional device and because AC power flows in both directions!

WIFI Mains Power Dimmer / Switch – [Link]

Simple SSR has zero-cross on/off switching


by Dan Meeks @ edn.com:

This Design Idea is a solid state relay (SSR) which uses a triac to switch AC loads at high current. There are plenty of simple SSR circuits available, but this may be the simplest circuit that achieves turn-on and turn-off only when the AC line voltage is near zero.

Simple SSR has zero-cross on/off switching – [Link]

Sleep easy with this desk fan speed reducer


by Michael Whybray:

Most desk fans I have come across have three speeds: Full Speed, Almost Full Speed, and Off – useless if you want just a gentle air movement, and far too noisy if you are trying to get to sleep (in your bedroom of course, not at your work desk!). The squirrel cage induction motors they use have switches to two or more windings – and possibly a capacitor – to reduce the drive current. But unless the drive frequency is also reduced, the torque and speed stability are poor, so minimal speed reduction is usually available on these fans. Using a triac to provide phase control of the voltage works poorly for the same reason, with the speed very sensitive to the triac firing phase angle and fan load, and has a tendency to stall.

Sleep easy with this desk fan speed reducer – [Link]

Triac based lamp dimmer


Triac based Indecent lamp dimmer is a simple circuit and it doesn’’t requires additional power supply, works directly with 110V AC or 230V AC. It is a low cost dimmer circuit for adjusting the light brightness of incandescent, Halogen Lamp, Light Bulb load up to 250 W.

Triac based lamp dimmer – [Link]

Renesas Computerized Electric Fan

Electric fans continue to sell at a steady pace, despite the widespread use of air conditioners. Recently, models have begun to appear on the market with features such as a variety of blade shapes, silent operation, and better air efficiency. Electric fans are also increasingly being used in concert with air conditioners to save electricity through better cooling efficiency.

Renesas offers microcontrollers such as the 4286 Group which is ideal for remote control function, the RL78 Family, the R8C Family, and the 78K Family which are ideal for system control, and TRIACs and photocouplers for peripheral components. We offer the best solutions for electric fan systems.

Renesas Computerized Electric Fan – [Link]

400VA AC Light Dimmer


This is a simple TRIAC AC load dimmer used to control the power of a resistive load such as incandescent lamp or heater element. The max load it can handle is 400VA. Such a circuit is often found on cheap commercial light dimmers and is proven to work reliable for the rated power.

400VA AC Light Dimmer – [Link]