AC Dimmer Circuit Using TRIAC

The circuit that we will be discussing today is a TRIAC dimmer, often known as an AC dimmer circuit by some of them. Today, we will study the operation of an AC dimming circuit, the appropriate operation of a TRIAC, and the significance of this highly valuable and crucial component. Will also demonstrate several AC dimming circuits and test them using motors, AC lights, and other AC devices.

What is a Triac?

A three-electrode, bidirectional AC switch that permits electron passage in both directions is called a TRIAC device. It is equivalent to having two SCRs connected in reverse parallel with gates attached between them. An SCR gate signal causes a TRIAC to conduct in both directions. TRIACs were created to provide a way to create better AC power controls.

TRIACS are available in a range of package configurations. A large range of voltages and currents are compatible with them. When compared to SCRs, TRIACs typically have lower current capabilities. Their typical operating range is less than 50A, therefore they cannot replace SCRs in high current applications. Because of their capacity to function with either a positive or negative voltage across their terminals, TRIACs are frequently regarded as being extremely adaptable. Using a TRIAC is a superior way to control low power in an AC circuit because SCRs have problems conducting current in only one direction.

First Method of AC Dimmer


AC Dimmer Circuit Using TRIAC

Circuit Schematic



C89621 DIAC
500KΩ Potentiometer
100nF/400V Capacitor
100nF/600V Capacitor
Resistor (100Ω, 4.7KΩ)
Connection Wire

Working Principle of First AC Dimmer Circuit

This circuit for an AC dimmer is entirely analogue. It merely has a few passive components and neither a microcontroller nor a digital component. This is the simple circuit that was previously stated. We provide resistors, capacitors, TRIAC (also known as DIAC), and other components. A potentiometer is required in order to adjust the firing pulse time, which also allows us to adjust the output power. The circuit used by this module is the same one.

What is the purpose of using this straightforward circuit to apply pulses to the TRIAC gate now that we don’t have a microcontroller? That needs to be examined. The DIAC component permits bidirectional current flow, but only if a specific voltage threshold is met. Thus, we turn off the power at the beginning and then apply the AC voltage at the input. The output is still zero even if the TRIAC is now off. The capacitor C1 will be charged by a tiny current passing via the potentiometer and R2 resistor. The TRIAC is activated when the voltage across its capacitor surpasses the DIAC’s limit value, allowing current to flow through its gate.

Following that, the TRIAC will permit current to flow while the output is on until the AC voltage’s polarity changes and the TRIAC is turned off. However, because the AC voltage is now placed to the negative side, the tiny capacitor is now charged with a negative polarity. Once more, the DIAC permits current flow and triggers the TRIAC gate when the capacitor reaches a predetermined limit value.

As you can see, those firing pulses at the TRIAC gate are produced simply by charging the tiny capacitor. The resistor and potentiometer values regulate how quickly the capacitor charges. The firing pulse is applied later and the output power decreases as the resistance value increases due to the slower charging process. For this reason, a potentiometer is employed, which has the ability to control this resistance value, making it easier to adjust the output power and enabling a quicker or slower charging process.

Second Method of AC Dimmer

Circuit Schematic



Bridge Rectifier
500KΩ Potentiometer
100nF/400V Capacitor
100nF/600V Capacitor
470Ω Resistor (x2)
3.3KΩ Resistor (x2)
Connection Wire

Working Principle of Second AC Dimmer Circuit

This module is different from the other one, yet it functions similarly. However, it differs slightly in that it makes use of a tiny full bridge rectifier. I’ve worked on the PCB, made some modifications, and am aware of how it functions. A diagram of it is shown above. Although it appears somewhat similar to the other one, you should be aware that when the TRIAC receives a lot of power, it heats up more due to power losses. To manage that heat, AC dimmers are equipped with a cooling system. For instance, the BTA24 has a maximum operating voltage of 800V and a maximum current of 25 amps. However, before attempting to create such a circuit, make sure to confirm the datasheet of each and every component.

Remember that the firing time will vary depending on the potentiometer and capacitor values. The charging procedure will be slower the larger the value of the capacitor. The used resistor is subject to the same rules. Typically, we include a fuse to regulate the highest power output.

This circuit, for instance, has a 20 amp fuse. Additionally, SMD resistors are utilised, as seen on the module’s PCB, however they are somewhat large and come in 25 12-packages because they can withstand higher power. Since we are working with AC here, the capacitor that is employed needs to be non-polarized and rated for high voltage.

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