Build your own digital dimmer - part 1

What is Dimmer?

A clasical method to varry ac voltage is achieved using transformers, and then precise control of the require rms load voltage. The newest method to do this, by presence a Triacs or inverse-parallel connected Thyristors.

Dimmer or ac phase control (see Figure 1) is generally known as a TRIAC-based converter (see Figure 1) which is used to vary the magnitude of ac voltage. To doing this, TRIAC must be sliced a part of the ac main waveform—so that a part of the output voltage waveform will be dissappear. Thus, the RMS of output voltage become decreased.

In phase control method a part of the sinusoidal voltage waveform is blanked out each half or full cycle. In the other way, burst-firing control create a complete half or full cycles are blanked out., burst-firing generates less in the way of harmonies since it switches at mains voltage zero, but it is not suitable for some loads. Phase control can be used for any of these applications, but burst firing is not suitable for mains frequency filament lamps due to lamp flicker, or for motor control due to fluctuating torque. Heater control is suited to burst firing due to the long thermal time constant of the heating element. Other applications of Triacs are in spot welding and as solid-state contactor.

Block diagram:

Figure 1. Dimmer block diagram

Circuit Operation:
The TRIAC is turned on by using low voltage short-duration pulse to the gate. Once ON, the gate loses control and the TRIAC remains ON until the load current falls to virtually zero, or at mains voltage zero. In the ON state (1V voltage drop in Triacs). To synchronize the firing pulse with source voltage, here we use the ZCD circuit. The rms load voltage is then varied using firing angle delay; the smaller the delay, the greater the rms load voltage.

Figure 2. Dimmer Circuit

Let’s we take a look the ilustration in Figure 2. From Figure 2, we can say that “ TRIAC firing pulses are slicing the ac main waveform in two portions; one for TRIAC and one for load (ouput)“. 

When we used dimmer?

Dimmers are commonly used for dimming a lamp, heater for coffe mechine, and ac motor speed control (e.g washing machine and ventilator)

   

How can be firing pulses sliced the ac main waveform?

In dimmer circuit, a TRIAC acts as a switch (see TRIAC basic). It’s OPEN and CLOSED the circuit according to the firing pulses. When TRIAC is fired, the voltage accross the TRIAC is equal to zero (or about 1V in practice). So, It will closed the circuit, then the the current will flow to the load.But, when the firing pulse is not exist, the TRIAC will block a part of the ac main voltage. Thus it  open the circuit and there is no current flow to the load.

 

Figure 3. Dimmer Works

How can dimmer reduce the rms output?

If the delay angle (alpha, )  also called firing delay angle (FDA) is increased, the rms output become reduced. This is determined by the equatin below (Fewson, 1998):

 

Where

Vrms is the rms of voltage waveform output

Es is the rms value of ac main

(alpha) is delay angle (in rad)

Phi () is 3.14 also in rad 

The visual relationships of Conduction anggle(theta q) and the output voltage rms is shown in Figure 4. Conduction angle is the time that TRIAC conduct (say ON state), which define by 180°-a.

 Figure 4. Characteristics

What about the avg voltage output?

Because of the positive and negative portion in output voltage waveform are equal, so the average value is not exist (or say that Vavg ouput is equal to 0). This is proven by the equation below.

Circuits:

This is the digital dimmer circuit shown in Figure 5.

 

Resouces:
Fewson, D. (1998). Introduction to Power Electronics. Oxford University Press, Inc., New York
AND8011 - ON Semiconductor