AC phase control converts a fixed ac input voltage to controllable ac output voltage. The output voltage (effective or rms value) is adjustable. The ac phase control can be found in many applications, such as:

• Light dimmer
• Egg incubator
• Ac voltage regulator
• Fan speed control
• Many more … (try list by yourself!) 😌

AC phase control also is known as dimmer or ac chopper. AC phase control circuit commonly used TRIAC or SCR to adjust the output voltage. AC phase control circuit only regulates the rms value of ac output voltage, not the frequency. The block diagram of ac phase control is shown in the following:

Fig. 1 Block diagram of AC Phase Control

Before go in depth, let take a look a short introduction about the principle of operation of ac phase control. I will explain it using a basic schematic of this circuit (single phase) as depicted in Fig. 2. In single phase, ac phase control composes by a single switch. The load in this example is resistive load. 

                                                Fig. 2 Single phase AC phase control circuit

Take a note: while the Switch (Sw) is open, no current flow trough Sw (i=0A), which mean that the output (or load) voltage is equal to zero (Vo=0V) and the voltage accross the Sw terminals is the same as the source (Vsw=Vo). Meanwhile, Sw is closed, the current flow through Sw to the load. The output Voltage (Vo) will be depended on the Vo= R(load).i. But, the voltage accross the switch is zero (Vsw=0V)

Fig. 3 Waveform

• Voltage v_s is a sinusoidal input to the basic circuit shown in Fig 2 (it can be
  considered as the mains 220VRMS, 50 Hz). During the positive half cycle of input
  voltage, the power flow is controlled by varying the delay angle of the thyristor MT1;
  and thyristor MT2 controls the power flow during the negative half cycle. The firing
  pulses of MT1 and MT2 are kept 180º (π radians) apart. We also can consider the firing delay angle in time delay with simple equation (2).

 Table 1. Converter a to t_d

time	Firing delay angle (FDA)		Time delay, td (ms) for f=50Hz
	a (°)	a (rad)
0	0	0	0
T/24	15	p/12	0.83
T/12	30	p/6	1.67
T/8	45	p/4	5
…	…	…	…
T/2	90	p	10

• The waveforms for the input voltage, output voltage and gating signals for TRIAC are shown in Fig. 3. The output voltage rms can be calculated as follow:

• Since the load is resistive, thus the output current is Io = Vo/R
• The output power can be calculated as Po= Vo. Io = IoxIoxR = VoxVo/R
• Since the Io = Iin, thus the input power and efficiency are calculated in (5) 
• The power factor is determined by (6)

In order to design the simple ac phase control circuit, we must consider several practical steps as follow:

• Step 1. Try to determine the maximum power rating, input voltage and output current rating for your application. For example, we want to design ac phase control which has ratings as bellow:
• Maximum Output Power (Po, max) is 100W. Note that for several applications which need a specific minimum power, such as egg incubator, you must define it here. For example, the minimum necessary voltage for heating the egg is 20W (just an approximation value), so you should consider this value carefully. Later, we can also take care of it from the control circuit. Thus, in this example, we only consider the maximum output power of our simple ac phase control circuit.
• Input voltage (Vin) = 220 V.
• Frequency (f) = 50 Hz (this is important for us to calculate the firing angle or time delay for control circuit).
• Maximum output current (Io, max) = 1 A (Remainder: the current rating is very important for calculating the rating of fuse or protection circuit). You can refer this website for more information of selecting fuse. Note that: in order to prevent damage to components, the selected fuse current rating should be large enough compare to current capability so that the fuse will not open under steady state conditions, yet will only open during an abnormal condition, such as overload or short-circuit. In practice, 150% to 200% of the maximum input current (steady state) at maximum load and minimum line input voltage.
• Step 2. Chose the power semiconductor that you prefer more. You can use TRIAC or SCR, no worries. 
• In this article, consider that you want to use TRIAC. To regulate the AC output voltage, you only need one (1) TRIAC. Back to the block diagram above, you can replace the box with TRIAC. I provide you the example of the use of TRIAC in ac phase control circuit. 

• Step 3. Select the main component (TRIAC or SCR) rating. If you are new in this design, you may read my first article in TRIAC basic. For our case, we need TRIAC which can handle 220V (red circle) with a current rating of 1A (red square) or more is better. You can download datasheet of BT131 from NXP semiconductor here. 

• Step 4. The next is how to control the TRIAC. You better to see my next article. :)

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