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. :)