Principle of Phase Angle Control
Top – Output Voltage
Bottom – Gate Drive Signal
The photo above clearly illustrates phase angle control: output voltage controlled by the gate drive signal applied to a thyristor. What is phase angle control? That is what I’m going to talk about in this article.
Phase angle control is a method of PWM applied to AC input voltages, usually the mains supply. Of course, the AC supply could be from a transformer or any other AC source, but the mains supply is the most common input – this gives the phase angle control method its greatest usefulness. It has of course become quite obvious from the title (and I’m sure most of you reading will already know this) that the purpose of phase angle control is to control or limit power to the load.
The power device used in phase angle controllers is a thyristor – mostly triacs or SCRs. (There are methods of phase controlling employing high frequency switching using a MOSFET or IGBT, but here I’ll talk about phase angle control with thyristors only). The power flow to the load is controlled by delaying the firing angle (firing time each half-cycle) to the power device.
We know that the thyristor is a latching device – when the thyristor is turned on by a gating signal and the current is higher than the holding current and the latching current, the thyristor stays on, until the current through it becomes sufficiently low (very close to zero). The thyristor turns off when current through it becomes zero, as happens at the AC mains zero crossing. This is the natural line commutation. (Another method of turning the thyristor off is by forced commutation. I won’t go into that now.) The assumption here is that the load is resistive and has little to no inductance. Of course, this is not always the case, as inductive loads are often used. However, I’ll work with this assumption for now.
I’ve added the circuit, code and simulation of an example later in this article. And that uses a triac as the power device. So, from now on, I’ll just refer to the triac instead of talking about a thyristor in general.
So, in phase angle control, a gate pulse is sent to the triac. This is sent at a time between one zero crossing and the next. Without the gate pulse sent to the triac, right after zero-crossing, the triac is off and no current flows through it. After a certain time, the gating signal is given to the triac and it turns on. The triac then stays on until the current through it becomes zero (natural line commutation). This is at the next zero crossing. For simplicity’s sake and as usually should be, assume that the current through the triac (when on) is larger than the latching current and the holding current. If you didn’t already know this, the latching current is the current that must pass through the triac right after it is turned on to ensure that it latches. The holding current is the current level through the triac below which the triac will turn off. So, the assumption that current through the triac is higher than the latching current and the holding current means that the triac stays on once it is fired on. It stays on until the current through it is zero.
This means that the voltage is supplied to the load for a fraction of the cycle, determined by how long the triac is on. How long the triac is on, is, in turn, determined by the delay time between the zero-crossing and the applying of the triac gating signal.
So, to sum it up, we adjust the voltage or power delivered to the load by delaying the trigger signal to the triac. One thing to remember is that, the delivered voltage and power are not linearly related to the firing phase angle.
There are two voltages here that we are concerned with – the RMS voltage and the average voltage. The RMS voltage governs the power output to resistive loads such as incandescent bulbs and resistive heaters. The average value relates to devices that function on the average voltage level. This is important because, when testing, your voltmeter will register the average voltage – and not the true RMS voltage – unless you have a “true RMS voltmeter”. Most inexpensive voltmeters are not true RMS meters but will respond to average value changes.
To clarify why power and voltage are not linearly related, let’s examine the formula relating the two.
So, assuming a constant resistance (be careful if you’re using incandescent lamps, since they are NOT constant resistance devices), power is directly proportional to the square of the voltage. So, if you half the voltage, the power is not halved, but is reduced to one-fourth the original power! One-fourth power with half the voltage!
Now let’s now go on to the design part – how we’re actually going to do this.
For the microcontroller, I’ve chosen the extremely popular PIC 16F877A. However, since this application requires only a few pins, you can easily use any other small microcontroller for this purpose, such as PIC 12F675.
The zero-crossing is done using the bridge-optocoupler method.