Silicon-controlled rectifiers (SCR) are solidstate semiconductor devices that are usually used in power switching circuits. SCR controls the output signal by switching it ‘on’ or ‘off,’ thereby controlling the power to the load in context. The two primary modes of SCR control are phase-angle fired—where a partial waveform is passed every half cycle—and zerocrossing fired—where a portion of the complete waveforms is passed to regulate the power.
In the phase-angle controller, the firing pulse is delayed to turn on the SCR in the middle of every half cycle. This means that every time a part of an AC cycle is cut, the power to the load also gets cut. To deliver more or less power to the load, the phase angle is increased or decreased, thereby controlling the throughput power.
There are several ways to control the firing angle of SCR. This article describes a microcontroller AT89C51-based phase-angle controller. A microcontroller can be programmed to fire SCR over the full range of half cycles—from 0 to 180°—to get a good linear relationship between the phase angle and the delivered output power.
Some of the features of this microcontroller-based phase-angle controller for SCR are:
1. Utilises the zero-crossing detector circuit
2. Controls the phase angle from 0–162°
3. Displays the phase angle on an LCD panel
4. LED indicators are used for displaying the status of SCR
5. Increases or decreases the phase angle with intervals of 18°
Basically, the zero-crossing detector circuit interrupts the microcontroller after every 10 ms. This interrupt commands the microcontroller to generate some delay (in the range of 1ms to 9 ms). The user can increase or decrease the delay in intervals of 1 ms using switches. the SCR is then fired through the opto-coupler. This repeats after every 10 ms.