Phase Control with Thyristors

An effective and widely used method of controlling the average power to a load through a triac is by phase control. Phase control is a method of using the triac to apply the ac supply to the load for a controlled fraction of each cycle. In this mode of operation the triac is held in an off, or open, condition at a time in the half-cycle determined by the control circuitry. In the on condition the circuit current is limited only by the load, i.e. the entire line voltage (less the forward drop of the triac) is applied to the load.

As one might expect, in spite of its usefulness, phase control has some disadvantages. The main disadvantage is the generation of electromagnetic interference (EMI) in triac applications. Each time the triac is triggered the load current rises from zero to the load-limited current value in a very short time. The resulting di/dt change generates a wide spectrum of noise that may interfere with the operation of nearby electronic equipment unless proper filtering is used.

The following figure shows the concept of the phase control:

During the first portion of each half-cycle of the sine wave an electronic switch is opened to prevent the current flow. At some specified phase angle (Alpha) this switch is closed to allow the full line voltage to be applied to the load for the remainder of that half cycle. Varying Alpha will control the portion of the total sine wave that is applied to the load (shaded area), and thereby regulate the power flow to the load.

The following schematic shows a digital dimmer with phase control from 10% to 95% of the ac sine wave:

click for larger image

The operational amplifier (LM339) is performing a zero-crossing function and, due to this, the integrated circuit (555) is being activated each time a zero-crossing condition occurs. Once activated the 555 will wait a short time before it triggers the transistor and, in turn, the triac (MAC9M). Because of the delay time the triac will be activated at different points (phase angles) of each half-cycle of the ac sine wave. The load will therefore receive only a portion of the current waveform, determined by the amount of delay time set. This delay time is selected by the adjustable resistor which, in this example, is 2.5 kW and is able to control almost 100% of the complete ac sine wave.

On the other hand, the kind of triac to be used in the dimmer circuit is determined by the load characteristics, taking into account the current, voltage, power, and physical characteristics (either an inductive or resistive load). It is very important to select the proper triac according to the load requirements.

In conclusion, the Digital Dimmer described here offers a very reliable control phase (from 10% to 95% of the ac sine wave). It can be an extremely useful and inexpensive option for lighting applications, or single-phase motor speed controls, or any other kind of application circuit where a reliable control phase is needed. Nevertheless, it is important to remember that the main disadvantage of using phase control in applications is the generation of electromagnetic interference (EMI.) Proper filtering needs to be used in order to eliminate EMI, otherwise the noise would interfere with the operation of nearby electronic equipment. Another factor that is important is that extreme environmental temperatures could cause poor operation of the dimmer and cause incorrect phase controls for the load. Therefore, it is necessary that the designer determine under what conditions the dimmer needs to operate in order to select the correct components.

This Tech Note has also been published in a slightly altered form as Application Note AND8011/D and is available from ON Semiconductor; http://www.onsemi.com.

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