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The Benefits of TRIAC-based Control

Tue, 08/31/2010 - 9:43am
Geoff Garcia and Alfonso Camacho-Carrillo, ON Semiconductor
Original equipment manufacturers (OEMs) in a broad spectrum of different industry sectors all face the same basic challenges when developing products that will be attractive to their customer base, and gain them market share. They need to heighten power efficiency levels (in order ensure they comply with increasingly stringent environmental legislation and to help customers to keep their utility bills low). Their products must have long and trouble free operation, so that the reputation of the brand is not marred. They need to, as much as possible, support compact and streamlined designs. Finally, it almost goes without saying, they must be competitive on price. A growing number of design engineering teams have now recognized the value of incorporating TRIACs into the system designs they develop. These offer a cost effective, compact and highly reliable way of providing control, and in many cases have supplanted traditional electromechanical relays.

TRIACs are effectively bidirectional electronic switches. They have greater efficiency characteristics, and fit into far smaller form factors than electromechanical alternatives. Unlike electromechanical devices, they don’t have moving parts and aren’t prone to oxidation of their contacts – both of which can seriously shorten the period over which traditional relays remain fully operational. Migration to TRIACs based control circuits will result in the creation of more compelling products which hit higher performance benchmarks. However, engineers need to have a good understanding of the various nuances involved in their use, in order to specify devices which are optimized for their particular application.

Motion control implementation
One of the most important application areas for TRIAC technology is in the control of motors, scaling the speed as required (see Figure.1). TRIAC devices are suitable for industrial, transportation and domestic system designs of this nature. 

Figure 1. Example of a three-phase motor control circuit

TRIACs are now being deployed into a variety of white goods and appliances used around the home. They provide the switching functions to control the motors in washing machines, power tools, dish washers and air conditioners, as well as refrigerator compressors. For food processors or handheld blenders they allow the speed to be set at different levels, so that a variety of tasks can be done, while in washing machines they allow the speed at which the appliance runs to be matched to the size of washing load, thus helping to save energy. In refrigerators, having different compressor speeds (rather than simply off and full speed) enhances appliance efficiency, and once again means that energy can be saved.

Figure 2 shows the typical three-phase motor arrangement utilized in a modern washing machine. TRIAC devices handling currents of 8 ARMS to 16ARMS are normally specified for this sort of application. Generally the motion control circuit will have a complement of three of these TRIACs which, once they are triggered, supply power to drive the three-phase motor that spins the washing machine’s drum. An optocoupler device supplies the signal current to each of the TRIACs. The three optocouplers ensure the motor keeps the same phase shift between lines as they incorporate zero crossing circuits.

TRIACs are generally connected to the line voltage. As well as powering the motor drive of the washing machine’s drum, they drive smaller solenoid valves through which the water intake and draining operations are carried out, as well as soap dispensing. As the TRIACs are connected to the line on the mains they can be driven directly from a microprocessor. The microprocessor triggers the appropriate TRIAC based on the cycle that has been selected by the user. The mains connection delivers an AC signal, and as this will cross 0 V it provides the means to have the TRIACs to return to their blocking state. The microprocessor then only needs to pulse the gate of each TRIAC when operation is required once again. TRIACs employed in this type of application will normally have blocking voltages (VDRM/VRRM) of around 800 V. 

Figure 2. Use of TRIAC devices in a washing machine circuit

Power drills and other handheld tools tend to have the following type of circuit for controlling of the motor speed (see Figure.3). The RC network is adjusted through use of a varistor activated by the TRIAC. As it is able to act across conducting angles from 5% to 95% of the overall cycle, the TRIAC can deliver high accuracy speed control. Because power tools often rely on induction motors, they require higher di/dt(c) values than those of other applications, as the voltage and current are out of phase with one another. The TRIACs specified will thus need a wider operating temperature range, as they could be exposed to a greater degree of heat. 

Figure 3. Use of TRIAC devices in the variable speed control circuit of a power drill

Dimming control implementation
8 A to 25 A current rated TRIACs are can be integrated into the dimming controls of lighting systems. Figure 4 is an example of the sort of circuit that will be used. Phase control is employed to control the dimming. The electronic switch is opened to prevent the current flow for some proportion of each half cycle of the AC signal. At a set phase angle, the switch is closed, with the full line voltage then being applied to the load for the rest of the half cycle. The operational amplifier (denoted OA) takes care of the zero crossing function, with the integrated circuit (denoted IC) being activated each time a zero crossing. 

Figure 4. Use of TRIAC devices in dimming control circuits


About ON Semiconductor
ON Semiconductor is a premier supplier of high performance, energy efficient, silicon solutions for green electronics. The company's broad portfolio of power and signal management, logic, discrete and custom devices helps customers efficiently solve their design challenges in automotive, communications, computing, consumer, industrial, LED lighting, medical, military/aerospace and power applications. ON Semiconductor operates a world-class, value-added supply chain and a network of manufacturing facilities, sales offices and design centres in key markets throughout North America, Europe, and the Asia Pacific regions. For more information, visit http://www.onsemi.com.
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