Triac controller

Silicon-controlled rectifiers (SCR). These arc basictilly thyristors and unless specified, a thyristor will mean an SCR Triacs... 

Fig. 64 Light dimmer and heating mantle triac power control.

A triac may be perceived as opposing SCR s (semiconductor controlled rectifier or silicon controlled rectifier) in parallel, each activated by a gate current (Figure 2.12). Current can flow in only one direction through the diodes in Figure 2.12. Electrical power from a wall socket, conventionally 110 or 220 ac volts is fed into the device. The SCR s act like diodes in the sense that current is allowed to flow in only one direction. During one half of the cycle, the current may flow through one... 

A cross-sectional view of one cell assembly of the differential photoacoustic apparatus is shown in Fig. 9. Optical access to the sample solution contained in a 1-cm cuvette (an all-quartz modification of a cuvette from Hellma Cell Inc. No. 221, which reduces the probability of cell fracture) is provided by 6.4 mm apertures in a thermostated aluminium housing. Temperature control is provided by four cartridge heaters (Hot Watt, 100 W/120 V) mounted at the four corners of the aluminium block and connected in series to a 10 A/120 V triac (LOVE Controls Corp). The power output of the triac is controlled by a proportional, integral,... 

True proportional control is obtained with solid-state switching of loads through SCRs, triacs, and similar solid-state devices. This allows a reduction of cycle time to the millisecond level. If the cycle time is reduced to one-half the power line period (8.3 ms for 60 Hz), then the proportioning action is referred to as stepless control or phase-fired control. A 50% phase-fired output is shown in Fig. 4.33. 

The thyristor is only a half-wave device (like a diode) allowing control of only half the available power in an a.c. circuit. This is very uneconomical, and a further development of this device has been the triac which is considered next. 

The triac was developed following the practical problems experienced in connecting two thyristors in parallel to obtain full-wave control, and in providing two separate gate pulses to trigger the two devices. 

Figure 3.107 Waveforms to show the control effect of a triac,...

Whenever a border limit switch is activated, the bi-stable multivibrator, sensing it, changes the phase of the triggering pulse to the triac which drives the rotation direction control coil of the motor. 

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