COURS THYRISTOR DIAC TRIAC PDF

SCRs are unidirectional one-way current devices, making them useful for controlling DC only. Because individual SCRs are more flexible to use in advanced control systems, these are more commonly seen in circuits like motor drives; TRIACs are usually seen in simple, low-power applications like household dimmer switches. A simple lamp dimmer circuit is shown in Figure below, complete with the phase-shifting resistor-capacitor network necessary for after-peak firing. TRIACs are notorious for not firing symmetrically. Generally speaking, this is undesirable, because asymmetrical firing results in a current waveform with a greater variety of harmonic frequencies.

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SCRs are unidirectional one-way current devices, making them useful for controlling DC only. Because individual SCRs are more flexible to use in advanced control systems, these are more commonly seen in circuits like motor drives; TRIACs are usually seen in simple, low-power applications like household dimmer switches.

A simple lamp dimmer circuit is shown in Figure below, complete with the phase-shifting resistor-capacitor network necessary for after-peak firing. TRIACs are notorious for not firing symmetrically. Generally speaking, this is undesirable, because asymmetrical firing results in a current waveform with a greater variety of harmonic frequencies.

Waveforms that are symmetrical above and below their average centerlines are comprised of only odd-numbered harmonics. Asymmetrical waveforms, on the other hand, contain even-numbered harmonics which may or may not be accompanied by odd-numbered harmonics as well. In the interest of reducing total harmonic content in power systems, the fewer and less diverse the harmonics, the better—one more reason individual SCRs are favored over TRIACs for complex, high-power control circuits.

Since the DIAC prevents any gate current until the triggering voltage has reached a certain, repeatable level in either direction, the firing point of the TRIAC from one half-cycle to the next tends to be more consistent, and the waveform more symmetrical above and below its centerline.

Not much more needs to be said about this device except for an important caveat concerning its terminal designations. From the equivalent circuit diagram shown earlier, one might think that main terminals 1 and 2 were interchangeable. These are not! Although it is helpful to imagine the TRIAC as being composed of two SCRs joined together, it in fact is constructed from a single piece of semiconducting material, appropriately doped and layered. The actual operating characteristics may differ slightly from that of the equivalent model.

Although the resulting circuit lacks the fine control ability of the more complex version with capacitor and DIAC , it does function: Figure below. According to the equivalent circuit diagram shown earlier in this section, the swap should make no difference.

The circuit ought to work: Figure below. However, if this circuit is built, it will be found that it does not work! The load will receive no power, the TRIAC refusing to fire at all, no matter how low or high a resistance value the control resistor is set to.

The key to successfully triggering a TRIAC is to make sure the gate receives its triggering current from the main terminal 2 side of the circuit the main terminal on the opposite side of the TRIAC symbol from the gate terminal. Don't have an AAC account? Create one now. Forgot your password? Click here. Latest Projects Education. Home Textbook Vol. Although the resulting circuit lacks the fine control ability of the more complex version with capacitor and DIAC , it does function: Figure below This circuit with the gate to MT2 does function.

The circuit ought to work: Figure below With the gate swapped to MT1, this circuit does not function. TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored. Published under the terms and conditions of the Design Science License.

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