SCRs and Triacs

A "fuse," as most readers know, protects against too much current flow by melting and thus breaking contact to the power source. It must be replaced after doing its protective job, while a "circuit breaker" (see index) can be reset after it has been "tripped" (operated to break contact), without replacing it. Sometimes a circuit breaker is an electromagnetic relay, and sometimes an SCR or Triac. 

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

Unlike an SCR, which is unidirectional, a triac is a bidirectional thyristor switch and conducts in both directions. It can be considered as composed of two SCRs, connected back to back with a single gate, as shown in Figure 6.22(a). Since the thyristor now conducts in both directions there is no positive (anode) or negative (cathode) terminals. 

The triac may, however, have some limitations in handling frequencies higher than normal. In such cases, they can be simulated by using two SCRs in inverse parallel combinations as illustrated in Figure 6.22(b). Now it is known as a reverse conducting thyristor. An SCR has no frequency limitations at least up to ten times the normal. The required voltage and current ratings are obtained by series-parallel connections of more than one thyristor unit. 

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 diac can be made with either three "layers," PNP, or with five, NPNPN. The former type is simply forced into avalanche like a Zener diode, and its characteristic curve was shown on page 153. (The PNPN diode s curve looks like half of that, with the other half being just a vertical line of high voltage but no current.) The triac requires an extra "layer" of N-type silicon that only covers part of the underlying P-type material, as shown at the top of Fig. 21.4. It operates something like an SCR or the two-transistor circuit that we made in the previous experimental section, on page 228, but it works in both directions. 

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