Triac circuit

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. 

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. 

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. 

Figure 3.108 Symbol for and appearance of a diac used in triac firing circuits.

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