Electrical Projects An SCR takes up little space and has no moving parts. However, it can only be used with dc-operated appliances, since it permits current to flow in only one direction.


Triac. A more useful device from the standpoint of the modern appliance designer is the triac. This device is really two diodes, back-to-back. When enabled by a small gate voltage, a triac will permit current to flow in both directions (hence the back-to-back diode configuration in the diagram), making it suitable for use in ac circuits. Unlike the SCR, however, the triac cannot be latched—switch S must be closed and gate voltage applied to the gate at all times for the triac to conduct. If there is no gate voltage, the triac should not permit current to flow through it.

The advantage of using SCRs and triacs, besides their small size and the fact that they have no moving parts, is that a light-duty, low-current switch (S) can be used to handle heavy current. This eliminates burned and pitted switch and relay contacts and the need for large, heavy switches.


Diac. A diac is similar to a triac, but does not have a gate. At low voltages, the diac will not conduct, but when the ac voltage exceeds its breakdown voltage (usually about 35 volts) it will conduct and pass ac.




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November 20, 2008