Control Devices 461
that spark and arc when changes in load current occur. Control of this
type, however, is only an on-off function. SCRs are widely used to control
the amount of electrical power supplied to a load device. Circuits of this
type respond well to 60 Hz ac.TRIAC POWER CONTROL
Ac power control can be achieved with a device that switches on and
off during each alternation. Control of this type is accomplished with a spe-
cial solid-state device known as a triac. This device is described as a three-
terminal ac switch. Gate current is used to control the conduction time of
either alternation of the ac waveform. In a sense, the triac is the equivalent
of two reverse-connected SCRs feeding a common load device.
A triac is classified as a gate-controlled ac switch. For the positive
alternation it responds as a PNPN device. An alternate crystal structure of
the NPNP type is used for the negative alternation. Each crystal structure
is triggered into conduction by the same gate connection. The gate has a
dual-polarity triggering capability.Triac Construction
A triac is a solid-state device made of two different four-layer crys-
tal structures connected between two terminals. We do not generally use
the terms anode and cathode to describe these terminals. For one alterna-Figure 17-7. SCR power control switch