User Manual

Page 12 of 21

Figure 4-2 Conduction of one SCR when turned on at the beginning of a half-cycle.

Because an SCR is a type of diode, it can conduct only during every other half-cycle of the

applied voltage. Therefore, SCR’s used to control AC power are usually installed in pairs,

connected in reverse-parallel, as shown in figure 4-3. One of the SCRs then can be fired

during the positive half-cycle and the other can be fired during the negative half-cycle.

Figure 4-3 Conduction of two SCRs connected in reverse parallel when turned on at the beginning of half-cycles.

Principles of Phase Angle Control

In phase angle controllers, the SCRs conduct during all or part of every half-cycle. When

delivering full power, the SCRs start conducting at the beginning of the AC half-cycles. When

delivering less than maximum power, the gating signal is de-layed so the SCRs start

conducting some time later than the beginnings of the half-cycles. Operation of a phase angle

controller at partial output is depicted in figure 4-4.

INPUT LINE VOLTAGE

FIRING

CIRCUITRY

CONTROL SIGNAL

GATING SIGNAL

OUTPUT LINE VOLTAGE

GATE

CATHODE ANODE