Precision Control Systems 915 - Manual (Page 15)

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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
INPUT LINE VOLTAGE
FIRING
CIRCUITRY CONTROL SIGNAL
GATING SIGNAL OUTPUT LINE VOLTAGE
SCR 1
GATING SIGNAL
SCR 2
SCR 1
SCR 2