Precision Control Systems 915 - Manual (Page 16)

Page 13 of 21
Figure 4-4 Conduction of two SCRs connected in reverse parallel when turned on after the beginning of the half-cycles.
The average power a phase angle controller delivers to its load is determined by the number of
electrical degrees by which the gating signal that turns on the SCRs is delayed past the
beginning of the half-cycles. The later in each half-cycle the SCRs fire, the less the time the
SCRs conduct during each half-cycle and, there, the less power is delivered to the load (see
figure 4-5). The o-5 VDC control signal received by the phase angle firing-circuit determines
the amount by which the gating signal is delayed.
INPUT LINE VOLTAGE
FIRING
CIRCUITRY CONTROL SIGNAL
GATING SIGNAL OUTPUT LINE VOLTAGE
SCR 1
GATING SIGNAL
SCR 2
SCR 1
SCR 2