Watlow QPAC Modular SCR Power Control Service - Manual (Page 12)

12
WATLOW QPAC Service Manual
Operation Theory, Chapter 3
Q33 Block
into 256 parts. When the control card sends out a pulse, U7A picks the correct half
cycle, and sends the pulse to U3D which starts counter #1 (U9) and counts to 1 for
a short delay. When U1 counts down, it sends a pulse to the output board to fire
SCR #1, and resets U3C, and sets U4A. See schematic 02-0798 on Page 35.
This starts counter #2 (U10) which counts to 85 (to give a 2.77ms delay). It sends
an output pulse to fire SCR #2, resets U4B, sets U4D, and starts counter #3.
Counter #3 (U11) counts to 85, sends an output pulse to fire SCR #3, (U11) counts
to 85, sends an output pulse to fire SCR #3, and resets U4C to finish the cycle.
U12 is used to detect the loss of L2 phase, (phase loss) and U13 is used to detect
the proper phase rotation. U7B, U7C, and U7D combine phase loss, phase rota-
tion, PLL, and the thermostat for emergency shut down (ESD).
Figure 8 -
Q33 Block Diagram
and Typical Load
Circuit