Watlow QPAC Modular SCR Power Control Service - Manual (Page 30)

30 WATLOW QPAC Service Manual
Troubleshooting, Chapter 5
Cards
QAF Card (08-5288)
Q1 makes up the FWZ signal which is inverted by U3A to give a high signal for all
of the half cycle but the zero cross notch. U3B, CR1, R19, and C5 detect missing
half cycles which then pulls C7 low with CR2. When C7 is low, the AF card is in
ESD. Q4 and Q3 make a linear ramp which is reset at the end of each half cycle
by the output of U3A. U1C accepts the input signal and performs the bias and
gain function. U1B adds the soft start from C9/R40, and U1D adds the line voltage
compensation. The output of U1D is then compared to the ramp by U1A to output
a square wave from U2B, providing ESD is high and it is during the high output
period of U3A. See Scope picture 12 (TP1 top inverted FWZ, TP2 ramp, TP3
comparator output, and the load current waveform (measured by a CT), scope pic-
ture 13 (TP4 top ESD, TP5 soft start dump, and Tp6 bottom soft start ramp). See
schematic 02-0656 on Page 40.
QAL Card (08-5411)
Q4 makes up the FWZ signal which is inverted by U3A to give a high signal for all
of the half cycle but the zero cross notch. U3B, CR5, R33, and C12 detect miss-
ing half cycles which then pulls C13 low with CR2. When C13 is low, the AF card
is in ESD. Q4 and Q3 make a linear ramp which is reset at the end of each half
cycle by the output of U3A. U1A accepts the input signal and performs the bias
and gain function. U1B adds the soft start from C4/R15, and U2B adds the signal
from the current limit circuit. U2C adds the line voltage compensation the output
of U2C is then compared to the ramp by U2D to output a square wave from U4C,
providing ESD is high and it is during the high output period of U3A. A current
transformer is used to monitor the output of the power control and provides an AC
signal to TB3. Its input is then converted to a DC signal by U5 which provides a
DC output equivalent to the true RMS value at the CT. U1D provides a gain of 4,
and U2C is a two mode amplifier providing both proportional and integral feedback
(for limiting the current) into U2B. U2A is a comparator to turn on the LED when in
limit. See scope picture 12 (TP1 top inverted FWZ, TP2 ramp, TP3 comparator
output, and the load current waveform, measured by a CT), scope picture 13 (TP4
top ESD, TP5 soft start dump, and TP6 bottom soft start ramp.) See schematic
02-0701 on Page 41.
QAD Card (08-5314)
U4 and U5 make up a phase locked loop (PLL) which is synchronized to the 50 or
60 Hz line. It produces an output 256 times the full wave frequency (120 X 256 =
30.72KHz) for the U5 and (100 x 256 = 25.6KHz) at 50 Hz. The input lines D0 to
D7 are used to set the power level desired. When the input lines are programmed
to give an output level, the address line is low (Pin 1), and the strobe line is pulsed
low, the octal D latch (U1) will be updated. The output of the latch goes through
the prom (U2) for inversion and to linearize the output curve. the output of the
prom is then used to program U3 which counts down from zero cross to whatever
power level is set in. See Scope Picture 14 (TP1 FWZ, TP2 +ZX180, and TP3
-ZX180. Scope 15 (TP2, TP3, and TP5 output. Picture 3 TP4 MCK (high frequen-
cy clock). See schematic 02-0671 on Page 42.