888-2408-002 4-47
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.7.5.6 Watchdog Timer
The watchdog timer, U4 is basically looking for two things. First, it must receive a
pulse from the micro-controller every 1.0mSec. If it does not it will activate the
RESET line at TP4, and the supplies will be shut off. It is also monitoring the
+20Vdc supply. If the voltage falls below the minimum limit of 12V, it will activate
the RESET line.
4.7.5.7 Analog Inputs
U7 acts as a linear amplifier and buffer for the analog inputs to the A/D converter
which is part of U6.
• SAMPLE 1 and SAMPLE 2 are the voltage samples from the Rectifier Boards.
Primarily they are used to control the power supply tap switching. However they
are also digitally filtered by the micro by two filters. The first is a 50 to 60 HZ
band pass filter whose output is used to detect a loss of a rectifier fuse on the
transformer secondary, a non-critical fault. The second is a 100 to 120 Hz band
pass filter which is used to detect a severe primary phase imbalance or loss of
phase. Loss of phase is a critical fault which will shut the power supply off
momentarily (see “Power Supply Controller Faults” in Section 6 of this manual).
• HEAT SINK THER(mistor) is the temperature sensor located on the rectifier
heatsink. If the heat sink temperature exceeds 100°C it will shut the power supply
off.
NOTE:
There are test points located after U7 which give a place to check the analog
voltages actually going into the A/D converter.
4.7.5.8 PS Controller Faults
For a complete listing of the faults associated with the PS Controller refer to Section
VI, Troubleshooting.
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