9600 Service Manual - gmecorp-usa.com

Power Distribution 27
Current Sensing
Charger current is sensed in the output line by T2. One side of T2 is tied to an ideal or
zero drop diode, comprised of U4, CR8, Q9, and associated circuitry. The other side is
connected to current to voltage converter U4. The output of U4 goes to U7 which drives
DS2, a yellow bar graph to indicate charger current output with a scale from 25 to 625
milliamps. When the output load current gets above 500 mA, comparator U5 output
switches high, turning Q7 on, and placing R26 in parallel with the series combination of
R9 and R10, changing the feedback divider such that the output voltage goes up by
about 12 VDC. This puts the charger in “High Charge Mode”.
The signal, HICHGMOD (high charge mode), is in an input from kV control circuitry on
the X-ray Regulator PCB that can also put the charger into high charge mode.
“HICHGMOD” goes high whenever the measured KVP is above 30 KVP. CHGISEN
(charger current sense) is sent to the Analog Interface PCB where charger activity can
be monitored.
Voltage Sensing
The charging voltage bargraph indicator, U6, is driven by the VOLTAGE SENSE line
which is adjusted by the combination of R11, R28, R29, and R32 (potentiometer). The
red bargraph indicator DS1, reads in 10 linear steps from 160 VDC to 250 VDC.
The Battery Charger output voltage is also sensed on the Generator Driver PCB at the
resistor network of R11 and R10. This signal, “+200VSEN”, is used in the pre-charge
cycle (see Boot Sequence section of this manual). It is also used to monitor the battery
charger status.
Overvoltage Protection
To guard against battery damage and/or venting, an overvoltage protection circuit is
provided. If the last segment (250V) of the voltage bargraph indicator lights, Q8 is turned
on, U1 output is set low, turning Q6 ON. This results in both activating the “shutdown”
control of the PWM chip and shunting the drive to Q1 to ground, resulting in charger
shutdown.
The charger can also be disabled by the Technique Processor PCB via PIO U27-18 on
the Analog Interface PCB, this happens during the boot sequence to check for the
presence and condition of the batteries and during a FILM exposure. A logic high asserts
Charger Disable and a logic low resets the disabled condition.
Under Voltage Protection
A low voltage cutout circuit is provided by Q10 and Q2 to prevent over-current conditions
in the event the battery is discharged below 160 VDC. If biasing for Q10 is lost by the
output line dropping near 160 VDC, Q2 is turned off and power from CR1 cannot get to
L2. With the Battery Charger in the “shut down” condition, the charge in the batteries is
slowly restored by a 5-20 mA current limited trickle charger comprised of C26, CR9, and