9600 Service Manual - gmecorp-usa.com

kV Generation 9
KV GENERATOR FAULTS
FAULT PAL OVERVIEW
Fault PAL U2 monitors the system for the following fatal fault conditions:
• Saturation Fault
• Overvoltage Fault
• Overload Fault
During any of these fault conditions, the FAULT signal which is fed to the HV Drive PAL
U7-9 prevents X-rays by disabling the HVDRA and HVDRB signals. The outputs of the
fault PAL U2 are also fed to the Technique Processor for software monitoring through
the Analog Interface PIO U27. When software detects a fault, the Technique Processor
shuts the system down by disabling the 24V interlock circuit. The Technique Processor
also displays the appropriate error message on the generator control panel display.
SATURATION FAULT
The Darlington power transistors Q1 and Q2 drive the primary winding of the high
voltage transformer contained within the tank assembly. Normally, the voltage drop
across the Darlington’s output, when turned-on, is minimal indicating a completely
saturated transistor.
However, if a Darlington transistor becomes weak (or if too much current passes through
a Darlington transistor) a relatively large voltage is dropped across its output during the
on-time. This excess voltage drop is detected by the system and defines a Saturation
Fault.
If a fault occurs, the message SATURATION FAULT is displayed on the Control Panel.
This is a fatal error resulting in a system shut down preventing X-rays.
Saturation Fault Circuit
The bridge rectifier VR1 and Q9 on the Generator Driver PCB samples the voltage drop
of which-ever of the two upper transistors of the H-Bridge is turned on at the time. This
voltage is referenced to ground level by Q9 at R19. In effect, the voltage across R19
represents the voltage across the Darlington power transistor. This voltage is fed
through CR23 to TP7 and then to TP3 and comparator U14 on the X-ray Regulator PCB.
VR1’s negative output samples the drop on the “lower half” of the H-Bridge for the same
purpose.
On the X-ray Regulator PCB, this sense voltage (SATDET) is monitored by comparator
U14-2. A reference voltage derived by R35 and R36 of approximately 8 VDC is applied
to U14-3. This voltage is the comparator’s threshold voltage which represents the
maximum allowable voltage drop across the Darlington’s output during their on-time. A