Physio Control LIFEPAK 9 - Page 46

LIFEPAK 9 defibrillator/monitor
outputs high. The selected transistors conduct, dropping a voltage across
their respective heating elements and activating the selected printhead dots.
Printhead Resistance Evaluation. The Printhead Resistance Evaluation
circuit is active only when the instrument is in motor speed calibration mode.
When the motor speed calibration test runs, the Display/Recorder
microprocessor starts energizing the printhead dots.
The measured printhead resistance used together with printhead temperature
and voltage during printing determines the optimum duty cycle of the
printhead. The printhead duty cycle is the time period that the printhead dots
are energized; this is regulated by the width of the printhead strobe pulse, SB
(see Figure 1-10). If the strobe pulse is too short, the print will be light; if it
is too long, the excessive heat on the dots will greatly shorten the life of the
printhead. The System microprocessor monitors the printhead resistance,
battery voltage, and printhead temperature, and calculates the optimum
printhead strobe width.
Monitoring the current through the printhead while increasing the number of
dots turned on determines the printhead resistance. As the number of dots
turned on increases, the combined resistance of the printhead decreases.
Comparator U2A monitors the load on the VHD line at pin 5, through current
sense resistor R27 and voltage divider R13 and R16. The VBATT line is
monitored at pin 4 through voltage dividers R10 and R15. As the load on the
printhead increases, the voltage across R16 decreases. When the voltage
across R16 drops lower than the voltage across R15, U2A switches low. This
directs the Display/Recorder microprocessor to stop sending strobe signals to
the Recorder Gate Array at the STROBE input. The Display/Recorder
microprocessor then determines the printhead resistance from the number of
dots turned on and passes this information to the System microprocessor.
Display PCB/CRT The Display PCB/CRT Assembly (Figure 1-11) is composed of a CRT and
Assembly (803706) the Monitor PCB Assembly. The Monitor PCB assembly processes video
information and synchronization signals from the Main PCB and displays the
information on the CRT. The incoming signals are Video 1, Video 2, and
synchronization signals Frame Sync and Raster Sync.
1-32 All Rights Reserved © August 1993 Physio-Control