BCI Capnocheck Plus - Capnocheck Plus Circuitry Breakdown; Battery Charger Circuitry; Power Supply and ON;OFF Control

Chapter 3: Capnocheck Plus Circuit Description
Chapter 3: Capnocheck® Plus Circuit Description
Battery Charger
The battery Charger is built around charging controller U17. This IC controls
the charging current in both fast charge and trickle charge modes, switching
from fast charge mode to trickle charge mode when the end of charge condition
has been reached. It senses charging current by measuring the voltage drop
across current sense resistor R63. Thermistors T3, T4 and the thermistor inside
battery pack in combination with resistors R59..R61 provide temperature
control, preventing fast charge in very cold or very hot environments, and, also,
preventing battery from overheating during fast charge. U17 controls the
switch mode voltage regulator U18 using current mirror Q8-Q9. This
completes the feedback loop.
The charging indicator circuitry is built around multivibrator Q12-Q13.
Depending on the FAST control signal (U17, pin 8), the green LED either stays
lit (in trickle charge mode) or blinks with a very low duty factor (in the fast
charge mode).
System Power Supply and ON/OFF Circuitry.
The system +5VDC power supply is built around a low drop voltage regulator
IC U20. MOSFET transistor O11 is used as a controlled element. Isolated
power for the Oximeter Slave Board is generated by the DC-DC voltage
converter built around U21, Q14 and transformer T5. The clock signal for the
converter is the microprocessor clock divided by the freguency divider inside
Us.
The Power ON/OFF circuitry is built using the voltage comparator located
inside the “watch dog timer” IC U19. When the STNBY/ON key is pressed, it
pulls the base of PNP transistor Q10 to “ground” through resistor R67.
Transistor Q10 pulls up EN (enable) input of voltage regulator U20 turning
system +5V on. From then the high voltage level on EN input is maintained by
the VCC voltage through resistors R68 and R69 until power is off.
Capnocheck Plus Service Manual 3-1