
12-19
Power Supply/Power Control Circuitry
Power supply circuitry is located on the auxiliary PCB and consist of the following subsections:
•
Batteries – Four 1.5 V alkaline "C" size batteries provide 4-6 VDC power.
•
Power control circuitry – Power control circuitry is connected to the batteries. It senses any press of the
measure button and switches on the power supplies. Reverse current limiting protects the N-20P from
damage if batteries are inserted incorrectly.
•
Power shut-off circuit – This circuit controls power to all circuits except the power control circuit. In
addition, a fuse protects the power supply from excessive current draw. The power supply is also protected
against electrostatic discharge and electromagnetic interference.
•
Power supply circuits consist of the following power supplies:
Regulated power supply
: Power supplied by the batteries is regulated at 5 VDC. This supply is used by all
of the digital circuitry and some of the SpO
2
analog circuitry.
Unregulated power supplies
: 5 VDC is connected by a switched capacitor network into unregulated
power supplies of -5 VDC, 10 VDC, and 12 VDC, all of which are used in the SpO
2
analog circuits.
High voltage power supply
: A voltage regulator/doubler converts battery voltage to 70 VDC; this increase
in voltage is needed by the display drivers as well as the display backlight.
Disposable
Batteries
Power
Control
Circuits
Measure
Button
Power
Shutoff
Circuits
(fuse;
EMI protect;
ESD protect)
Power
Supply
Circuits
Microprocessor
Display Drivers
Display Backlight
Main PCB
AUX PCB
AUX PCB
AUX PCB
SpO
Analog
Section
(main board)
2
-5 VDC
+12 VDC
+10
VDC
4-6 VDC
+5 VDC
+70 VDC
Figure 12.6 Power Supply Block Diagram