Welch Allyn PneumoCheck 61000, Service Manual

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26 Pneumocheck Handle and Printer/Charger Service Manual 610132 Rev. E
The battery charge is supplied by U14 which is a voltage
regulator wired as a 12ma current source. D6 and D7
provide reverse polarity protection.
Transistor T1 is the source for the current used in the
spirometer if it is turned on while plugged into the
printer. Its output voltage is regulated by the zener D13
to a voltage which will reverse bias D18 while at the
same time insuring that the voltage reaching U8 does
not exceed its design limit of 10v. This means that the
tested voltage at TP1 must lie between 7.7 and 9.6v
with 14-20v applied to the DC jack.
R22 and R23 divide the zener voltage to provide the
sense voltage so that the processor can tell when DC is
applied. This voltage should be between 4.20 and 4.85
volts with DC applied to the charging jack. This sense
voltage is taken to pin 35 of the CPU and pin 1 of the U1.
The raw power for the spirometer comes from either the
battery via D18, or if DC is supplied, via T1. It goes
through the ON/OFF switch SW1 to the 5v regulator
119. U9 also supplies the low battery warning if Vcc
comes out of regulation. This will occur when the
battery voltage drops below about 6.3 volts.
Power for the analog sampling circuitry is taken directly
from the battery so that it is impossible to test patients
while powering the unit from the DC jack. T5 and T6
form a software controlled switch to turn analog power
on only while actually needed and thus conserve
battery power. This voltage will be on when CPU pin 2
is high. The voltage at the collector of T6 will be 6 to 10
volts when on, and can be easily tested at pin 1 or U1.
This switched analog voltage is inverted by U8, which
should have a visible charge pump oscillating at eight
kilohertz across C18. It will develop at least -6 volts
across C20.
The switched analog voltage and its inversion are
regulated by U7, T3, and T4 to p/- 4 volts VA+
and VA-. R18 and R19 divide Vcc to provide a guide
voltage. U7A and T3 use this voltage to give VA+. U7B
and T4 invert VA+ to provide a matching VA-.
3. Spirometer Power
Circuit Description
Section 3 - Theory of Operation