International Biomedical AirBORNE A750i, Руководство по обслуживанию, Страница: 84 / 112

SECTION 10: CIRCUIT DESCRIPTION
Part No. 715-0080, Rev. B - 82 -
BATTERY CHARGER
The battery charger is powered any time the incubator is plugged into a source of AC power. The
charger maintains a constant output voltage of 13.7 volts in the trickle charge mode or limits the
charge current to 5 amps in the current mode. A fully charged battery has a voltage between 13.5
volts and 13.8 volts. The voltage regulator for the battery charger consists of Q5 , Z6 , and its
associated components. A 200 millivolt reference within Z6 establishes the voltage at the noninverting
input of Z6 . The amplifier output drives Q5 at the level necessary to cause the voltage at the inverting
input of Z6 to be equal to the reference voltage. By adjusting the potentiometer R1 , the ratio of the
voltage divider string R32 , R1 , and R34 can be varied to set the voltage at TP1 to equal 13.7 volts.
The battery charging is interrupted when the heater is on to prevent excessive current draw on the
transformer. This is accomplished by means of R35 and CR15 . When the heater on signal at TP12
goes high, the heater is turned on and causes a current through R35 . The current increases the
voltage at the inverting input of Z6 to exceed the 200 millivolt reference level, causing Z6 to turn off
Q5 . During the portion of the duty cycle when the heater signal at TP12 output is low, the heater is
turned off and battery charging resumes. Battery charging will be totally interrupted if the duty cycle of
the heater reaches 100%. With average line voltages the maximum duty cycle is limited to less than
65%. When the battery is depleted, maintaining a voltage of 13.7 volts at TP1 would force a charging
current high enough to damage the battery. Therefore, a current limiting circuit is provided. This
circuit is formed by Q6 , Z4 , and associated components. Zener diode CR14 establishes a reference
voltage and the potentiometer R14 is adjusted to provide a reference of negative 50 millivolts at TP14
with respect to the AC rectified voltage. The resistor R28 is connected to the AC rectified voltage and
supplies the battery charging current. As long as the voltage across R28 is less than 50 millivolts or
the charging current is less than 5 amps, Z4 keeps Q6 turned off. When the voltage across R28
exceeds 50 millivolts, Z4 causes the drain current of Q6 to be increased until the voltage drop across
R27 matches the voltage drop across R28 . The drain current of Q6 flows through R34 producing an
additional voltage to the inverting input of Z6 . The output of Z6 responds by lowering the voltage to Q5
and the battery charging current. The control loop responds to current rather than voltage and
regulates to maintain a constant current of 5 amps. The current regulation is maintained until the
battery terminal voltage rises sufficiently to cause TP1 to rise above 13.7 volts and voltage regulation
resumes.