Part No. 715-0080, Rev. B
- 79 -
SECTION 10: CIRCUIT DESCRIPTION
The theory of operation of the incubator’s electronic circuits is described. The individual schematics
for the control board and display board should be referred to when reading the theory of operation.
CONTROL BOARD
This section describes the theory of operation of the incubator’s control board.
TEMPERATURE REGULATION
The air inside the incubator is heated and its temperature is sensed by the components of the air flow
tray. The air flow tray contains the circulating fan, the temperature sensors, the heater, and the heater
switching transistor. The heater power is controlled with pulse width modulation. There are two
separate thermistors in the airflow tray. The first or primary thermistor is used to measure and control
the incubator’s temperature. The second or secondary thermistor is used for display control and an
independent indicator of high temperature.
On the incubator control board are three identical temperature sensor circuits (
Z3B
,
Z9B
,
Z10B
, and
their associated components). Two of these circuits (
Z3B
and
Z10B
) convert the two thermistors’
nonlinear resistance values into a voltage proportional to the temperature of the thermistors. The third
bridge circuit (
Z9B
) converts the voltage output of the setpoint circuit on the display board into a
voltage proportional to the temperature setting.
The difference between the measured temperature and the setpoint temperature is amplified by
Z10A
producing an error voltage.
Z7B
compares this error voltage to a triangular wave produced by a 555
timer oscillator circuit,
Z5
,
R60
, and
C9
. This comparison produces a square wave, the duty cycle of
which is proportional to the error voltage. This square wave is then used to drive the base of the
heater switching transistor. Diode
CR12
limits the maximum value of the error voltage seen by
comparator
Z7B
. The transistor amplifier formed by
Q4
,
R54
,
R55
,
R56
,
R57
, and
C16
, causes this
maximum value limit to decrease approximately as the square of the applied heater voltage. This
results in a relatively constant maximum heater power even though the heater voltage may vary. The
applied heater voltage will vary due to changing AC line conditions, varying states of charge of the
internal battery, or variations in the external DC voltage.
Содержание AirBORNE A750i
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