Theory of Operation
Chapter 3: Theory of Operation
Isolette® Infant Incubator (Model C2000) Service Manual Page 3 - 23
3
used. These errors produce the same error signal to the controller as
for low or high impeller speeds.
The controller’s +12 V supplies the power for the IMD circuit through a
301 Ω resistor. This resistor and the load of the IMD circuit form a
voltage divider that sets the LONG signal voltage that remains constant.
U1, a voltage regulator, supplies +5 V DC to the Hall effect sensor (U3)
and the microcontroller (U2). As the magnets pass above the Hall effect
sensor, its open-drain output, U3-2, goes low, detecting the magnets’
field. When the magnets’ field is removed, the oscillator is used as a
reference. When an error condition is detected, U2-2, the
microcontroller’s output, drives low, which accomplishes the following:
• Turns off Q1.
• Releases a portion of the load on the short signal, allowing it to rise
above the LONG signal voltage.
R2, the switch portion of the SHORT signal load, and R4, the unswitch
position of the SHORT signal load, adjust to provide an approximate
1 V swing between the error and non-error outputs. The capacitors, C1
and C2, filter the system’s supply.
The IMD P.C. board supports in-circuit programming (ICP) of the
microcontroller. Programming is done after the unprogrammed
microcontroller is populated with all the other components by
connecting a programmer to the pads labeled VPP, +5 V, CLK, DTA,
and GND.
Fan Motor
The controller sets the fan motor speed if the watchdog is not tripped.
The microprocessor supplies a pulse-width modulation (PWM) signal to
an optocoupler for isolation. The output connects to an integrator circuit
that converts the PWM signal to an analog signal for the motor
controller. The motor incorporates the Hall effect sensors for monitoring
and control. One of the Hall effect sensor outputs is fed to the
microprocessor for measuring the motor speed. If the watchdog timer
trips, the fan motor speed is maintained at 1500 rpm ± 450 rpm.
The controller provides an alarm to indicate a failure of the fan to rotate.
When this occurs, the heater and humidifier disable, and an audible
alarm with a visual indication activates.
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