GE Giraffe OmniBed Carestation CS1, Service Manual

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Chapter 1: Functional Description
Relay Board
Service Manual 1-3
The overtemperature circuit compares the air temperature to a reference level, generates an interrupt, and
turns off the heat if the air temperature is higher than the reference level. The overtemperature circuit
requires varying its voltage levels to accommodate various thermistor measurements. This is because the
calibration is digital (no potentiometer).
The watchdog circuitry monitors the 80C188 microprocessor, and monitors the +5V and +5VSTBY voltages.
It generates the interrupt signal and power failure signal to the 80C188 microprocessors. The audio circuit
includes a 8752 microcontroller that reads a wavetable located in a PROM and sends the table to a digital
audio circuit and amplifier. The high priority (HP) and other alarm signal lines select an output at the correct
frequencies.
Three OR gates are combined to generate the error signal. The inputs to the circuit are overtemperature,
power failure, and system failure. This circuit generates an error signal that turns off the heater and sounds
the HP alarm. This circuit is independent of the microcontroller.
1.2 Relay Board
The Relay Board includes 2 safety relays, which close to supply mains power to the heater and motor
circuits. Safety relay 1 is wired in series with the primary coil of the isolation transformer for the incubator
and radiant warmer heaters. Safety relay 2 closes the mains supply to the humidifier isolation transformer
and the transformer for the elevating base and canopy motors. Control signals for the two relays originate
on the Control Board.
The Relay Board interfaces the DC control signals to the two chassis mounted solid-state relays (SSRs),
which control the incubator and radiant warmer heaters individually. The control signals for the two heater
SSRs originate on the Control Board.
The Relay Board includes a SSR for the humidifier. The SSR output is wired in series with the humidifier
heater. The humidifier SSR control signal originates on the Control Board.
There is one current sense circuit for the incubator and radiant warmer heaters and an additional one for
the humidifier heater. These circuits consist of a small signal transformer that produces a current
proportional to the current through the heater circuits. The current is rectified and measured. The
subsequent comparator then generates a digital level based on a specified current level. This results in a
signal to the Control Board representing the state of the heater (on or off).
The two line compensation circuits consist of a signal transformer connected to the mains voltage. The
secondary of this transformer feeds a full wave rectifier and capacitor. The resulting DC voltage is
proportional to mains voltage, and it is measured on the Control Board.
The line frequency circuit consists of a full wave rectifier and a comparator. This circuit generates a digital
pulse with frequency twice that of the line frequency (50 or 60 Hz). The output signal is provided to the
Control Board.
The Relay Board provides the +5v standby power supply to the entire Giraffe system. A +5V regulator
generates the +5V standby from the diode OR combination of the 12V power supply or the PF
Battery. If there is no mains power, then +12v is not present, and the battery will generate the +5V standby.
When +12V is present, the battery is biased out of the circuit with the diode and is merely being trickle
charged though a resistor.
The battery charges only when the device is powered on.