Relay Board - GE Giraffe Service Manual

Chapter 1: Functional Description
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 heater. Safety
relay 2 closes the mains supply to the humidifier isolation transformer and the transformer for the e-base
motor. Control signals for the two relays originate on the Control Board.
The Relay Board interfaces the DC Control signal to the chassis mounted solid-state relay (SSR), which controls
the incubator heater. The Control signals for the heater SSR originates 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 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 line compensation circuit consists 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 provide 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 system +12V power supply or the backup 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 unit is powered on.
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