Ohmeda Ohio Care Plus, Operation, Maintenance, Service Manual And Installation Instructions

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6/Service
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The microcontroller is programmed to internally pull this line high. When the fill tray
is installed this line is a low logic level; when the fill tray is removed this line is a high
logic level.
Safety Relay Circuit
Relay S2 is a mechanical relay that disconnects the neutral line from the heater if
the watchdog times out or when the fill tray is removed from the humidifier. The main
control of the safety relay is a NAND gate output U7A. One input (pin 1) of U7A is
directly connected to the watchdog circuit. The other input (pin 2) of U7A is connected
to pin 2 of J5.
The fill tray assembly shorts pins 1 and 2 of J5. J5 (pin 1) is pulled high by resistor
network R47 (pin 4). When the tray assembly is properly installed, pin 2 of U7A is
pulled high. When the tray assembly is removed or improperly installed, pin 2 of U7A
is pulled low. The resistor R46 is used to insure the signal line is low when the tray
assembly is removed. The NAND gate U7B is configured as an inverter. One input
(pin 5) is tied to 5 v the other input (pin 4) is connected to the output of U7A (pin 3).
The output (pin 6) of U7B drives transistor Q2 and its associated biasing network.
Q2 is a darlington transistor used as a switch to energize relay coil S2. Resistor R1
sets the base biasing current and resistor R2 is used to ensure the transistor turns off.
The diode D5 is installed to protect Q2 from over-voltage during field collapse of the
relay coil.
Audio Prompt Circuit
The audio transducer SP1 produces a tone at 2200 ± 300 Hz. The capacitor C23 is
used suppress the frequency oscillations generated by the audio transducer. Resistor
R56 controls the output volume of the audio transducer.
The audio transducer is controlled by transistor Q3. Q3 is a darlington transistor which
is used as a switch. Resistor R7 set the base biasing current, and resistor R8 is used
to insure the transistor turns off. One input of NAND gate U7D (pin 13) is connected to
the output of the watchdog circuit. The other input of U7D (pin 12) is tied high through
resistor pack R47 (pin 1) and is connected to P0.0 (pin 39) of the microcontroller. With
both inputs of U7D at a high logic level (normal output of watchdog is high), the output
will be low and the audio transducer will be off. If either or both inputs of U7D go to
a low logic level, the output will be high, and the audio transducer will turn on. This
allows both the watchdog circuit and the microcontroller to independently turn on the
audio transducer.
Solid State Relay Circuit
The solid state relay (SSR) S1 control line is pulled high through resistor pack R47
(pin 3). Port P0.2 (pin 37) of the microcontroller is used to control the SSR. A high logic
level output of port P0.2 will turn on the SSR and allow the heat to turn on; a low logic
level output of port P0.2 will turn the heat off.
Q1 is used as a switch to turn S1 on and off. Resistor R3 sets the base biasing current
and R4 is used to ensure the transistor turns off. The SSR is a zero referencing solid
state relay to minimize surges and transients during power switching of the relay. Pin
3 of the SSR is connected to the 5 volt line; pin 4 is connected to the collector of Q1.
When Q1 turns on, the SSR will conduct between its pins 1 and 2 when the signal
at pins 1 and 2 cross the zero voltage point. When Q1 turns off, the SSR will stop
conduction between its pins 1 and 2 when the signal at pins 1 and 2 cross the zero
voltage point. RV1 is MOV used to protect the output of the SSR from high voltage
noise.