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In this way, a patient airway temperature greater than 41 °C causes both heaters to be shut down. A watch dog failure, which is a high output from U5g, is communicated via isolating diode D11 and resistor R31 to the inverting input of U10c. This also ensures that both heater control circuits are clamped off in the event of a processor crash. 4.2.8 HEATERPLATE OVERTEMPERATURE Thermistor HP senses the heaterplate temperature. Should the heaterplate exceed the maximum heaterplate limit (default 110 °C) during normal operation, power to the heaterplate is discontinued until it cools below this limiting temperature. Under certain operating conditions, for increased safety, lower maximum heaterplate temperatures can be selected. (See Appendix A3: Programming Control Options.) Also mounted on the heaterplate is a mechanical 118 °C cut out which will give protection should the electronic control system fail. This mechanical cut out is connected in series with the power transformer primary and thus causes complete shut down of the humidifier should a serious fault develop. See § 3.5.1. 4.2.9 WATCH DOG During operation the processor is continually monitored for correct function. This is done by changing the state of output port PB7 of U4 every 16.6 or 20 ms (power line frequency interrupt). This square wave, buffered by U5b, is fed to the charge pump circuitry C16, D4, D5 and C15 which ensures that pin 13 of U5g is held hard up. Should this square wave be interrupted for any reason, C15 will discharge through R3 giving a high output from pin 12, which drives the trap interrupt input of U3. This high trap input forces re-initialisation of the processor which may or may not be successful depending on the nature of the fault. The purpose of this is to ensure that if the processor should fail, and is unable to re-initialise, it fails in a safe mode. The watch dog circuit is tested every time the humidifier is turned on by deliberately halting program execution and waiting for initialisation via the watch dog. To indicate that the processor has failed the high output of U5g also turns on Q3 which lights the ‘see manual’ LED10 and also turns off the display I.C. U7 to blank the digital display. Ref. 185040646 MR730/720/700/480 Technical Manual . Revision F . Issued March 2001 11 4.3 POWER BOARD 4.3.1 REVISION D/F PCBS A bridge rectifier (D13 to D16) supplies power to the 5 V regulator Q7 mounted on the heat sink. This circuit is protected by a 1 A fuse (F1) mounted on the circuit board. The 5 V negative supply stabilized by Q8, is derived from the voltage doubling network D17, D18. The Heater Wire (HHC) is controlled by triac Q9 which is mounted on the heat sink. Q9 is switched by the optoisolator U11 which in turn is driven by Q10. Q10 is controlled directly from an output port of U4 on the control board. The resistor R52 connected to the anode of Q9 provides a signal through (HHS) to U5d and U4. This signal, with processor interpretation, provides the information to determine if the Heater Wire is connected or open circuit and also the integrity of fuse F2, the 4 A fuse supplying power to the Heater Wire. Triac Q11 (mounted on the heatsink) is controlled by optoisolator U12, which in turn is controlled by Q12, and HPC. Transistors Q5 and Q6 are a RS232 level driver for the serial data interface. 4.3.2 REVISION H AND J PCBS A bridge rectifier (D13 to D16) supplies power to the 5 V regulator Q7 mounted on the heat sink. This circuit is protected by a 1 A fuse (F1) mounted on the circuit board. The 5 V negative supply stabilized by Q8, is derived from the voltage doubling network D17, D18. The Heater Wire (HHC) is controlled by triac Q9 which is mounted on the heat sink. Q9 is switched by the optoisolator, U11, which in turn is driven by Q10. Q10 is controlled directly from an output port of U4 on the control board (HHC). Resistor R9, C6 form a snubber on the triac. Capacitors C8, C32 balance the heater wire with respect to ground. Heater wire sensing is accomplished through the optoisolator U13 and associated components R51, R52, D21, R66 and R67. R51 and R52 limit the current through the LED when the triac is on or off. With the triac off and a heater wire connected then the optoisolator will remain off. Otherwise if a heater wire is not connected then the optoisolator will be turned on and off in synchronisation with the secondary cycle. The output of the optoisolator feeds U4 on the control board. This signal, with processor interpretation, provides the information to determine if the Heater Wire is connected or open circuit and also the integrity of fuse F2, the 4 A fuse supplying power to the Heater Wire. Q11 controls power to the heaterplate and is under control of optoisolator U12 and transistor Q12 via an output port (HPC) on U4 on the control board. Resistors R64, R57 limit the gate current to the triac and with C33 form a snubber for the triac and the optoisolator. Transistors Q5 and Q6 are an RS232 level driver ...

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