SERVICE MANUAL
Medi-Therm II
THEORY OF OPERATION
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If the machine is in
MANUAL
mode, the blanket water temperature as sensed
by the blanket water probe is used as the feedback signal for controlling the
water temperature to the
MANUAL
mode set point temperature.
If the machine is in
AUTO
mode, the patient temperature as sensed by the patient
probe connected to the patient probe jack is used as the feedback signal for
controlling the patient temperature to the
AUTO
mode set point temperature. The
machine accomplishes this by adjusting the water temperature.
For water temperature control, the microprocessor control system outputs a
pulse train to each solenoid valve. The pulse train to the hot solenoid (and also
heater) is the complement of the pulse train to the cold solenoid. The pulse train
duty cycle depends on the magnitude and sense of the control signal calculated by
the microprocessor. That is, while the solenoids are each either on or off, the
ratio of on time to off time is proportional to the calculated control signal
amplitude. For large differences between set point and probe temperatures, the
output to each solenoid valve will be either on or off. For differences approaching
zero, the outputs to the solenoid valves (and heater) will switch on and off, with
the on and off times automatically adjusted to maintain a probe temperature
equal to the set point.
The circulating pump is energized whenever the unit is in
AUTO
or
MANUAL
modes.
The control latch U51 on the control/display board is the interface between the
microprocessor (via the data bus at address 0FF7FH) and the peripheral drivers
on the power supply board.
Interface circuitry on the power supply board consists of U1, U2, U7, U8, Q3,
Q4, Q8, Q9, and associated components. U1, U2, U7, and U8 are optically
coupled triac drivers used to control their respective triacs (Q3, Q4, Q8, and
Q9); these combinations provide electrical isolation between the low voltage
microprocessor control circuits and the line voltage circuits.
The heater, pump, hot solenoid valve, and cold solenoid valve are individually
controlled by the microprocessor through latch U51 on the control/display
board. A high signal on the data line from the microprocessor causes a low signal
on the appropriate output line of U51 which then sinks current from the power
supply board to activate the peripheral devices.
Pin 11 of U51 on the control/display board and U8 and Q9 of the power
supply board control the cold solenoid valve while pin 9 of U51 on the
control/display board and U7 and Q8 of the power supply board control the
hot solenoid valve. Pin 8 of U51 on the control/display board and U2 and Q4
of the power supply board control the circulating pump. Pin 7 of U51 on the
control/display board and U1 and Q3 of the power supply board control
power to the heater.
6.4.5 BLANKET / PATIENT
TEMPERATURE
CONTROL
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