Ohmeda 3000 - Control Board

1/Functional Description
The heater control circuit uses a zero crossing opto-
isolator triac driver to isolate the line voltage from
the low voltage circuits. Operation of the heater
control and other line voltage controls differ only in
the type of isolator used and the use of snubber cir-
cuits. When a logic “High” signal is sent to the hea-
ter control circuit from J12 pin 9 the output of the
isolator will not switch “On” until the ac signal of
the heater crosses the zero potential from a nega-
tive voltage. After the input line from the microcon-
troller goes “Low”, heat will not switch “Off” until
the first zero crossing preceded by the negative half
cycle. This provides zero crossing control of the hea-
ter switching. The time that the heater is “On" de-
pends on the percent heat desired (controllable in
5% increments).
The microcontroller also monitors the line voltage
and
adjusts the number of ac cycles that the heater
is switched “On”. This provides heater compensa-
tion. If the line voltage is not at the nominal value
these two functions result in 60 levels of heat.
Relay
The relay circuit is used to switch “Off" the heater
in the event of a triac or microcontroller failure.
Under normal conditions the input line from J12 pin
10 is a logic “High”, 2.4 volts minimum. A logic
"High" signal on the input from the control board
switches “On” the FET causing the relay coil to en-
ergize and close the switch. If the FET input is a
“Low” from the control board, (0.5 volts max.) the
FET switches “Off” and the relay contacts open.
The signal at J12 pin 10 comes from U1 on the con-
trol board which is a part of a logic/timing circuit in-
dependent of the microprocessor.
A minimum voltage of 7.2 volts is reguired to ener-
gize the relay coil. Therefore the minimum allowable
voltage for the 8 volt unregulated supply is 7.32
volts since the FET has an internal voltage drop of
0.12 volts.
B. Control Board
This is a functional description for the Infant
Warmer System Control Board Part No. 0631-5033-
700. Refer to Schematic No. 0676-0326-000 for a de-
tailed circuit diagram.
The control board contains electronic circuitry invol-
ved with the measurement, control, computation,
memory, logic, and decision making functions of the
Infant Warming System. The principle IC on this
board is the 8031 single component, 8-bit microcon-
troller. The 8031 has: an internal read/write memory
(RAM)of 128 bytes, 32 I/O lines configured as four 8-
bit parallel ports, two 16-bit timers, a five source
two priority nested interrupt, a programmable serial
VO port, and an on-chip oscillator with clock cir-
cuitry. The program memory is stored in a 2764 64k
bit (8k x 8) UV EPROM. An octal transparent latch
(74LS373) is connected to address inputs of the
EPROM to permit the use of the bi-directional data
bus port of the microcontroller for addressing the
EPROM and receiving program instructions.
Four ICs with a network of precision resistors are
used to interface the microcontroller. The tempera-
ture sensor, calibration resistors, or line voltage
1-2
scaler are selected by an MC14051B 8 Channel Mul-
tiplexer. An LM-10 precision reference with adjusta-
ble reference buffer, and on-board operational
amplifier furnishes a stable reference supply. This is
reguired by the temperature measurement circuits
and the ADC 3711 Analog to Digital Converter. An
8243 I/O expander is used to interface the microcon-
troller with the multiplexer and the A/D converter.
The control board is also equipped with several IC's
that form the triac watchdog circuit, watch-dog
timer, and the audio alarm tone generator. The
audio transducer for the alarm signals and its driver
circuit are also included on the control board.
The operation of the circuits listed in the preceding
paragraphs will be explained in detail below.
Analog to Digital Converter
Temperatures are measured using a negative tem-
perature coefficient thermistor that is calibrated for
specific resistance values and interchangeability.
Analog voltage signals inversely proportional to
temperature are derived from a voltage divider net-
work consisting of a 5.76k + 0.1% resistor in series
with the temperature sensor. The voltage source for
the measuring circuit is obtained from the LM-10's
internal precision reference source of 200 mV
amplified to a nominal 1.0 volts by the reference
buffer of the LM-10. The Op-amp portion of the LM-
10 provides an adjustable reference of 2.0 volts
nominally, which is required by the A/D converter,
U6. In addition to the patient probe, there are three
other
voltage divider networks on the control board.
Two have fixed output and are used for calibration
check points of the A/D system at 25.0 and 37.9 de-
grees C. The third divider network is unused.
A separate input to the control board A/D circuit
comes from the line voltage monitor network lo-
cated on the power supply board.
The outputs of all the voltage dividing networks are
connected to individual switch input terminals of
U13, the MC14051B Analog Multiplexer. The
MC14051B contains eight normally open switches
with a common output terminal. The common out-
put of the MUX (pin 3) is tied directly to the analog
input (pin 9)of the A/D converter. The microcontrol-
ler selects which sensor is to be measured by toggl-
ing the control lines, pin 11A, pin 10B, and pin 9C of
the MUX via the 8243 #2, US. The following table
shows the digital codes used to select the individual
switches of the MUX:
Control On Pin
Inputs Switches Number
ABC
000 χο 13 calibration value 25C
001 xt 14 calibration value 37.9C
010 X2 15 line voltage monitor
011 X3 12 unused
100 X4 01 patient probe
101 X5 05 unused
110 X6 02 unused
111 X7 04 unused
Note: Inhibit terminal (pin 6) of the MUX has no ef-
fect on the switch selection because it is tied "Low"
through R19 (200 ohms).