SuteTemp 767 Technical Manual Rev B

Technical Manual

Note: This Technical Manual applies to the M767 Suretemp® Thermometer with Axillary Algorithm, serial numbers 3192665 and above. © 1999 by Welch Allyn. All rights reserved. No part of this manual may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, without prior consent in writing from Welch Allyn. Printed in the USA SureTemp is a trademark of Welch Allyn, Inc. All rights reserved.

TABLE OF CONTENTS WARRANTY ...4 SPECIFICATIONS ...5 OPERATIONAL CHARACTERISTICS ...5 Introduction...5 Basic System Description ...5 SAFETY & WARNINGS...7 SELF TESTS ...7 Instrument Reset/Self Tests...7 Display Test ...8 Probe Warmer Circuit Self Tests ...8 THERMOMETER MODES ...9 Normal Mode ...9 Monitor Mode...9 Biotech Mode...10 Biotech Mode Program Table ...10 Temperature Recall ...11 F/C Conversion...11 PREVENTATIVE MAINTENANCE ...12 CLEANING AND STERILIZATION...12 Routine Cleaning ...12 ETO Gas Sterilization Procedure...12 CALIBRATION TESTING ...13 Calibration Key Procedure ...13 M9600 Procedure ...13 THEORY OF OPERATION ...13 Temperature Measurement A/D Converter ...14 Theory of Operation...14 Circuit Description...15 Probe Warming ( Oral probes only) ...16 Theory of Operation...16 Circuit Operation...16 Probe Identification Logic ...17

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Welch Allyn Model 767

Basic Function ...17 Circuit Operation...17 Reset/Self Tests ...18 Power Supply ...18 Other Components ...18 Microcontroller ...18 Microprocessor Clock ...18 Liquid Crystal Display ...19 Backlight (Optional) ...19 Horn ……………………………………………………………………………………………………….19 Probe Switch...19 Serial Communications Port...19 Mode Button ...19 Thermometer Modes (Theory of Operation)...20 Monitor Mode...20 Normal Mode ...20 TROUBLESHOOTING...21 Error Codes ...21 Error Codes Table ...21 Equipment Required...22 Terminology...22 Troubleshooting Table ...23 FIELD SERVICEABLE REPAIRS...27 FIELD SERVICEABLE PARTS ...28 Model 767 Enhanced Replacement Parts...28 Replacement Parts-Circuit Board Assemblies ...29 THERMOMETER DISASSEMBLY ...31 THERMOMETER REASSEMBLY ...35 ACCESSORIES ...36

Technical Manual

TABLE OF FIGURES FIGURE 1 - THERMOMETER DIAGRAM ...6 FIGURE 2 - M767 DISPLAY ...8 FIGURE 3- SYSTEM BLOCK DIAGRAM ...14 FIGURE 4 - PROBE LOGIC...17 FIGURE 5 - THERMOMETER ASSEMBLY DRAWING ...32 FIGURE 6 - PRINTED CIRCUIT ASSEMBLY ...33 FIGURE 7 - SYSTEM SCHEMATIC...34

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Welch Allyn Model 767

WARRANTY 3-YEAR LIMITED WARRANTY ON NEW INSTRUMENTS: Instrumentation purchased new from Welch Allyn, Inc. is warranted to be free from original defects in material and workmanship under normal use and service for a period of three years from the date of first shipment from Welch Allyn. This warranty shall be fulfilled by Welch Allyn or its authorized representative repairing or replacing at Welch Allyn's discretion, any such defect, free of charge for parts and labor. Welch Allyn should be notified via telephone of any defective product and the item should be immediately returned, securely packaged and postage prepaid to Welch Allyn. Loss or damage in shipment shall be at purchaser's risk. Welch Allyn will not be responsible for loss associated with the use of any Welch Allyn product that (1) has had the serial number defaced, (2) has been repaired by anyone other than an authorized Welch Allyn Service Representative, (3) has been altered, or (4) has been used in a manner other than in accordance with instructions. 1 YEAR LIMITED WARRANTY ON PROBES AND PROBE COVERS: Welch Allyn warrants probe covers and probes to meet Welch Allyn’s specifications for the Product at the time of purchase and to be free from original defects in material and workmanship under normal use and service for a period equal to 1 year from the date of first shipment of such Product to the customer by or on behalf of Distributor. THIS WARRANTY IS EXCLUSIVE AND IN LIEU OF ANY IMPLIED WARRANTY OR MERCHANTABILITY, FITNESS FOR PARTICULAR PURPOSE, OR OTHER WARRANTY OF QUALITY, WHETHER EXPRESSED OR IMPLIED. WELCH ALLYN WILL NOT BE LIABLE FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES.

The information in this manual has been carefully reviewed and is believed to be accurate; however, no responsibility is assumed for inaccuracies. Furthermore, this information does not convey to the purchaser of Welch Allyn devices any license under the patent rights to the manufacturer.

Technical Manual

SPECIFICATIONS Case Dimensions:

5.1 inches high x 4.1 inches wide x 4.0 inches deep.

Case Material:

ABS Plastic.

Weight:

1.0 lbs

Input:

Welch Allyn Thermistor Probe (Oral P/N 02678-000 (4 ft. cord), P/N 02678-100 (9 ft. cord) Rectal P/N 02679-000 (4 ft. cord), P/N 02679-100 (9 ft. cord)).

Display range:

84.0o F to 108.0o F (28.9o C to 42.2o C)

Laboratory Accuracy:

± 0.2o F in Monitor Mode.

Clinical Accuracy:

Meets the proposed ASTM clinical test criteria.

Push buttons:

Mode button

Probes:

Interchangeable Oral, Rectal

Power Source:

Welch Allyn M767 Wall Transformer.

Display Type:

Liquid Crystal Display, 3½ digits plus special icons.

Operating temperature:

60.8 o F-104o F (16o C to 40o C) @ 15% to 95% RH non-condensing per ASTM E1112-86.

Storage Temperature:

-13o F-120o F (-25o C to 50o C) @ 15% to 95% RH non-condensing per ASTM E1112-86.

OPERATIONAL CHARACTERISTICS Introduction The Welch Allyn 767 SureTemp is a wall mounted thermistor thermometer to be used for measuring patient temperatures ranging from 84 °F to 108 °F (28.9 °C to 42.2 °C). Oral and rectal temperatures are taken using Normal or Monitor operating modes. Axillary temperatures are taken with an oral probe with the unit operating in the Axillary mode. In the Normal mode, the thermometer’s microprocessor “predicts” body temperature in about 4 seconds for oral temperatures, about 15 seconds for rectal temperatures, and about 10 seconds for axillary temperatures. The Monitor mode continuously displays the temperature of the probe which will reflect the patient’s actual temperature in about 3 minutes and will continue for as long as the probe remains in place.

Basic System Description The thermometer system consists of four main components: the external AC power supply, the thermometer instrument, the probe, and the probe cover. The M767 Thermometer can use either a Welch Allyn Model 767 domestic (120V 60HZ) or international (230V 50Hz) Wall Transformer or a Welch Allyn Model 77305 AC Adapter for its external AC power supply. These power supplies are available from Welch Allyn. The main instrument’s operation is described below. The Model 767 is easily connected to the Model 767 Wall Transformer. Two screws are used to secure the Model 767 to the Model 767 Wall Transformer.

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Welch Allyn Model 767

Welch Allyn’s M678/679 thermistor probes used with 767 thermometers can be identified by color combinations as follows: Handle Color

Top Color

Connector Color/Type

Model #

Probe Type

Fog White

Blue

Fog White, Latching

M678

Oral/Axillary

Fog White

Red

Fog White, Latching

M679

Rectal

The Model 767 instrument’s connector receptacle configuration and size disallows insertion of any but Model 678/679 probes and older, previous M767 probes. The probe covers are unchanged from previous Welch Allyn thermistor thermometer models and are compatible for use with all of Welch Allyn’s thermistor based thermometers. 9 SureTemp

1

7 2

3 6 5

4

1.

Display

6.

Probe

2.

Probe storage channel

7.

Probe handle collar

3.

Dual probe cover storage well

8.

Ejection button

4.

Probe connector receptacle

9.

Mode selection button

5.

Probe connector receptacle latch

FIGURE 1 - THERMOMETER DIAGRAM

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Technical Manual

SAFETY & WARNINGS CAUTION: DISCONNECT THE SYSTEM FROM THE MAINS SUPPLY BEFORE CONNECTING THE 767 SURETEMP THERMOMETER. USE THE 767 SURETEMP WITH WELCH ALLYN 3.5V 767 SERIES WALL TRANSFORMER ONLY. DO NOT USE IN THE PRESENCE OF FLAMMABLE ANESTHETICS. Caution: Federal (USA) law restricts this device to sale by or on the order of a physician. In order to obtain accurate and reliable temperature-taking results and ensure patient safety, it is important that this booklet be read thoroughly prior to using the instrument. If you have any technical or clinical questions concerning the thermometer’s use and/or care, please contact our Customer Service Department at (800) 854-2904. •

Single-use disposable probe covers, available from Welch Allyn, will limit patient crosscontamination. The use of any other probe cover or the failure to use a probe cover may produce temperature errors and will invalidate the instrument’s warranty.

•

Oral probes (blue ejection button) are to be used for taking oral and axillary temperatures only. Rectal probes (red ejection button) are to be used for taking rectal temperatures only. The use of the wrong probe will produce temperature errors.

•

The thermometer case is not waterproof; do not drip fluids onto it.

•

Do not use this instrument for any purpose other than that specified in this booklet. Doing so will invalidate the instrument’s warranty.

•

Do not use the thermometer if you notice any signs of damage to the probe or instrument. Contact our Customer Service Department for immediate assistance.

SELF TESTS Instrument Reset/Self Tests Note: If a problem is reported with an instrument, it is preferred that the user investigate operation before the unit is reset. This is because once the unit is reset, the error buffer which stores internal error messages will be erased. The M767 Wall Transformer Unit must be turned off for at least 1 second to reset the internal microprocessor electronics. 1. Remove the probe from the probe storage well and unplug the probe connector from the instrument by depressing the locking tab and pulling on the connector body. Do not pull on the cord. 2. While watching the display, turn the power to the Wall Transformer unit on and observe the power up self test. The self test includes several internal microprocessor self tests, instrument electronics tests and the display test. If there are internal electronics problems detected by the self tests, the error “|X|” icon will be displayed and an audible tone will sound. Refer to the Error Codes in the Troubleshooting section for an explanation.

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Welch Allyn Model 767

Display Test The display test begins with each display segment and icon being individually lit in brief and rapid succession. Immediately after this, all display segments and icons are simultaneously illuminated briefly followed by a display of the software revision in this instrument. The beeper also briefly sounds at the beginning of the test. At the end of the test, the display goes blank.

FIGURE 2 - M767 DISPLAY Note: If a probe is installed during this power up time, the probe type will be displayed as the last item before the display blanks. At this point, there should be no probe connected to the instrument. If there is no display, any missing segments, or no beeper, refer to the Troubleshooting section. Probe Warmer Circuit Self Tests Proper instrument functionality should be verified first as described in the Instrument Reset/Self Tests section (above) before a probe is installed. With a properly functioning instrument, the probe can be run through the self test. If the instrument is functioning properly and a probe is installed, the instrument will initiate the probe warmer self test during the instrument reset self test and whenever a probe is plugged in. If an instrument has passed the Instrument Reset/Self Tests section, install the probe connector and observe the display of the probe type. For the M767, the probe type will display for approximately 2 seconds. · If the display does not show “OrL” or “Aly” with an Oral/Axillary probe plugged in, or “rEC” with a Rectal probe plugged in, there is a problem with either the probe or the probe connector in the instrument. · If the display goes blank after the probe type display, the probe has passed its tests and the instrument is ready for use. Do not withdraw the probe during this self test. · If the display shows the malfunction icon |X| and/or the ‘Broken Probe’ icon, refer to the Troubleshooting section. Note: Handle the probe only by the probe handle, not the metal shaft. When removing a probe, disconnect the connector by pressing the locking tab and pulling on the connector body. Do not pull it by the cord. If there are any problems with probe initialization, refer to the Troubleshooting section.

Technical Manual

THERMOMETER MODES Normal Mode After instrument and probe warmer self tests, the system is ready for use. Normal mode operation is the rapid mode of temperature taking. This is the default mode and is automatically selected when the probe is withdrawn from the storage well. 1. Upon withdrawal of the probe from its storage well, every segment on the display will be illuminated. Watch for the display to change from the all segments test to the probe type display; “OrL/Aly” or “rEC”, depending on the probe and algorithm type , followed by °C or °F, whichever is selected. This display might take several seconds to appear. At the same time that the probe type is displayed, a short beep will sound. 2. At this point, load a probe cover and take a temperature. To change between the Oral and the Axillary algorithms; place the instrument in Ready mode as described above, and press the Mode button for approximately two seconds. Observe that the display changes between “Orl” and “Aly” every time the Mode button is pressed for more than two seconds. Note: It is possible that the display will switch from the probe type display to the “walking segments” display and back again several times before the probe is inserted in the mouth. This is acceptable operation and will not adversely effect the temperature taken. With correct use, the patient’s temperature will be displayed in about 4 seconds. The instrument will beep to signal completion of the Normal mode temperature cycle. Note: The thermometer reads the probe temperature immediately upon removal from the storage well. If the probe was just placed into the storage well from a previous temperature and immediately extracted, insufficient time may have passed to allow the probe to cool to room temperature. This will result in the instrument determining room temperature to be higher than actual. This could result in the instrument switching to Monitor mode immediately if it detects room temperature to be above 93.0°F (33.9°C). For best results, the user should wait at least 30 seconds between Normal mode temperatures. If the probe is left out of the storage well after completion of a Normal mode temperature and has not been placed into Monitor mode, the unit will shut down after 30 seconds to conserve power. Simply replace the probe in the storage well to prepare for the next temperature.

Monitor Mode The instrument can be placed in Monitor mode by pressing and holding the Mode switch for two seconds after a predictive (normal) temperature has been taken. The instrument will also automatically switch to Monitor mode under the following conditions: · If the prediction algorithm has not been activated for more than 60 seconds after taking the probe out of the storage well. ·

If the instrument determines that room temperature is above 93.0°F (33.9°C).

· If the thermometer is unable to predict an oral temperature after 15 seconds due to improper technique such as excessive probe movement.

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Welch Allyn Model 767

Monitor mode will be indicated on the display by a capital M in the bottom right corner. Monitor mode provides a direct readout of the probe temperature. This mode of operation has the ability to provide long term monitoring of a patient’s temperature. Unlike Normal mode, Monitor mode will follow a temperature as it rises or falls. The typical slow rise in temperature when Monitor mode is used is due mainly to the mouth temperature slowly recovering from placement of the colder (room temperature) probe. The probe itself is actually very fast at rising to the temperature of its surroundings, usually within a few seconds. Because of this slow mouth recovery, the recommended time to wait before recording a Monitor mode oral temperature is 3 minutes. The recommended time to wait before recording a Monitor mode rectal temperature is also 3 minutes. Similarly, the recommended waiting period for an axillary temperature taken in Monitor mode is 5 minutes. Monitor mode is also useful in testing the accuracy of the combined probe/instrument system when the probe can be warmed to a known temperature, as with a Welch Allyn Model 9600 Calibration Tester or in a circulating water bath set to a known temperature. Note: The instrument will shut off automatically if the probe temperature remains below 84.0°F (28.9°C) or above 108.0 °F (42.2 °C) for more than 5 minutes. If Monitor mode does not display expected temperatures or exhibits other problems, refer to the Troubleshooting section.

Biotech Mode To enter this special program mode: 1. Ensure an oral or rectal probe is installed in the instrument. 2. Press and hold the Mode button, and at the same time remove the probe from the probe well. Press the Mode button approximately 2 seconds to move sequentially through the various program categories. Changes within each category can be made by momentarily pressing and releasing the Mode button. Upon entering the Biotech mode, the features and selections described in the table below become available. A number is provided in the flag area to indicate which Biotech category is displayed. To exit Biotech mode at any time and return the unit to normal operation, insert the probe into the probe well. The Biotech mode will automatically time out after 5 minutes of inactivity. If you cannot enter the Biotech mode, refer to the Troubleshooting section.

Biotech Mode Program Table Program Category

Settings

1. Software Version

Observe the display, The display should show “r X.X” where “X.X” is a number such as 2.3. This can be helpful when discussing operation with Welch Allyn customer support. Press the Mode button for more than two seconds to advance to the Default Algorithm.

Technical Manual

2. Default Algorithm Oral/Axillary Modes

Three settings are available to set the default predictive algorithm: oral(OrL), axillary(ALY) or last prediction(LSt). To change the default algorithm, momentarily press the Mode button to advance to the next algorithm. The instrument will be placed in the selected algorithm after the probe has been returned to the probe well. The power-up setting is the oral predictive algorithm. Press the Mode button for more than two seconds to advance to the Battery Voltage.

3. DC Regulator Voltage

This section displays the internal DC Regulated voltage with 10 mV resolution. (This should be about 5 volts).

4. Predictive Temperature Counter

This section displays the count of the number of predictive temperature actuations that have occurred since last cleared (in 100’s). Resetting power to the Wall Transformer Unit will clear the counter. The power-up setting is 0.

5. Error Log

The instrument will save the last 10 error messages that occur and display those messages in a last in first viewed sequence. While in error log, the display reads E x.x. To display the next error, momentarily press the mode button. See the Troubleshooting section of this manual for a description of the various error that might appear in the error log. Note: Resetting power to the wall transformer will clear the error log of all error messages.

Temperature Recall Whenever the instrument is asleep (in Low Power mode), pressing and releasing the Mode button will cause the most recent predicted temperature to be displayed for 5 seconds. An “A”, “O”, or “R” will be displayed near the lower right corner of the LCD, designating Axillary, Oral and Rectal temperatures for the recalled temperature. Pressing the Pulse Timer button will interrupt the temperature recall function. Note: No Monitor temperatures will be saved for recall. When a temperature is recalled, the mode in which it was obtained (axillary, oral or rectal) will be shown independent of the present mode. If the last temperature cannot be recalled, refer to the Troubleshooting section.

F/C Conversion When a final temperature is displayed (in Normal, Recall, or Monitor mode), pressing and releasing the Mode button will toggle the temperature between °F/°C. Note: A recalled temperature will be displayed in whichever scale (°F/°C) is selected at the time of recall. This can also be changed during display. If pressing the Mode button does not change the scale of the displayed temperature, refer to the Troubleshooting section.

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Welch Allyn Model 767

PREVENTATIVE MAINTENANCE The following suggested preventative maintenance is recommended to maximize uninterrupted service on the Model 767 Thermometer Units which are in service on a regular basis should have the following preventative maintenance performed every 6 months: 1.

Visually inspect the thermometer for physical damage which might cause future product failure.

2.

Clean the unit per instructions in your Directions for Use manual supplied with the thermometer and/or per the instructions below.

3.

Perform the power up display test, the startup display test, and the Calibration Testing procedure found in the Model 9600 Operation Manual.

CLEANING AND STERILIZATION Routine Cleaning Refer to the Directions for Use for routine cleaning

ETO Gas Sterilization Procedure When no other form of decontamination such as a germicidal wipe is acceptable, a low temperature, (not to exceed 120°F(48.9°C)) ETO gas sterilization cycle may be used. Refer to your institution’s standard operating procedure for the length of the cycle. This type of sterilization may cause some hazing of glossy plastic surfaces and should be used only when absolutely necessary. 1. Disconnect the Model 767 Thermometer from the power supply. 2. Ensure that the probe is removed from its storage channel and disconnected from the instrument. 3. Remove any probe covers from the probe cover storage well 4. Wrap the thermometer in a standard sterilization type packaging such as a Baxter Tower Dualpeel Sterilization Pouch. 5. ETO gas sterilize at a temperature not to exceed 120°F(48.9°C) and aerate. 6. Remove the sterilization packaging. 7. Before applying power, allow the probe and instrument to stabilize to room temperature for at least one hour. 8. Verify proper calibration of the thermometer and probe using the Welch Allyn Model 9600 Calibration Tester.

Technical Manual

CALIBRATION TESTING The Calibration Key provides a convenient means of testing the thermometer for accuracy.

Calibration Key Procedure 1. Extract the probe and disconnect it from the thermometer. 2. Insert the Calibration Key into the probe connector receptacle on the thermometer and observe the display. The display should read CAL for two seconds, and then go blank. 3. Insert a probe shaft into the probe storage well and remove it to initiate a temperature taking cycle. Wait for the display test and then observe the display. The display must read between 97.1°F(36.2°C) and 97.5°F(36.4°C) inclusive for the calibration of the instrument to be correct. 4. Remove the Calibration Key and reinstall the probe connector plug. 5. Then install the probe into the probe storage well. Note: The Calibration Key test does not test the probe. To do so requires the use of the Welch Allyn Model 9600 Calibration Tester. If the reading from the Calibration Key is not within the specified range or you are having other problems with the use of the Calibration Key, refer to the Troubleshooting section.

M9600 Procedure The Model 9600 Calibration Tester provides a convenient means of testing the entire thermometer system (instrument and probe). · The 9600 Calibration Tester must be warmed up and stable at one of the two available temperature settings. ·

The unit under test must be in Monitor mode with no probe cover loaded onto the probe.

The probe is inserted into the small hole in the dry heat well of the 9600 Calibration Tester and allowed to settle for a minimum of 2 minutes to the final temperature. The reading on the thermometer must be within the range specified on the 9600 Calibration Tester. Refer to the Model 9600 Calibration Tester Operation Manual for complete instructions. Note: All Welch Allyn and Diatek thermometers (thermistor and infrared ear thermometers) can be checked in the Model 9600. If you are having problems with the use of the Model 9600, refer to the Troubleshooting section in the Model 9600 Operation Manual.

THEORY OF OPERATION This section describes in detail the operation of the various subcircuits in the M767 Thermometer. Refer to Figure 3 for a block diagram representation of the circuits and Figure 6 for the circuit schematic.

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Welch Allyn Model 767

MODEL 767 PCA Test Port J3

NEC UPD78064 uP

A/D Converter Circuitry

16 bit Timer

CPU

Serial Port

Serial I/O

Mode Button LCD Driver

Voltage Reference Circuitry

Horn Driver

Horn

Backlight Circuit Liquid Crystal Display

Probe Warmer Circuitry

Probe Logic Circuitry

AC TO DC CONVERTER CIRCUITRY

J8

J7

PROBE ASSEMBLY Probe Connector

M767 WALL TRANSFORMER

Four Conductor Coil Cord

Thermistor

Probe Warmer

FIGURE 3- SYSTEM BLOCK DIAGRAM

Temperature Measurement A/D Converter The primary function of the temperature measurement A/D converter circuit is to convert a measured resistance into a 16-bit word, which is then used by the microprocessor to calibrate itself and calculate a temperature.

Theory of Operation The A/D circuit is made up of 4 major parts: · The resistance to be measured (either the probe thermistor, the PTBCAL resistor, or the HICAL resistor). · The timing capacitor (C21, 0.33uF) and associated driver circuitry (Q1-Q4, Q9, R15, and R30). ·

A low bias dual voltage comparator (U2, TLC555).

·

A 16 bit timer internal to the microprocessor.

Technical Manual

In effect, this is a single slope converter which measures the time constant of the measured resistance combined with a fixed capacitance. The time constant is measured by counting the time it takes for the voltage to decay from a fixed initial voltage level to a fixed lower voltage. The ratio of this pulse width and the pulse width corresponding to a known calibration resistance (R13, 11.55K) is used to calculate the measured resistance. Once the resistance is known, the corresponding temperature is calculated using the thermistor temperature equation. Because the M767 uses this ‘ratio cal’ method for measuring the thermistor resistance, the device is immune to a number of gain errors which can be measured and corrected using software.

Circuit Description Initially, the microprocessor simultaneously discharges both sides of the capacitor. This is accomplished by bringing A/D TRIGGER high which turns on Q9, pulling one side of capacitor C21 to ground. This also turns on Q1, Q2, and Q3 via PROBE_SEL, PTBCAL_SEL, and HICAL_SEL, which allows the other side to discharge through the thermistor R8(12.1K) and R13(11.55K). Once capacitor C21 has been fully discharged, the probe thermistor is then selected by switching the PTBCAL and the HICAL resistor paths off. The microprocessor then forces the A/D TRIGGER signal to go low turning on Q4, which lifts one end of capacitor C21 up to VREF(2.4v). Because there can be no instantaneous voltage drop across capacitor C21, the other side of the capacitor immediately goes to 2.4v. This exceeds the threshold voltage(1.6v) of U2(TLC555), causing the output A/D_OUT to go low. At this point, the capacitor begins to discharge from 2.4v to 0v through the thermistor resistor. The output of the comparator goes high again when its input reaches the trigger voltage(0.8 volts). This produces a pulse of length equal to the time it takes for the capacitor to discharge from 1.6v to 0.8v through the thermistor. The microprocessor measures this pulse width using an internal 16-bit timer and then the same A/D conversion is performed using the HICAL resistor as the measurand resistance. The microprocessor performs the following calculation that ratios these two pulse widths to determine the exact resistance of the thermistor: Rthermistor

( PWthermistor ) . ( Rhical) PWhical

The microprocessor then uses the following equation to convert the measured resistance into a temperature: 1

Temperature_in_Kelvin Ra

Rb . ln ( Rt )

Rc . ( ln ( Rt ) )

3

Q4 and Q9 and the base resistors R15 and R30 form the level shifter and drive circuit for the fixed end of the timing capacitor. R15 (4.7K) is selected to give adequate saturation on-resistance. R30 (10K) is selected so that a float on the input will not cause damage from cross-conduction if the input is left floating, yet supply enough current to the timing capacitor during recovery. Q4 and Q9 exhibit only a few millivolts of saturation voltage, which does not affect the accuracy of the A/D as described above. · C21 is the surface mount timing capacitor. This capacitor is selected for low dielectric absorption, hence the high voltage rating (50V).

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Welch Allyn Model 767

· D4 and R16 protect the comparator from input undervoltage when the timing capacitor (C21) is discharged. D4 also reduces the recovery time by limiting the voltage at the beginning of recovery to 0.6 volts instead of 2/3VREF. · C5 provides the first line of defense from EMI (Electro Magnetic Interference) coming in on the probe. R33 and C18 protect Q1 from rectifying EMI in the substrate diode. R16 protects the comparator input from overcurrent. C32, C33, C34, C35, and C37 are also needed for protection from EMI.

Probe Warming ( Oral probes only) Probe characteristics vary somewhat due to normal production process variations. It is desirable to warm the probe as efficiently as possible from a time-to-ready standpoint and from a temperature stability standpoint when the probe is up to temperature. The probe warming process is a closed loop feedback control system incorporating PWM (pulse width modulation) control. The probe warmer circuitry is used to heat the probe tip prior to taking a temperature reading in order to speed the convergence of the prediction algorithm. This allows quicker temperature readings. A fail safe hardware shutoff circuit is included to ensure the heater will shut off in the event of a software failure.

Theory of Operation The microprocessor sends pulses via /HTRC to drive the probe heater resistor which heats the probe tip. A temperature of about 93°F(33.9°C) is maintained prior to taking a temperature. A software algorithm calculates the width of the HTRC pulse as a function of the difference between the probe temperature and 93°F(33.9°C), and as a function of the probe temperature rate of change. It provides an initial pulse to rapidly heat up the probe tip to the 90°F(32.2°C) region and then supplies progressively shorter pulses as the probe temperature converges to about 93°F(33.9°C). Once 93°F(33.9°C) is reached, software continues to send a “control” pulse to maintain the temperature.

Circuit Operation The warmer circuitry consists of Q5, Q13, Q14, C1, C2, L1, D1, R1, R2, R3, R4, R5, R31, R32, and the heater resistor (27 ohms) connected across J1-2 and J1-5. Line /HTRQ is pulled low by the uP which enables Q13 to turn on. /HTRC is then pulsed low which brings the base of Q5 low via capacitor C1. Q5 turns on which in turn enables Q14 on. Base current from Q14 flows through R31 (1K) and R32 (1K). Most of this current flows through the emitter of Q5 while some flows through R1 (3M) to satisfy the diode drop of Q5’s BE junction. The base current of Q5 along with the current through R1 flows into C1(1uF), charging it up. This sets up the mechanism for the hardware shutoff. As this capacitor charges up, the base voltage of Q5 approaches the emitter voltage and the transistor shuts off, thereby shutting down the probe warmer. As long as Q5 remains on, Q14 has a base current flowing which allows current to flow from its collector through R2 (4.7K) and the heater resistor. With about 150mA flowing through it ([VCC-2VCEsat]/29 ohms), the heater resistor heats up the probe tip.

Technical Manual

During normal operation, software turns the warmer circuit on and off. The width of the pulse on HTRC determines how long Q5 is turned on, thereby determining how long the heater is heating. Once the HTRC pulse goes high again, the base of Q5 is pulled high turning it off, and the capacitor discharges to VCC through D1. Q13 and Q14 are selected for their low saturation on voltage. D1 is a diode clamp used to keep the base of Q5 from attaining a much higher voltage than VCC. R4(47K) and R2(4.7K) in combination with R3(47K), serve as pull down resistors ensuring that the processor feedback lines (U1-31 and U132) go low immediately upon warmer component shut off. C2 serves as an RFI suppression component. C35 is not populated.

Probe Identification Logic Basic Function The probe logic circuitry is needed to determine which type of probe is connected to the device. This allows the microprocessor to initiate the proper algorithm for converting a thermistor measurement into a temperature. Figure 4 shows the probe logic diagram and corresponding circuitry. PROBE LOGIC SIGNAL

ORAL

RECTAL

CAL KEY

NO PROBE

PROBE_0

0

0

1

1

PROBE_1

0

1

0

1

VCC

1

R10 1M

R11 1M 2

Probe Connector PCB

1

NO CUT FOR ORAL

2

PROBE 1

To Microprocessor

CUT HERE FOR RECTAL

J1-6

PROBE 0

J1-3 1 C19 0.01UF

2

1

2

C20 0.01UF

CUT HERE FOR CAL KEY

FIGURE 4 - PROBE LOGIC

Circuit Operation R10, R11 (both 1 MW pullup resistors), J1-6(PROBE_1), J1-3(PROBE_0) and J1-5(GND) are used to provide logic 0 or 1 inputs to U1-17 and U1-18 depending on whether a probe or cal key has been plugged into the Model 767 Enhanced probe connector receptacle. When a probe has been plugged in, the software determines whether it is Oral, Axillary, Rectal, or a Cal Key as follows: · When J1-3 (CAL) and J1-6 (RCTL) are both connected to J1-5 (GND) the software determines that the probe is an Oral probe;

17

Welch Allyn Model 767

· When J1-3 (CAL) is connected to J1-5 (GND), but J1-6 (RCTL) is not connected to J1-5 (GND), the software determines that the probe is Rectal probe; · When J1-6 (RCTL) is connected to J1-5 (GND), but J1-3 (CAL) is not connected to J1-5 (GND), the software determines that the probe is a Calibration Key; · When neither J1-3 (CAL) or J1-6 (RCTL) are connected to J1-5 (GND), the software determines that no probe has been plugged in. C19 and C20, both 0.01 uF capacitors, are bypass capacitors used to filter out spurious noise to the microprocessor on the probe input lines J1-6(PROBE_1) and J1-3(PROBE_0).

Reset/Self Tests Upon power up, (assuming that the electronics have been discharged sufficiently by turning off power to the 767 Wall Transformer Unit for at least 1 second) the microprocessor receives a power up reset signal from the components associated with the reset line at U1-12. When power is applied to the instrument, U4 (Econo reset chip) waits approximately 200 milli-seconds before providing an active low reset to the microprocessor. When the reset signal is complete, the microprocessor launches a series of self checks which include RAM test, ROM test, instruction set test, self calibration tests (electronics accuracy test, hi cal, low cal), probe warmer circuitry tests, probe test, and ambient temperature test. Any failures here will cause a specific error code to be displayed to assist with debugging.

Power Supply Power is drawn from an 8VAC power supply. This AC voltage is converted to DC by a full wave bridge rectifier (D7, D8, D9, and D10), regulated by a low dropout 5 volt regulator (U3), and filtered by capacitors (C18, C24, C30, C28, C29 and C13). The regulated voltage will range from about 4.75 volts to 5.25 volts DC. This +5VDC (VDD) is further filtered to create an analog +5 volt branch (VCC).

Other Components Microcontroller A NEC UPD78064 or UPD78063 single chip microcontroller in a QFP package (U1) is used for signal digitizing, data processing, program memory addressing and storage, and I/O interfacing. The microcontroller also includes an LCD controller/driver which allows internal conversion of CMOS logic levels to a data format capable of driving the Model 767 LCD. In this application, the microcontroller is running at approximately 2.5 MHz, which is achieved by using a 4.9152 MHz Crystal (X1).

Microprocessor Clock The clock for the microprocessor is generated by X1 and capacitors C26 and C27, which form a 4.9152 MHz. oscillator circuit. The microcontroller is running at approximately 2.5 MHz, which is achieved by internally dividing the frequency of oscillation by two.