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DS2760 Thermocouple Kit (#28022)
1-Wire
®
Thermocouple Interface
Introduction
Thermocouples provide a low-cost, reliable means of measuring temperature over a wide range. The
challenge when using a thermocouple is accurately measuring the very low Seebeck output voltage
(fractional to low millivolts) from the element, and providing for cold junction temperature compensation.
The Dallas/Maxim DS2760 High Precision Li+ Battery Monitor is very easily configured into an effective
thermocouple interface. The Parallax DS2760 Thermocouple Module capitalizes on this application and
provides a complete connection between the BASIC Stamp and a standard thermocouple element.
Features
1-Wire interface allows multiple devices with just one Stamp IO pin
Cold Junction measurement: 0°C to +127°C (0.125°C resolution)
Low power consumption:
-- Active current: 90
A
max
-- Sleep current: 2
A
max
®
Packing List
Verify that your DS2760 kit is complete in accordance with the list below:
DS2760 Thermocouple Module
(3) Thermocouple elements:
-- (1) K-type (Chromel / Alumel)
-- (1) J-type (Iron / Constantan)
-- (1) T-type (Copper / Constantan)
This documentation
#550-28022
#800-00011
#800-00012
#800-00010
Note: DS2760 demonstration software may be downloaded from www.parallax.com.
Parallax, Inc.
•
DS2760 Thermocouple Kit (#28022)
•
01/2004
1
Connections
Before connecting the DS2760 Thermocouple Module to the BASIC Stamp you will need to prepare a
thermocouple element, and then connect it to the cold junction port of the module. Start by carefully
removing about one inch (250 mm) of the outer sleeve from each end of the element. From each lead on
the temperature measurement end, remove about
½ inch (125 mm) of insulation and then carefully twist
together (using pliers if necessary) and trim as shown in Figure 1.
Figure 1: Thermocouple Junction
On the cold junction (DS2760 module) end of the element, remove only
¼ inch (60 mm) of insulation from
each lead. Route these leads through the bottom of the thermocouple module PCB and insert snuggly
into the pin sockets as shown in Figure 2.
Figure 2: Cold Junction Connection to DS2760 PCB
Use this table to ensure that you make the proper thermocouple connections to the module. If the leads
are reversed, the measured temperature will be incorrect.
Type
K
J
T
Materials
Chromel / Alumel
Iron / Constantan
Copper / Constantan
SNS
Red
Red
Red
Vss
Yellow
White
Blue
Finally, the DS2760 Thermocouple Module is connected to the BASIC Stamp as shown in Figure 3 below
®
(Note that the module includes a 4.7 K pull-up on the 1-Wire data line).
Figure 3: DS2760 Connections to BASIC Stamp
Vdd
Data IO
Vss
2
Parallax, Inc.
•
DS2760 Thermocouple Kit (#28022)
•
01/2004
BASIC Stamp Application
The following BASIC Stamp application will run on either the BS2p or BS2pe and demonstrates how easy
measuring wide-range temperatures can be when using the DS2760 Thermocouple Module. Other
Stamps will require a Serial-to-1-Wire protocol converter, as well as code to manage the large tables
across program slots, and are not covered in this document.
A little background: When two dissimilar metal wires are joined, a voltage will be developed across the
open end that is proportional to the temperature difference between the joined and open ends. This
effect was discovered by Thomas Seebeck in 1821. Through empirical testing, voltage tables have been
established that correspond to the thermocouple junction temperature. These tables, however, use a
cold junction (voltage measurement point) reference of zero degrees Celsius, forcing electronic devices to
employ cold junction compensation.
Using the DS2760 we can measure the Seebeck voltage from the thermocouple with a resolution of
15.625 microvolts, then measure the cold junction temperature with a resolution of 0.125 degrees
Celsius. A simple table look-up using the cold junction temperature will give us the cold junction
compensation voltage. This is combined with the Seebeck voltage and, using a modified binary search
algorithm, we can determine the compensated temperature from the thermocouple data table.
' =========================================================================
'
'
File...... DS2760TC_Demo.BPE
'
Purpose... Thermocouple temperature measurement using the DS2760
'
Author.... Parallax, Inc. (Copyright 2004, All Rights Reserved)
'
E-mail.... support@parallax.com
'
Started...
'
Updated... 19 JAN 2004
'
'
{$STAMP BS2pe, KTablePos.BPE, JTablePos.BPE, TTablePos.BPE}
'
{$PBASIC 2.5}
'
' =========================================================================
'
'
'
'
'
-----[ Program Description ]---------------------------------------------
This program lets a BS2p or BS2pe read the temperature from the Parallax
DS2760 thermocouple module. User input of thermocouple type (K, J, or T)
and temperature display is via the DEBUG window.
' -----[ Revision History ]------------------------------------------------
' -----[ I/O Definitions ]-------------------------------------------------
OW
PIN
8
' 1-Wire buss pin
Parallax, Inc.
•
DS2760 Thermocouple Kit (#28022)
•
01/2004
3
' -----[ Constants ]-------------------------------------------------------
ReadNet
SkipNet
RdReg
CON
CON
CON
$33
$CC
$69
' read OW net address
' skip OW net address
' read register
' -----[ Variables ]-------------------------------------------------------
idx
type
char
vIn
tmpCJ
tCuV
sign
cjComp
tempC
tempF
tblLo
tblHi
eePntr
testVal
error
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
Nib
Nib
Byte
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Word
Bit
' loop counter
' device type
' display byte/char
'
'
'
'
in millivolts
device temp in C
thermocouple millivolts
TC sign bit
' temp compensation
' temp in Celsius
' temp in Fahrenheit
' table pointers
' test value from table
' 1 = out of range
' -----[ EEPROM Data ]-----------------------------------------------------
' -----[ Initialization ]--------------------------------------------------
Stamp_Check:
#IF ($stamp < BS2P) #THEN
#ERROR "This program requires BS2p or BS2pe"
#ENDIF
Check_Device:
OWOUT OW, %0001, [ReadNet]
OWIN OW, %0010, [SPSTR 8]
GET idx, char
IF (char <> $30) THEN
DEBUG "No DS2760 found."
STOP
ENDIF
'
'
'
'
get serial number
store in SPRAM
read device type
if not $30, wrong device
' stop program
4
Parallax, Inc.
•
DS2760 Thermocouple Kit (#28022)
•
01/2004
Menu:
DEBUG CLS,
"===============================", CR,
" DS2760 Thermocouple Interface ", CR,
"===============================", CR,
CR,
"Select TC Type (1 - 3)", CR,
CR,
"(1) K - Chromel/Alumel", CR,
"(2) J - Iron/Constantan", CR,
"(3) T - Copper/Constantan", CR,
CR,
">>> "
DEBUGIN DEC1 type
IF (type < 1) OR (type > 3) THEN Menu
DEBUG CRSRXY, 0, 3, CLRDN
STORE type
Show_SN:
DEBUG CRSRXY, 0, 4, "Device SN... "
FOR idx = 0 TO 7
GET idx, char
DEBUG HEX2 char
NEXT
Show_Type:
DEBUG CRSRXY, 0, 6, "TC Type..... "
LOOKUP (type - 1), ["KJT"], char
DEBUG char
'
'
'
'
get selection
validate selection
remove selections
point READ to table
' -----[ Program Code ]----------------------------------------------------
Main:
DO
GOSUB Read_TC_Volts
GOSUB Read_CJ_Temp
READ (tmpCJ * 2), Word cjComp
' combine cjComp and tCuV
'
IF sign THEN
' TC below cold junction
IF (tCuV < cjComp) THEN
cjComp = cjComp - tCuV
ELSE
cjComp = 0
ENDIF
' read Seebeck voltage
' read cold junction temp
' get compensation voltage
' limit to 0C
Parallax, Inc.
•
DS2760 Thermocouple Kit (#28022)
•
01/2004
5
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