Production Testing of Thermistors Using the Model 2400
Contents
1. 5 JOE 1 Reference Ch 2 N Ch 3 T gt To other an channels 39 4 Ch 40 wy ELOT TO TTN PIHE PLEAS Output In Out HI Model 2400 In Out LO Figure 2 Switching multiple thermistors to Model 2400 using 2 wire method ae a a aoe aga ak Reference ae ae SS Model 7001 701 Mainframe 9 e o Card 2 i with 7011 Ch 1 wy i Ch 41 Card 1 ape oe eo Ch 2 i Sy i Ch 42 EO e Cy e ge ie Ch 3 i 7 i Ch 43 eo Lg __4 A A To other H oe tu Y channels Se am bi Oy 4 Ch 40 T i Ch 50 5 L te e A aR AA RAC oR ITEAN AEE EA UR See or Output Output In Out HI In Out LO Model 2400 Sense HI Sense LO Figure 3 Switching multipple thermistors to Model 2400 using 4 wire method To measure the resistance of the reference thermistor close Channels 1 and 41 apply the specified current and measure the resistance Open Channels 1 and 41 then close Channels 2 and 42 to begin testing the first thermistor Ry 1 Repeat this procedure for all the thermistors The actual number of thermistors may vary depending on the system Switching systems can be configured for any number of thermistors and for various electrical specifications When used with the appropriate scanner cards each Keithley Model 7002 Scanner Mainframe can switch up to 400 therm2. Fax 01 60 11 77 26 Landsberger Strasse 65 82110 Germering 089 84 93 07 40 Fax 089 84 93 07 34 Unit 2 Commerce Park Brunel Road Theale Reading Berkshire RG7 4AB 0118 929 7500 Fax 0118 929 7519 Flat 2B Willocrissa 14 Rest House Crescent Bangalore 560 001 91 80 509 1320 21 Fax 91 80 509 1322 Viale San Gimignano 38 20146 Milano 02 48 39 16 01 Fax 02 48 30 22 74 2FL URI Building 2 14 Yangjae Dong Seocho Gu Seoul 137 130 82 2 574 7778 Fax 82 2 574 7838 Postbus 559 4200 AN Gorinchem 0183 635333 Fax 0183 630821 Kriesbachstrasse 4 8600 D bendorf 01 821 94 44 Fax 01 820 30 81 Keithley Instruments Taiwan 1FL 85 Po Ai Street Hsinchu Taiwan R O C 886 3 572 9077 Fax 886 3 572 9031 No 1838 7012KDCI
3. a source and measuring instrument in one unit it is the same size as a half rack DMM and is just as easy to use This application note describes how to configure a ther mistor production test system using the Model 2400 SourceMeter instrument A short program that s included configures the instrument to output a single short current pulse and measure the resistance Another program provided measures the resistance of an external temperature device and then calculates the tempera ture In some production applications the temperature reference and the thermistors need to be switched to only one SourceMeter instrument For these cases a test configuration that includes programmable switching instruments is provided Production Testing of Thermistors Using the Model 2400 SourceMeter Instrument Test Description The DC resistance value of a thermistor is measured at one or more specific temperatures typically 25 C To get accurate measurements the test method must ensure self heating of the thermistor is minimized This can be done by using the lowest possible DC or pulsed test current Testing the thermistors in an oil bath also reduces heating effects and ensures temperature stability In some cases however the thermistors may be tested in air in which case lower current or pulsed current is used In either case the oil or air temperatures are typically measured because measuring and compensating for the temperature is usually e
4. length 24 status PRINT USING C VAL LEFTS A length CALL send 24 OUTP OFF status END turn output OFF programmable front rear switching so the reference tempera ture can be measured at the rear terminals and the resistance of the unknown thermistor can be measured at the front terminals In this way it s possible to take a temperature compensation measurement automatically without the need for a second instrument If timing is critical keep in mind that the sum of both the switching and settling times can be in the tens of milliseconds When the 2400 is switched from one input to the other the output is automatically turned off ensuring cold switching The relay used to switch between front and rear inputs Keithley Part No RL 163 has a mechanical life expectancy of 108 operations If the contact life of the RL 163 relay is not long enough then a separate scanner system can be used with the SourceMeter instrument to switch the input either to the thermistor under test or the temperature sensing element This is discussed further in the section titled Switching Multiple Thermistors To illustrate measuring temperature using the SourceMeter instrument a thermi linear component consisting of both a thermistor composite and a resistor set can be used as a tempera ture sensing element This particular device linearly relates its resistance to a temperature over a specific temperature
5. measure resistance manual mode auto zero off remote sense off conversion status trigger once turn output ON CALL send 24 SENS FUNC RES status CALL send 24 RES MODE MAN status CALL send 24 RES RANG 10E3 NPLC 1 0 status REM set source CALL send 24 SOURCE FUNC MODE CURR status CALL send 24 SOURCE CURR RANG 10E 6 status CALL send 24 SOURCE CURR 1E 6 status CALL send 24 SOURCE CURR MODE FIXED status CALL send 24 SOURCE DELAY 0 status REM global paramters CALL send 24 SYST AZERO 0 status CALL send 24 SYST RSEN 0 status REM set up math expression for temperatur CALL send 24 CALC MATH DEL ALL status CALL send 24 CALC MATH EXPR NAME TEMP status CALL send 24 CALC MATH EXPR res 4593 39 32 402 CALL send 24 CALC MATH NAME TEMP status CALL send 24 CALC MATH UNITS C status CALL send 24 CALC STATE ON status REM set trigger CALL send 24 ARM COUNT 1 status CALL send 24 TRIG COUNT 1 status CALL send 24 TRIG SOURCE IMM status CALL send 24 TRIG DELAY 0 status CALL send 24 OUTPUT ON status REM get temperature reading CALL send 24 INIT status CALL send 24 CALC DATA status AS SPACES 80 CALL enter A
6. Number 806 KEITHLEY Application Note Series Introduction Thermistors are devices that exhibit a change in resistance with a change in temperature The resistance of an NTC negative temperature coefficient thermistor decreases with an increase in temperature whereas the resistance of a PTC positive tempera ture coefficient thermistor increases with an increase in tempera ture Thermistors have many applications including their use as temperature sensors resettable fuses power indicators and current limiters Due to the wide variety of applications for thermistors the specific production tests performed on these devices often depend on the application One common production test regardless of the application is resistance versus temperature The range of resistance of a thermistor may vary from lt 1Q to as high as IMQ In most cases however the resistance is in the kilo ohm range While digital multimeters are often used to measure the resis tance these instruments do not usually allow programming the magnitude and duration of the test current To keep the test current low when using a DMM the resistance measurement must often be made on a higher range which decreases sensitiv ity However the Model 2400 SourceMeter instrument measures a wide range of resistance and has an adjustable test current This instrument measures resistance directly by using a separate current source and voltmeter Even though it combines both
7. asier than trying to control it A reference thermistor which has a similar composition to the thermistors being tested is often used to make the temperature compensation measurement Once the temperature is known the control software can correct for the actual resistance measurement of the thermistor The resistance value is compared to a standard value then the thermistor is either sorted with an automatic handling machine and placed in the proper bin or reground to tolerance Test Procedures Using the Model 2400 to Measure the Resistance of a Thermistor Depending on the magnitude of resistance being measured either a two wire or four wire method is used The 4 wire method prevents any lead resistance from being added into the measure ment For measurements in the kilo ohm ranges a two wire approach is usually sufficient Figure 1 illustrates making a two wire connection from the SourceMeter to a thermistor Current Source Figure 1 Using the Model 2400 to measure resistance of a thermistor The lowest possible test current should be used to avoid thermistor self heating This is especially true for testing in air and for NTC thermistors which are very sensitive to temperature changes The SourceMeter instrument allows programming both the magnitude and duration of the test current The magnitude of the test current can be programmed from 50pA to 1A The instrument can generate pulses as short as 2ms while still ensuri
8. bin The SourceMeter instrument has four digital output lines that can be used to set up bit patterns to produce TTL level signals Typical Sources of Error Lead Resistance A common source of resistance measurement error is the series resistance of the test leads running from the SourceMeter instrument to the thermistor This series resistance is added into the measurement when making a two wire connection See Figure 5 The effect of lead resistance is particularly detrimental when long connecting cables and high currents are used because the voltage drop across the lead resistance becomes significant compared to the measured voltage Current Source Figure 5 Two wire technique To eliminate this problem use the four wire remote sensing method rather than the two wire technique With the four wire method Figure 6 a current is forced through the thermistor using one pair of leads and the voltage across the thermistor is measured through a second set of leads As a result only the voltage drop across the thermistor is measured Model 2400 r Current Source Voltmeter Figure 6 Four wire technique Thermoelectric EMFs Thermoelectric EMFs may cause measurement problems especially when testing the thermistors in air One way to avoid these thermally generated voltages is to use the instrument s offset compensated ohms mode This mode works by first taking a measurement with the source current on th
9. ction manual resistance mode 2 volt range 20 volt compliance souce current source clear auto 4ms delay time CALL send 24 SENS FUNC OFF ALL gpib status CALL send 24 SENS FUNC RES gpib status CALL send 24 SENS RES MODE MAN gpib status CALL send 24 VOLT RANG 2 NPLC 0 01 gpib status CALL send 24 VOLT PROT 20 gpib status REM set source CALL send 24 SOURCE FUNC MODE CURR gpib status CALL send 24 SOURCE CLEAR AUTO ON gpib status CALL send 24 SOURCE CURR RANG 100E 6 gpib status CALL send 24 SOURCE CURR 10E 6 gpib status CALL send 24 SOURCE CURR MODE FIXED gpib status CALL send 24 SOURCE DELAY 004 gpib status REM set output to ZERO mode CALL send 24 OUTPUT SMODE ZERO gpib statuss REM set trigger CALL send 24 ARM COUNT 1 gpib status CALL send 24 TRIG COUN 1 gpib status CALL send 24 TRIG SOURCE IMM gpib status CALL send 24 TRIG DELAY 0 gpib status M xtake reading L send 24 READ gpib status L enter DATAS lenght 24 gpib status INT DATAS R C C P Depew REM turn output OFF CALL send 24 SOURCE CLEAR AUTO OFF CALL send 24 OUTP OFF gpib status output 1 pulse no delay before puls gpib status turn off so
10. en taking another with the source current off The second measurement is then subtracted from the first measurement canceling out any voltage offsets The corrected measurement is automatically determined by using the two point measurement method and is expressed mathematically as V V L 1 where I is the source current set to a specified level Offset Compensated Ohms I is the source current set to zero it could be set to any level V is the voltage measured at V is the voltage measured at L This feature can be enabled from the front panel by first placing the instrument in the source current mode then pressing CONFIG and then FCTN Selecting OFF COMP OHMS will bring up the menu making it possible to select the two desired source currents one of which should be zero If programming the instrument via the bus the Offset Compensated Ohms function is configured using the CALC1 Subsystem Alternative Solutions If controlling the duration and magnitude of the test current is unimportant then a DMM can be used to make these measure ments The Models 2001 and 2002 DMMs offer two high and two low limit values which can be tied to the status of any of four digital outputs When used with a component handler this allows these DMMs to be used to sort or grade thermistors When measuring thermistors with values less than 100Q the Model 2010 Low Noise Multimeter may be used because it offers a low power measuremen
11. istors using a two pole method or up to 200 thermistors using a four pole method Multiple mainframes may be used for larger systems Test System Configuration Component Handler _ Mechanical Digital 1 O Connection 2400 SourceMeter Instrument Test Leads Test Fixture Figure 4 Thermistor production test system A block diagram of a Model 2400 based production test system for thermistors is shown in Figure 4 The thermistor is placed in a temperature controlled test fixture with connections to the instrument When triggered the instrument outputs a current and measures the voltage drop across the thermistor then automatically calculates the resistance The instrument compares this resistance measurement to a pre specified limit then sends a signal via the digital I O port to the component handler which assigns the thermistor to the appropriate bin As Figure 4 illustrates the Model 2400 has both an IEEE 488 output and a digital I O port The digital I O port can send signals directly to and receive signals from an automatic handler machine Up to five limit values one compliance two high and two low values can be programmed into the instrument The limit values will determine the Pass Fail and High Low status of subsequent measurements Once a measurement is made and the tolerance is determined the digital I O port sends a TTL level signal to the automatic handling machine to route the thermistor to the appropriate
12. ng an accurate measurement To meet the time limitation when outputting pulses in the millisecond range the SourceMeter instrument must be pro grammed to run as fast as possible which involves disabling features of the instrument that slow it down such as autoranging auto zeroing the front panel display and filters The program mable trigger delay is set to 0 seconds The source delay time controls the duration of the pulse The program listed below was developed to output 104A for 5ms into a thermistor with a resistance of 10kQ One resist ance measurement is taken and displayed on the screen of the computer Program Name is 2400res bas SINCLUDE ieeeqb bi This program uses Microsoft QuickBasic 4 5 and the KPC 488 2AT IEEE Interface Card CALL initialize 21 0 CALL transmit UNT UNL MTA LISTEN 24 SDC UNL UNT CALL send 24 RST gpib status DATAS SPACES 800 REM global paramters CALL send 24 SYST AZERO 0 gpib status CALL send 24 SYST RSEN 0 gpib status CALL send 24 DISP ENABLE OFF gpib status CALL send 24 FORM ELEM RES gpib status REM set measure compliance gpib statusS auto zero off local sense 2 wire turn display off resistance reading only turn all functions off turn on resistance fun
13. nsulation layer fails e Use high reliability fail safe interlock switches to disconnect power sources when a test fixture cover is opened e Where possible use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards e Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury It is the responsibility of the test system designers integra tors and installers to make sure operator and maintenance personnel protection is in place and effective Keithley Instruments Inc 28775 Aurora Road Cleveland Ohio 44139 e 440 248 0400 Fax 440 248 6168 1 888 K EITHLEY 534 8453 www keithley com BELGIUM Keithley Instruments B V Bergensesteenweg 709 B 1600 Sint Pieters Leeuw 02 363 00 40 Fax 02 363 00 64 CHINA Keithley Instruments China FRANCE Keithley Instruments Sarl GERMANY Keithley Instruments G mbH GREAT BRITAIN Keithley Instruments Ltd INDIA Keithley Instruments GmbH ITALY Keithley Instruments s r l KOREA Keithley Instruments K orea NETHERLANDS Keithley Instruments B V SWITZERLAND Keithley Instruments SA TAIWAN Copyright 2001 Keithley Instruments Inc Printed in the U S A Yuan Chen Xin Building Room 705 12 Yumin Road Dewai Madian Beijing 100029 8610 6202 2886 Fax 8610 6202 2892 3 all e des Garays 91127 Palaiseau C dex 01 64 53 20 20
14. range The program for the SourceMeter instrument on the previous page automatically converts the resistance of the device as expressed in ohms to a temperature measurement in degrees Celsius by using the instrument s built in math capability Example Programs To obtain a digital file of either of the Example Programs listed access Keithley s World Wide Web site Switching Multiple Thermistors Some applications require switching both the temperature sensor and the thermistors to a single SourceMeter instrument Figure 2 shows the input output of the instrument being switched to either one reference thermistor or to any of the 39 thermistors of unknown value In this switching example a two wire technique is used The scanning is accomplished using one Model 7001 Scanner Mainframe and one Model 7011 40 Channel Switch Card To measure the reference thermistor Channel 1 is closed and the resistance is measured Open Channel 1 then close Channel 2 to measure the resistance of the first thermistor Ry 1 This process is repeated for all the thermistors Figure 3 shows a scanner system using a four wire tech nique For scanning thermistors with lower resistances lt 100Q a four wire technique is used to prevent the lead resistance from being added into the measurement Note that two 2 pole relays are used to connect each thermistor to the instrument Model 7001 Mainframe with one Model 7011 Scanner Card 7
15. t mode on the 10Q and 100Q ranges All Keithley 2000 Series DMMs 2000 2001 2002 and 2010 offer two and four wire ohms measurement functions and optional plug in scanner cards for switching multiple thermistors If the thermistors are in the milliohm range the Model 2750 Multimeter Switch System provides resistance measurements with 10uQ resolution and includes a built in switch mainframe for multi channel operation Specifications are subject to change without notice All Keithley trademarks and trade names are the property of Keithley Instruments Inc All other trademarks and trade names are the property of their respective companies KEITHLEY Test System Safety Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels It is also possible under single fault conditions e g a programming error or an instrument failure to output hazardous levels even when the system indicates no hazard is present These high voltage and power levels make it essential to protect operators from any of these hazards at all times Protec tion methods include Design test fixtures to prevent operator contact with any hazardous circuit e Make sure the device under test is fully enclosed to protect the operator from any flying debris Double insulate all electrical connections that an operator could touch Double insulation ensures the operator is still protected even if one i
16. urce The duration of the test current is controlled using the SOURCE DELAY command In the program listing note that the source delay was programmed for 4ms even though the actual pulse is about 5ms in length Depending on the setup of the 2400 there is roughly lms of overhead time when both sourcing a current and taking a voltage measurement A discussion for calculating the source delay measure timing cycle can be found in the Model 2400 User s Manual Using the Model 2400 to Measure Temperature It s also necessary to measure the temperature of the air or oil in which the thermistors are tested This temperature measure ment is often made with a thermistor that has a similar composi tion to the thermistors being tested This similarity is important in order to ensure that the reference thermistor and the thermistors under test will have the same temperature time constant How ever in some cases a more accurate temperature sensor such as an RTD may be used The Model 2400 has a dual channel input Program name is 2400temp bas SINCLUDE ieeeqb bi This program uses Microsoft Quickbasic 4 5 and the KPC 488 2AT IEEE CALL initialize 21 0 CALL transmit UNT UNL TA LISTEN 24 SDC UNL UNT CALL send 24 RST status REM set function Interface Card status
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