Therm is Tor Design Guide

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Ntc thermistor DesiGN GUiDeF O R D I S C R E T E C O M P O N E N T S & P R O B E S


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OUR MISSION: Troug tamwork, to aciv industr’s confidnc as t

igst quait producr of tmpratur snsors in t word.

i NDUstrY ’ s PArtNer iN QUAL i tY AND PerFormANce ™

Ntc thermistor DesiGN GUiDe For DiscretecomPoNeNts & Probes

What is a thermistor? ..........................................................................................3

How to use a thermistor .....................................................................................5

Wh use a thermistor? ........................................................................................6

How do I use a Thermistor? ...............................................................................7How much resistance do I need? .....................................................................8

What’s a curve and which curve do I choose? .............................................9

What is Therma Time Constant? (Mi-PRF 23648) ..................................10

What is Therma Dissipation Constant? .......................................................11

What is Se Heating? ........................................................................................11

How do I design a probe? .................................................................................12

Insuation Properties ..........................................................................................13

Conversion Tabes ................................................................................................14

Frequent Asked Questions .............................................................................15

New Products .......................................................................................................16

How sma can ou make a part? ...................................................................17

sPeciAL serVices .........................................................18

Since 1977, Quait Thermistor, Inc. has designed and manuactured PTC and NTC thermistors

o superior quait. Miions o QTI TM Brand thermistor temperature probes have been used or

mission critica appications rom deep beow the oceans’ surace to the outer reaches o space.

Our state-o-the-art manuacturing aciit ocated in Boise, Idaho combined with our high-voume

assemb pant in Mexico ensure no project is to sma or arge or us to accommodate.

This NTC thermistor design guide has been thoughtu prepared to address some o the most common

temperature reated questions acing design engineers. I ou have additiona questions, pease ee ree

to contact us. We woud be happ to work with ou on our appication.

CONTENTS



8 0 0 - 5 5 4 - 4 7 8 4 w w w . t h e r m i s t o r . c o m

An NTC thermistor is a semiconductor made rom metaic

oxides, pressed into a sma bead, disk, waer, or other shape,

sintered at high temperatures, and then coated with epox or

gass. The resuting device exhibits an eectrica resistance that

has a ver predictabe change with temperature.

Thermistors are wide used or temperature monitoring, contro

and compensation. The are extreme sensitive to temperature

change, ver accurate and interchangeabe. The have a wide

temperature enveope and can be hermetica seaed or use in

humid environments.

The term “thermistor” originated rom the descriptor therma

sensitive Resistor. The two basic tpes o thermistors are the

Negative Temperature Coeicient (NTC) and Positive TemperatureCoeicient (PTC).

Thermistors are avaiabe as surace mount or radia and axia

eaded packages. The eaded parts can then be either over

moded or housed in a variet o shapes and materias.

Athough this design guide ocuses on NTC (Negative

Temperature Coeicient), thermistors are aso avaiabe in

Positive Temperature Coeicients.

··

Pronunciation: ther--ter, thur-u-sterOrigin: 1935–40Function: nounEtmoog: a res

An electrical resistor whose resistance varies

rapidly and predictably with temperature and

as a result can be used to measure temperature.

TheRMISTOR STyleS

Aia ladd (PTC)

RTh42

RTh22 PTC

QTG12 PTC

OTG10 PTC

Surfac Mount

1206

0805 NTC & PTC0603

0402

NTC Radia ladd

QTMC

QTlC

Bar Di

QTC11 NTC

QTC11 PTC

WHAT IS A THERMISTOR?



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N t c t h e r m i s t o r D e s i G N G U i D e

Thermistor From Wikipedia, the ree enccopedia

A thermistor is a tpe o resistor used to measure temperature

changes, reing on the change in its resistance with chang-

ing temperature. The Thermistor was irst invented b Samue

Ruben in 1930, and has U.S. Patent #2,021,491.

I we assume that the reationship between resistance and

temperature is inear (i.e. we make a irst-order approximation),

then we can sa that:

∆R = k∆T

Where

∆R = change in resistance t

∆T = change in temperature

k = irst-order temperature coeicient o resistance

Thermistors can be cassiied into two tpes depending on the

sign o k. I k is positive, the resistance increases with increas-

ing temperature, and the device is caed a positive temperature

coeicient (PTC) thermistor. I k is negative, the resistance

decreases with increasing temperature, and the device is caed a

negative temperature coeicient (NTC) thermistor. Resistors that

are not thermistors are designed to have the smaest possibe

k, so that their resistance remains amost constant over a wide

temperature range.

sna ha quanIn practice, the inear approximation (above) works on over a

sma temperature range. For accurate temperature measure-

ments, the resistance/temperature curve o the device must be

described in more detai. The Steinhart-Hart equation is a wide

used third-order approximation:

where a, b and c are caed the Steinhart-Hart parameters, and

must be speciied or each device. T is the temperature in Kevin

and R is the resistance in ohms. To give resistance as a unction

o temperature, the above can be rearranged into:

where and

The error in the Steinhart-Hart equation is genera ess than

0.02°C in the measurement o temperature. As an exampe, tpi-

ca vaues or a thermistor with a resistance o 3000Ω

at roomtemperature (25°C = 298.15 K) are:

a =1.40 x 10-3

b =2.37 x 10-4

c =9.90 x 10-8

cndun dlMan NTC thermistors are made rom a pressed disc or cast chip

o a semiconductor such as a sintered meta oxide. The work

because raising the temperature o a semiconductor increases

the number o eectrons abe to move about and carr charge

- it promotes them into the conducting band. The more charge

carriers that are avaiabe, the more current a materia can con-

duct. This is described in the ormua:

I = n • A • v • e

I = eectric current (ampere)

n = densit o charge carriers (count/m3)

A = cross-sectiona area o the materia (m2)

v = veocit o charge carriers (m/s)

e = charge o an eectron

The current is measured using an ammeter. Over arge changes

in temperature, caibration is necessar. Over sma changes in

temperature, i the right semiconductor is used, the resistance

o the materia is inear proportiona to the temperature. There

are man dierent semiconducting thermistors and their rangegoes rom about 0.01 Kevin to 2,000 Kevins (-273.14°C to

1,700°C). QTI range -55 to 300ºC.

Most PTC thermistors are o the "switching" tpe, which means

that their resistance rises sudden at a certain critica tempera-

ture. The devices are made o a doped pocrstaine ceramic

containing barium titanate (BaTiO3) and other compounds. QTI

manuactures siicon based PTC thermistors that are .7%/ºC.

Thermistor Symbol

WHAT IS A THERMISTOR?




HOW TO USE A THERMISTOR

The NTC thermistor is best suited or precision temperature mea-surement. The PTC is best suited or temperature compensation.

The NTC thermistor is used in three dierent modes o operation,

which services a variet o appications.

ran-Vu-tpau md - B ar the

most prevaent. These circuits perorm precision temperature

measurement, contro and compensation. Unike the other appi-

cations this method depends on the thermistor being operated in

a “zero-power” condition. This condition impies that there is no

se-heating.

The resistance across the sensor is reative high in comparison

to an RTD eement, which is usua in the hundreds o ohmsrange. Tpica, the 25°C rating or thermistors is rom 10Ω up

to 10,000,000Ω. The housing o the thermistor varies as the

requirements or a hermetic sea and ruggedness, but in most

cases, there are on two wires going to the eement. This is pos-

sibe because o the resistance o the wire over temperature is

considerab ower than the thermistor eement. And tpica

does not require compensation because o the overa resistance.

cun-ov-t md – This depends on the dissipa-

tion constant o the thermistor package as we as eement’s

heat capacit. As current is appied to a thermistor, the package

wi begin to se-heat. I the current is continuous, the resis-

tance o the thermistor wi start to essen. The thermistor cur-rent-time characteristics can be used to sow down the aects

o a high votage spike, which coud be or a short duration. In

this manner, a time dea rom the thermistor is used to prevent

ase triggering o reas.

This tpe o time response is reative ast as compared to

diodes or siicon based temperature sensors. In contrast, ther-

mocoupes and RTD’s are equa as ast as the thermistor, but

the don’t have the equivaent high eve outputs.

Vlag-Vu-cun md - Votage-versus-current

appications use one or more thermistors that are operated in a

se-heated condition. An exampe o this woud be a ow meter.The thermistor woud be in an ambient se-heated condition.

The thermistor’s resistance is changed b the amount o heat

generated b the power dissipated b the eement. An change

in the media (gas/iquid) across the device changes the power

dissipation actor o the thermistor. The sma size o the therm-

istor aows or this tpe o appication to be impemented with

minima intererence to the sstem.



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rlun - larg cang in rsistanc for a sma

cang in tmpratur

Another advantage o the thermistor is its reative high resis-

tance. Thermistors are avaiabe with base resistances (at 25°

C) ranging rom tens to miions o ohms. This high resistance

reduces the eect o resistance in the ead wires, which can

cause signiicant errors with ow resistance devices such as

RTD’s. An exampe o this is the traditiona RTD, which tpica

requires 3-wire or 4-wire connections to reduce errors, caused

b ead wire resistance; 2-wire connections to thermistors are

usua adequate.

The thermistor has been used primari or high-resoution mea-surements over imited temperature ranges (-55° to 150°C). The

cassic exampe o this woud be a medica appication where

the user is on concerned with bod temperature. However,

widespread improvements in thermistor stabiit, accurac, and

interchangeabiit have prompted increased usage o thermistors

in a tpes o industries.

cThermistors are b ar the most economica choice when it

comes to temperature sensors. Not on are the ess expensive

to purchase, but aso there are no caibration costs during insta-

ation or during the service ie o the sensor. In addition, i

there is a aiure in the ied, interchangeabe thermistors can beswapped out without caibration.

spdDue to their sma size, thermistors can respond ver quick to

sight changes in temperature. Caution must be taken when

designing probes because materias and mass pa an important

roe in the reaction time o the sensor. See section on “Therma

Time Constant” and “How do I design a probe?” or urther

detais.

N calan rqudProper manuactured thermistors are aged to reduce drit

beore eaving the actor. Thereore, thermistors can provide astabe resistance output over ong periods o time.

WHy USE A THERMISTOR?




HOW DO I USE A THERMISTOR?

wa an y “inangaly”

“cuv akng”?A thermistor can be deined as having an interchangeabiit

toerance o ±0.1°C over the range rom 0° to 70°C. This means

that a points between 0° and 70°C, are within 0.1°C o the

nomina resistance vaues or that particuar thermistor curve.

This eature resuts in temperature measurements accurate to

±0.1°C no matter how man dierent thermistors are substi-

tuted in the appication.

wa an y ‘”Pn mad”?

A standard thermistor is caibrated and tested at 25°C to a toer-

ance o ± 1%, 2%, 5% or ± 10%. Since these thermistors on

have one controed point o reerence or ‘point matched’ tem-

perature, the resistance at other temperatures are given b the"Resistance vs. Temperature Conversion Tabes" or the appropri-

ate materia curve. The resistance vaue at an temperature is

the ratio actor times the resistance at 25°C.

In addition to the industr

standard o point matching

thermistors at 25°C, Quait

Thermistor can point match

to a speciic temperature

range. Exampes o this

woud be the reezing point

o water (0°C) or human

bod temperature (37°C).

AVAiLAbLe iNterchANGeAbLe toLerANces

0Cº to +70ºC

A3 = +/- 1ºC

B3 = +/- 0.5ºC

C3 = +/- 0.2ºC

D3 = +/- 0.1ºC

-20Cº to +50ºC

A2 = +/- 1ºC

B2 = +/- 0.5ºC

C2 = +/- 0.2ºC

0Cº to 100ºC

A4 = +/- 1°C

B4 = +/- 0.5°C

C4 = +/- 0.2°C

+20Cº to +90ºC

A5 = +/- 1ºC

B5 = +/- 0.5ºC

C5 = +/- 0.2ºC

+20Cº to +50ºC

A6 = +/- 1ºC

B6 = +/- 0.5ºC

C6 = +/- 0.2ºC

D6 = +/- 0.1ºC

Closed end tube with

flange, ideal for

rivet mounting.



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HOW MUCH RESISTANCE DO I NEED?

With an NTC thermistor, resistance decreases as the temperature

rises. One main actor in determining how much resistance ou

need at 25°C is to cacuate how much resistance ou wi have at

our critica temperature range.

I the tota wire resistance is a substantia percentage o the

resistance change at our critica temperature range, ou shoud

consider increasing our base resistance at 25°C.

Determine i the resistance change at our critica temperature is

arge enough to compensate or an other errors in our sstems

design. I not, ou shoud increase our base resistance at 25°C.

exAMPle –1,000 Ω curv Z trmistor at 25°C

Btwn –29°C and –28° C, tr is a rsistanc cang of

990 oms. Btwn 118° and 119° C, tr is on a rsistanc

cang of 1.1 oms.

Resitance R/T@25ºC Part# Curve

500 QTMC-1 Z

2,250 -7 Z

2,500 -8 Z

3,000 -9 Z

5,000 -11 Z

10,000 -14 Z

20,000 -19 Z

1,000 -27 y

2,000 -28 y

100,000 -43 V

Resitance R/T@25ºC Part# Curve

1 Meg -65 P

9.8 Meg -70 R

100 -78 X




WHAT’S A CURVE AND WHICH CURVE DO I CHOOSE?

I ou reca our deinition o a thermistor (An eectrica resistor

making use o a semiconductor whose resistance varies sharp

in a ver predictabe manner with temperature.) We can use the

Stein-Hart Hart equation to predict how the thermistor reacts

to temperature. I we pot these points on a graph, it orms a

repeatabe curve. Thermistor manuacturers can ater the chem-

istr o a thermistor, thereb changing the sope o a curve.

your curve seection shoud be based on how steep the curve is

or our critica temperature range, size constraints and the

target resistance vaue. Since a thermistor is based on buk

resistivit, the size o the sensor m not be easibe or ourappication. Unike the RTD and Thermocoupes, thermistors do

not have industr standards or their curves. However, most

thermistor manuacturers have curves that are simiar. An

exampe o this is Quait Thermistors ‘Z’ curve it’s b ar the

most common curve in the industr and most major thermistor

manuacturers have a ver simiar curve oerings.

Curve ZCurve W

Curve X

Curve Y

Curve V

Curve S

Curve M

Curve R

4

R M u l t i p l i e r

Temperature (ºC )

3

2

1

0

0 10 20 30 40 50 60 70

RESISTANCE VAlUE IS AlSO A FUNCTION OF CURVE



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THERMAl TIME CONSTANT

The therma time constant is the time required or a thermis-

tor to change to 63.2 percent o the tota dierence between

its initia and ina bod temperature when subjected to a step

unction change in temperature under zero power conditions.

The United States Department o Deense has a ver speciic

method or measuring the therma time response o a thermistor

(see Mi Spec 23648)

Pace thermistors in a sti air controed chamber (chamber tem-

perature: 25°C ±1°C) with a minimum voume o 1,000 times the

thermistor bod and test ixture.

4 Se heat the thermistor to 75°C. Aow 15 minutes (max-

imum) or stabiization o thermistors.

4 Prepare to measure time rom the instant the power is

cut to the time the bridge indicator passes through the

nu point (43.4°C)

4 Record this time: This is the time constant o the thermis-

tor is register a 63.3% change in temperature.

ta’ g, DD span al pn

a a an a a 32° ang!

Some thermistor manuacturers choose to use a 50°C change.

Be sure and consut the product speciications when making a

comparison.

THERMAl CONDUCTIVITy

Heat moves through a materia at a speciic rate. The rate it

traves depends on the materia itse: some materias aow

heat to move quick through them, some materias aow heat

to move ver sow through them. Beow is a ist o dierent

materias and how the conduct heat.

mAteriAL thermAL coNDUctiVitY (w/m K)

Siver - Best 429

Copper (pure) 401

God 317

Auminum (pure) 237

Brass (70Cu-30Zn) 110

Titanium 21.9

316 Stainess Stee 13.4

PEEK pastic 1.75

Therma Conductive Epox 1.25

UHMW pastic 0.42

ba ng a p aal ad lly n

nduvy. cn an, , ng and

analy a all ky a.

WHAT IS THERMAl TIME CONTANT? (Mi-PRF 23648 & Mi-PRF 32192)




WHAT IS THERMAl DISSIPATION CONSTANT?

WHAT IS SElF HEATING?

THERMAl DISSIPATION CONSTANT

The therma dissipation constant o a thermistor is the power

required to raise the thermistors bod temperature b 1°C. The

dissipation constant is expressed in units o mW/°C (miiWatts

per degree Centigrade).

Dissipation Constant can be aected b:

4 Mass o the thermistor probe

4 How the probe and sensor are mounted

4 Therma dnamics o the environment

The dissipation constant is an important actor in appica-

tions that are based on the se-heating eect o thermistors.

Speciica, the change in resistance o the thermistor due to

change in dissipation constant can be used to monitor eves or

ow rates o iquids or gasses. As an exampe as the ow rate

increases, the dissipation constant o the thermistor in a uid

path wi increase and the resistance wi change and can be

correated to the ow rate.

Stated another wa, the dissipation constant is a measure o

the therma connection o the thermistor to its surroundings. It

is genera given or the thermistor in sti air, but sometimes in

we-stirred oi.

SElF-HEATING EFFECTS

When current ows through a thermistor, it generates heat,

which raises the temperature o the thermistor above that o its

environment. This o course wi cause an error in measurement

i not compensated or. Tpica, the smaer the thermistor, the

ower the amount o current needed to se-heat.

The eectrica power input to the thermistor is just

P E = IV

where I is current and V is the votage drop across the thermis-

tor. This power is converted to heat, and this heat energ is

transerred to the surrounding environment. The rate o transeris we described b Newton's aw o cooing:

P T = K (T (R) - T 0)

where T(R) is the temperature o the thermistor as a unction o

its resistance R, T0 is the temperature o the surroundings, and

K is the dissipation constant, usua expressed in units o mi-

iwatts per °C. At equiibrium, the two rates must be equa.

P E = P T

The current and votage across the thermistor wi depend on

the particuar circuit coniguration. As a simpe exampe, i the

votage across the thermistor is hed ixed, then b Ohm's law

we have I = V / R and the equiibrium equation can be soved or

the ambient temperature as a unction o the measured resis-

tance o the thermistor:

T 0 = T (R) -

The dissipation constant is a measure o the therma connection

o the thermistor to its surroundings. It is genera given or

the thermistor in sti air, and in we-stirred oi. Tpica vaues

or a sma gass bead thermistor are 1.5 mw/°C in sti air and

6.0 mw/°C in stirred oi. I the temperature o the environ-

ment is known beorehand, then a thermistor ma be used to

measure the vaue o the dissipation constant. For exampe, the

thermistor ma be used as a ow rate sensor, since the dissipa-

tion constant increases with the rate o ow o a uid past the

thermistor.

V 2

KR



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Another probem with seecting materia based on therma con-

ductivit aone is that i the mass o high conductive probe

housing can actua act ike a heat sink and pu additiona heat

out o the sstem. This can obvious create measuring inac-

curacies.

To oset this, ou can combine dierent materias whie design-

ing our probe. A ow therma conductive housing with a

sma high conductive probe tip is a good soution.

In some cases, our appication ma require a sow therma timeresponse. An exampe o this woud be an outdoor sign that dis-

pas the temperature. A arge over moded probe wi insuate

the thermistor and even out quick uctuations in temperature

changes.

CONFINED SPACE

Due to a thermistors miniature

size, the can be potted intoamost an size housing. Current,

the smaest avaiabe thermistor

is 0.023” max diameter. Hoow-

tube rivets, set screws, hpodermic

needes and direct epox attach are

some common methods or con-

ined space thermistor appications.

lIQUID

For iquid appications, it’s best

to use a threaded probe. Possib,

with some tpe o eastomeric seaike an o-ring. QTI aso oers a

compete ine o NPT probe hous-

ings. Some appications require

over moding the thermistor into

the pastic housing o the product.

Another option is to use a gass

encapsuated bead. It provides a

hermetic sea that is as cose to

‘waterproo’ as Mother Nature wi

et us. Remember the Titanic?

GAS/AIR

Gas and air appications have a

variet o choices. Probes can

be surace mounted in the ow

stream or the can be projected

into the air stream b means o a

cosed or open-end tube. When

measuring gas or air under pres-

sure, we recommend using some

tpe o thread/o-ring combination.

SURFACE

B ar the most common method

or surace measurement is thering ug. Due to the sma size o

the thermistor eement, it can be

potted into most ring ug barres.

Be careu that the wire gauge

does not exceed the inside dimen-

sion o the barre. Another option

or surace measurement is direct

attachment o a thermistor using a

stainess stee disc.

HOW DO I DESIGN A PROBE?




WIRE INSUlATION PROPERTIES

hala- PVc- tfn- Ply- tzl thermAL PVc e-ctFe myla Kyna PFA suln FeP Kapn tFe etFe

Maximum ContinousRating (Cº) 105 135 105 135 260 150 200 200 260 150

low Temperature (Cº) -50 -100 -60 -70 -200 -100 -200 -200 -200 -100

Non-Fammabiit Ver Good Exceent Ver Good Exceent Exceent Good Exceent Exceent Exceent Exceent

Soder Resistant Good Ver Good Ver Good Ver Good Ver Good Ver Good Exceent Exceent Exceent Exceent

Smoke Moderate Sight Moderate Sight None Moderate None None None Sight

hala- PVc- tfn- Ply- tzl eLectricAL PVc e-ctFe myla Kyna PFA suln FeP Kapn tFe etFe

Voume Resistivit

(ohm-cm) 1012 1013 1016 2x1014 1018 5x1016 2x1018 1018 1012 1016

Dieectric Strength (1 mi fm)

VPM, 1/8” thick 350 490 350 450 430 400 430 420 430 400

Dieectric Constant 5.70 2.60 3.50 7.70 2.06 3.13 2.00 2.40 2.00 2.60

Dissipation Factor

(1 kHz) .09 .002 .03 .02 .0002 .001 0.4 .001 .0002 .0008

Capacitive Frequenc

Stabiit Fair Exceent Good Poor Exceent Good Exceent Exceent Exceent Exceent

hala- PVc- tfn- Ply- tzl mechANicAL PVc e-ctFe myla Kyna PFA suln FeP Kapn tFe etFe

1.68 (67%Densit (gm/cc) 1.36 1.68 1.48 1.76 2.15 1.24 2.18 poimide) 2.20 1.70

Tensie, psi 4,000 7,000 15,000 6,000 4,000 10,000 2,700 17,000 2,500 6,500

Eongation, % 250 200 50 250 300 100 250 75 225 100-400

Abrasion Resistance Fair Fair Good Exceent Good Exceent Good Exceent Good ExceentCut-through Resistance Good Good Exceent Exceent Fair Exceent Fair Exceent Fair Exceent

Bondabiit Good Good Good Good Good Exceent Good

hala- PVc- tfn- Ply- tzl eNViroNmeNtAL PVc e-ctFe myla Kyna PFA suln FeP Kapn tFe etFe

100 200 approx.100Nucear Radiation Fair megarads Fair Exceent Fair Good Fair megarads Fair megarads

UV Radiation Fair Exceent Fair Exceent Exceent Fair Exceent Exceent Exceent Exceent

hala- PVc- tfn- Ply- tzl

chemicAL PVc e-ctFe myla Kyna PFA suln FeP Kapn tFe etFe Water Absorbtion 0.7% .01% .06% .04% .03% .05% .01% .8% .01% .1%

Acids Good Exceent Good Ver Good Exceent Good Exceent Fair Exceent Exceent

Akai Good Exceent Poor Ver Good Exceent Good Exceent Fair Exceent Exceent

Acoho Fair Exceent Fair Ver Good Exceent Fair Exceent Ver Good Exceent Exceent

Ceaning Sovents Sight

(tri-chor, carbon, tetr) Swe Exceent Good Ver Good Exceent Crazes Exceent Ver Good Exceent Exceent

Aiphatic Hdrocarbons Sight

(gasoine, kersosene) Swe Exceent Fair Ver Good Exceent Good Exceent Ver Good Exceent Exceent

Aromatic Hdrocarbons Sight

(benzene, touene) Swe Exceent Fair Ver Good Exceent Crazes Exceent Ver Good Exceent Exceent

long Term Stabiit Fair Exceent Good Ver Good Exceent Ver Good Exceent Exceent Exceent Exceent



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CONVERSION TABlES

eQUiVALeNt tAbLes Dal/n/

sandad sud tnalsud sz Da hl Da.U.s. () in. () in. ()

#2 .0866 .090M2 (2.18) (2.29)

#4 .112 .118(M2,5) (2.84) (3.00)

#5 .125 .127(M3) (3.18) (3.23)

#6 .138 .146(M3,5) (3.51) (3.71)

#8 .164 .173(M4) (4.17) (4.39)

#10 .190 .198(M5) (4.83) (5.03)

1/4” .250 .270(M6) (6.35) (6.86)

5/16” .312 .330(M8) (7.92) (8.38)

3/8” .375 .385(M10) (9.53) (9.78)

1/2” .500 .520(M12) (12,7) (13.21)

5/8” .625 .650(M16) (15.88) (16.51)

3/4” .750 .810(M18) (19.05) (20.57)

Da Da o p osz in 1000 p k

20 AWG 0.032 0.813 10.15 33.29

21 AWG 0.029 0.724 12.80 41.98

22 AWG 0.025 0.645 16.14 52.94

23 AWG 0.023 0.574 20.36 66.78

24 AWG 0.020 0.511 25.67 84.20

25 AWG 0.018 0.455 32.37 106.17

26 AWG 0.016 0.404 40.81 133.86

27 AWG 0.014 0.361 51.47 168.82

28 AWG 0.013 0.320 64.90 212.87

29 AWG 0.011 0.287 81.83 268.40

30 AWG 0.010 0.254 103.20 338.50

31 AWG 0.009 0.226 130.10 426.73

32 AWG 0.008 0.203 164.10 538.25

33 AWG 0.007 0.180 206.90 678.63

34 AWG 0.006 0.160 260.90 855.75

35 AWG 0.006 0.142 329.00 1,079.12

36 AWG 0.005 0.127 414.80 1,360.00

37 AWG 0.005 0.114 523.10 1,715.00

38 AWG 0.004 0.102 659.60 2,163.00

2.0 mm 0.008 0.203 169.39 555.61

1.8 mm 0.007 0.178 207.50 680.55

1.6 mm 0.006 0.152 260.90 855.75

1.4 mm 0.006 0.152 339.00 1,114.00

1.25 mm 0.005 0.127 428.20 1,404.00

1.12 mm 0.004 0.102 533.80 1,750.00




FREQUENTly ASKED QUESTIONS

How does aging affect thermistor stability?“Thermometric drit” is a speciic tpe o drit in which the

drit is the same amount o temperature at a temperatures o exposure. For exampe, a thermistor that exhibits a -0.02°C shit

at 0°, 40° and 70°C (even though this is a dierent percentage

change in resistance in each case) woud be exhibiting thermo-

metric drit. Thermometric drit: (1) occurs over time at varing

rates, based on thermistor tpe and exposure temperature, and

(2) as a genera rue, increases as the exposure temperature

increases. Most drit is thermometric.

How do thermistors fail?

SIlVER MIGRATION

This aiure can occur when one or more o the oowing

three conditions are present: constant direct current bias, high

humidit, and eectrotes (disc/chip contamination). Moisture

inds its wa into the thermistor and reacts with the contami-

nant. Siver (on the thermistor eectrodes) turns to soution,

and the direct current bias stimuates siver crsta growth

across the thermistor eement. The thermistor resistance

decreases, eventua reaching zero O (short) (probab the

most common aiure mechanism).

MICRO CRACKS

Thermistors can crack due to improper potting materias i a

temperature change causes potting materia to contract, crush-ing the thermistor. The resut is a thermistor that has erratic

resistance readings and is eectrica “nois.”

FRACTURE OF GlASS ENVElOPE

Tpica caused b mishanding o thermistor eads, this aiure

mechanism induces ractures in the gass coating at the ead/

thermistor interace. These cracks ma propagate around the

thermistor bead resuting in a catastrophic upward shit in

resistance. Mismatching o epoxies or other bonding materias

ma aso cause this. Careu handing and the proper seection

o potting materias can eiminate this aiure.

AGING OUT OF RESISTIVE TOlERANCE

I thermistors are exposed to high temperatures over time,

sometimes reerred to as “aging,” their resistivit can change.Genera the change is an upward change in resistivit, which

resuts in a downward change in temperature. Seecting the

proper thermistor or the temperature range being measured

can minimize the occurrence o this aiure. Temperature

ccing ma be thought o as a orm o aging. It is the cumua-

tive exposure to high temperature that has the greatest inu-

ence on a thermistor component, not the actua temperature

ccing. Temperature ccing can induce shits i the compo-

nent has been buit into an assemb with epoxies or adhesives,

which do not match the temperature expansion characteristics

o the thermistor.

What happens if my application exceeds thetemperature rating?

Intermittent temperature incursions above and beow the oper-

ating range wi not aect ong-term survivabiit. Encapsuate

epox tpica begins to break down at 150°C and the soder

attaching eads to the thermistor bod tpica reows at about

180°C. Either condition coud resut in aiure o the thermistor.

Are thermistors ESD sensitive?

Per MIl-DTl-39032E, Tabe 1, thermistors b deinition are not

ESD sensitive.

What is the resolution of a thermistor?

There is no imit to the resoution o a thermistor. The imitationsare in the eectronics needed to measure to a speciied resou-

tion. limitations aso exist in determining the accurac o the

measurement at a speciied resoution.

Are QTI thermistors RoHS compliant?

(What i I don’t want a ead ree part?)

Quait Thermistor maintains two separate manuacturing ines

to meet the speciic environmenta needs o our customers. One

ine is dedicated to RoHS compiance and the other is main-

tained or traditiona tin/ead parts or miitar, aerospace and

medica appications.

Does the ength o wire impact the accuraco a thermistor?With a thermistor, ou have the beneit o choosing a higher

base resistance i the wire resistance is a substantia percentage

o the tota resistance. An exampe o this woud be a 100-ohm

thermistor vs a 50,000 ohm thermistor with 10’ o 24 AWG wire.

Tota wire resistance = 10’ x 2 wires x 0.02567 ohms per oot =

0.5134 ohms

The amount of drift over a period of time is dependent on the aging

temperature. Please note that not all thermistor manufactures age at

the same temperature so drift data may be different. This chart shows

typical drift when parts were aged at 125°C.



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Quait Thermistor, Inc. a eader in thermistor innovation

is peased to announce the Therma Bridge. The Therma

Bridge incorporates a bridge resistor with the thermistor

providing a more inear signa or conditioning. B incorporating

a bridge resistor with a thermistor in a singe precision assem-

b, temperature sensing is impemented without the need or

caibration, potentionmeters, precision externa components and

with no concern or cocking and bus issues.

Temperature is determined b a ratio o the

input versus output votage across the sensor

aowing inexpensive and precise tempera-

ture measurement capabiit or near anMicroprocessor based

sstem. With wide avaiabe embedded

mixed signa processors and A-D converters,

Design Engineers can easi condition the

non-inear signa o NTC thermistors.

FEATURES AND BENEFITS OF USING THE

THERMAl BRIDGE:

•Operatingtemperaturerangeof-55

to 150ºC

•Accuracyupto+/-.2ºC

rom 0–70ºC

– Up to +/-1ºC rom -55 to 100ºC

– Up to +/-1.5ºC rom -55 to 150ºC

•Availableinmanyprobeconfigurationsorasacircuit

board mounted component

•Highstabilitywithnocalibrationrequired

•Longsensorlife-span

•Dynamicresponseforeaseofmeasurement

•Wideoperatingvoltagerange,upto48VDC

•Monolithicthermistorsensorexhibitsnegligible

capacitance and inductance

•Noerrorintroducedduetonoise,andrandomnoise

se-cances

•Lowpowerconsumption,170uWmaximum

-55

20

0

40

60

80

100

-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125

V o u t ( % V i n ) C

NEW PRODUCTS




HOW SMAll CAN yOU MAKE A THERMISTOR?

Part Number Bead Dia. Resistance Toerance

QT06002-524 .023" 10,000 +/- 0.1ºC (0ºC to 70º)

QT06002-525 .023" 10,000 +/- 0.2ºC (0ºC to 70º)

NANo tUbe 0.023" mAx oD epox fed poimide tube with insuated #38 AWGsoid nicke eads, parae bonded, 6" (76.2 mm)

Part Number Bead Dia. Resistance

QTMB-14 .038" 10,000

QTMB16 .038" 15,000

miNi beAD 0.038" mAx oD epox coated bead with #34 AWGPo-non insuated bifar eads, twisted pair, 6" (152.4 mm). Toerance+/- 0.2ºC (0ºC to 70º)

c o N F i N e D s P A c e t h e r m i s t o r s & t e m P e r A t U r e P r o b e s

• Exceptiona ast therma response time•Suitableforsmallertemperatureprobehousings

•Customandsemi-customproductsmaybespecied

•Availableinpointmatchedandinterchangeabletolerances


QT06002-529 .031" 2,252

QT06002-530 .031" 3,000

QT06002-531 .031" 5,000

QT06002-532 .031" 10,000

micro tUbe 0.031" mAx oD epox fed poimide tubewith pourethane non insuated #32 AWG soid copper eads, twistedpair, 6" (152.4mm). Toerance +/- 0.2º (0ºC to 70º)


QT06002-526 .037" 2,252

QT06002-533 .037" 3,000

QT06002-527 .037" 5,000

QT06002-528 .037" 10,000

miNi tUbe 0.037" mAx oD epox fed poimide tube withpourethane non insuated #32 AWG soid copper eads, twisted pair, 6"(152.4mm). Toerance +/- 0.2º (0ºC to 70º)

ReSISTANCe

Temp(ºC) 2,252 3,000 5,000 10,000

0 7,355 9,798 16,330 32,6605 5,720 7,620 12,700 25,400

10 4,481 5,970 9,950 19,900

15 3,538 4,713 7,855 15,710

20 2,813 3,747 6,245 12,490

25 2,252 3,000 5,000 10,000

30 1,815 2,417 4,029 8,058

35 1,471 1,960 3,266 6,532

40 1,199 1,598 2,663 5,326

45 984 1,310 2,184 4,368

50 811 1,081 1,801 3,602

55 672 896 1,493 2,986

60 560 746 1,244 2,488

65 469 625 1,041 2,082

66 453 603 1,005 2,010

67 437 582 971 1,941

68 422 563 938 1,875

69 408 544 906 1,812

70 394 525 876 1,751



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Qualified Test Lab

To ensure the quait o our QTI

brand thermistors, Quait

Thermistor has an extensive test

ab or a wide range o testing

services. In addition, this aciit is avaiabe or customers or

the oowing services:

•Powerburn-in

•Temperaturecycling

•Moisturetesting

•Shockandvibrationtesting

•Temperaturecharacterization

•Space-levelscreening

•QCIMilitarytesting

Custom Design

With a u sta o experienced temperature appication

engineers, Quait Thermistor can provide custom design services

at an step aong the design process. Experts in temperature

measurement, compensation, and contro, Quait Thermistor

engineers can work with our in-house engineers or contractors,

or as a u-support design team to sove our appication.

•Components

•Probes

•Boards

•Systems

•Controlandsignal

conditioning

Private Labeling

The QTI brand is recognized in man industries or high-quait

manuacturing and measurement accurac and reiabiit.

However, in situations where private abeing is required, Quait

Thermistor wi provide components with no abe or with our

abe to ensure the integrit o our branding strateg.

•Yourdesign,yourlabel

•Ourdesign,yourlabel

•Yourdesign,theQTIlabel

Assembly

Quait Thermistor oers expert, time component and board

assemb services in our we-equipped Tecate, Mexico, aciit.

In addition, to ensure product is deivered on time, the aciit’s

capabiit is mirrored at our Idaho pant.

•Highly-trainedassemblers

•High-volumeproduction

•Competitiveprices

•Probeassembly

•PTCandNTCdevices

SPECIAl SERVICES



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For more information on QualityThermistor, Inc., or on QTI brandthermistors, probes, and engineeringservices, contact Technical Support.

Quait Thermistor, Inc.2108 Centur Wa

Boise, ID 83709

www.thermistor.com

800-554-4784 U.S.208-377-3373 Wordwide208-376-4754 [email protected]

QTI, leach Guard, and Hdroguard are

trademarks o Quait Thermistor, Inc.

Therm is Tor Design Guide

Documents

Transcript of Therm is Tor Design Guide