Introduction to NTCs - Philips Semiconductors / NXP Semiconductors

1995 Mar 31 2

Philips Components

NTC Thermistors Introduction to NTCs

GENERAL

Deﬁnition and composition

Negative Temperature Coefficient thermistors (NTCs) are

resistive components, of which the resistance decreases

as temperature increases. They are made from

polycrystalline semiconductors, the composition of which

is a mixture of chromium (Cr), manganese (Mn), iron (Fe),

cobalt (Co) and nickel (Ni).

Manufacture

The manufacturing process is comparable to that of

ceramics. After intensive mixing and the addition of a

plastic binder, the mass is shaped into the required form,

e.g. pressing (discs), and fired at a temperature high

enough to sinter the constituent oxide. New technologies

have led to the sawing of isostatic pressed wafers, the

compositions of which are very stable, with as a result,

high accuracy and high reproducibility.

Electrical contacts are then added by burning them in with

silver paste or by other methods, such as evaporation.

Finally, leads (isolated or not) are fitted. Different

encapsulations are possible, depending on the size of the

ceramic and the application of the component.

Miniature NTC thermistors are made by placing a bead of

oxide paste between two parallel platinum alloy wires and

then drying and sintering. The platinum alloy wires are

60 µm in diameter and spaced 0.25 mm apart. During

sintering, the bead shrinks onto the wires to make a solid

and reliable contact. Miniature NTC thermistors are

usually mounted in glass to protect them against

aggressive gases and fluids.

Relationship of resistance with temperature

The conductivity (σ) of the material is its capacity to drive

a current when a voltage is applied to it. As the current is

driven by carriers that are free to move (i.e. which are not

bound to atoms), then it follows that the conductivity will be

proportional to the number of carriers (n) that are free and

also to the mobility (µ) that those carriers can acquire

under the influence of electrical fields.

Thus:

where e is the unit of electrical charge stored by each

carrier.

Both n and µ are functions of temperature. For µ, the

dependency on temperature is related to the interactions

of a carrier with other carriers and with the total net amount

of vibrating atoms, the vibration varying with temperature.

σne×µ×=

It can be shown that:

For n, the dependency on temperature can be explained

as follows: electrons are bound to atoms by certain

energies. As one gives the electron an energy equal to, or

greater than, the binding energy (e.g. by raising its

temperature), there is a probability that the electron will

become free to move. As for many semiconductors, this

probability has the form of the well-known

Maxwell-Boltzmann distribution. Thus:

The total temperature dependency of the conductivity is:

In practice, the exponential factor is the most important.

Remembering that resistivity is the inverse of conductivity,

the following can be derived:

where

where A and B are parameters depending on each

component (resistivity and shape).

Shape of an NTC curve and determination of B-value

In Fig.1, the resistance is plotted as a function of the

inverse of the temperature. Even in semi-logarithmic

scale, it can be seen that this curve is not a straight line.

This is due to the fact that A and B are not perfectly

constant with temperature. However, over a wide range of

temperatures, it may be assumed that these parameters

are constant. If this range is defined between T1 and T2,

and it is assumed that the curve for this range could be

approximated with a straight line, the slope of which will be

B, this last value between T1 and T2 can be found as

follows:

The resistance value is measured at T1 and T2:

and

µTc–e×÷ q2kT⁄–

ne÷q

1kT⁄–

σTc–e×÷ q1q2

+()kT⁄–

RAe

BT⁄

×=

log R A B

----

R1Ae

⁄

×=

2Ae

⁄

×=

1995 Mar 31 3

Philips Components

NTC Thermistors Introduction to NTCs

Dividing yields:

Hence:

In practice, B varies slightly with increasing temperature.

The temperature coefficient of an NTC may be derived

from:

For the different materials, the constant B may vary

between 2000 and 5500 K; e.g. a value of 3600 K yields

α=−4%/K at a temperature of 300 K.

A and B are assumed to be constant between

T1and T2.

In practice, most NTCs are specified with a reference

value at 25 °C and a constant B-value between 25 °C and

85 °C. For commodity reasons, the curves printed in this

handbook show the resistance as a function of

temperature, instead of its inverse.

------- eBT

⁄BT

⁄–()

log R1log R2

–B1

------ 1

------

–





log e=

Bln R1R2

⁄()

⁄1T

⁄–

---------------------------------

α1

---- dR

--------

×B

------

–==

⁄





Fig.1 Resistance as a function of the inverse of

temperature for a typical NTC.

handbook, halfpage

3.82.6 3.0

MBD918

3.4

1/T (10 x K )

(Ω)

Voltage (V)/Current (I)-characteristic description

Figure 2 shows the relationship between the current

through and the voltage drop across the NTC thermistor

heated by this current to a temperature much higher than

the ambient temperature.

With very small values of current, it can be seen that the

curve remains straight, following an isoresistive line.

Remembering that an isoresistive line is in fact an

isothermal line (R = f[T]) it indicates that the power

consumption is too small to register a distinct rise in

temperature.

For higher current intensities, the temperature rises by the

Joule-effect (P = V ×I). The equilibrium temperature is

reached when the power dissipated by the NTC is in

equilibrium with the power applied to it. It can be seen that

as the dissipated power is dependent on the environment,

the equilibrium will also depend on it and thus the

V/I-characteristic too. The characteristic shown in Fig.2

was measured at a constant ambient temperature after

equilibrium had been reached.

Assuming that:

•a constant temperature is present throughout the body

of the thermistor;

•the heat transfer is proportional to the difference in

temperature between thermistor and surrounding

medium (which is true for low temperatures);

then, in case of equilibrium:

where T0 is the ambient temperature and δ the dissipation

factor (defined in Chapter “Speed of response”).

From this relationship, it is obvious that the temperature of

the component will be that of its surroundings if the

power P (W) applied to the component is equal to zero

(power-off value). If the applied power is not very low

(≥0.01 W), then T is no longer equal to T0 and will be

strongly dependent on δ (power-on conditions).

Because it is not possible to define δ without any doubt

(δis not only dependent on the component itself, but also

on special housing if any, convection, turbulence, etc.), all

components are specified with their power-off values.

To choose a component that will be used in a ‘power-on’

application, it is necessary to determine δ in that

application.

WVA×δTT

–()==

1995 Mar 31 4

Philips Components

NTC Thermistors Introduction to NTCs

V/I-CHARACTERISTIC

Fig.2 Voltage as a function of the current characteristics of a NTC thermistor.

handbook, full pagewidth

103

MBD919

102

10110 1

102

(V)

I (mA)

50 C

100 C

o200 C

300 C

400 C

50 mW

0.1 W

0.5 W

1 W

5 W

10 W

10 Ω

50 Ω

100 Ω

500 Ω

1 kΩ

5 kΩ

10 kΩ

50 kΩ

10 mW

5 mW

0.5 mW

1 mW

SPEED OF RESPONSE

Thermal time constant

The thermal time constant is an indication of the time that

a component needs to reach thermal equilibrium. This

constant depends on two important parameters.

One parameter is the thermal capacity (H) of the

component, i.e. the energy that must be applied to the

component in order to raise its temperature by 1 Kelvin (or

the energy that the component must loose in order to lower

its temperature by 1 Kelvin). The units are thus quoted in

Joules/Kelvin. The second parameter is called the

dissipation factor (δ). If the temperature of a component

rises, it will tend to dissipate energy. This dissipation will

depend on the surroundings and also on the component

itself. The dissipation factor is defined as the ratio of the

change in power dissipation with respect to the resultant

body temperature change (units in W/K).

If a step change in temperature is applied to a component

e.g. from high (T1) to low (T0) temperature, the energy lost

by the component (−HdT) is equal to the energy dissipated

by it (δ[T −T0]dt):

HdT–δTT

–()dt=

This equation yields:

where the thermal time constant (τ) is defined as the ratio

of the heat capacity (H) of the thermistor with respect to its

dissipation factor (δ).

The temperature value when the time elapsed (t) is equal

to τis given by the formula:

This equation gives the following definition:

The thermal time constant is the time required for the

temperature of a thermistor to change by 63.2% of the

difference between its initial and final body temperatures

(in accordance with “

IEC 539

”: 85 °C and 25 °C

respectively), when subjected to a step function

temperature change.

It is entirely dependent on the component design. The

thermal time constant depends on δ, which varies for

different media.

–T

–()e

tτ⁄–

------ T0

–– 1e

1–

–





0.632==

1995 Mar 31 5

Philips Components

NTC Thermistors Introduction to NTCs

The thermal time constants referred to in the data sheets

are measured as follows; the method used depends on the

application:

•By cooling in air under zero power conditions (Tc)

•By heating or cooling, transferring the thermistor from

ambient temperature (25 °C) to a bath with fluid with a

higher or lower temperature under zero power

conditions (Tr, termed ‘response time’ in the data

sheets).

Tolerances in the nominal NTC speciﬁcation

As already mentioned, an NTC thermistor is normally

specified by giving a reference value (generally R25) and

the B-value (B25/85). Unfortunately, the manufacturing

process dictates that identical components cannot be

guaranteed, so there are some tolerances.

These tolerances can mean an upward or downward shift

in the resistance value, equal at all temperatures due to,

for example, variations of mechanical dimensions. The

entire curve moves equally up or down (see Fig.3).

This tolerance is usually indicated by giving the shift at the

reference temperature; for example, R25 =10kΩ±5%.

A tolerance also exists on the slope of the curve. Because

the B-value is an indication of that slope, it is normally

indicated as a tolerance on B25/85. This is covered mainly

by variations in the material composition and the effect of

sintering on the material (see Fig.4).

The effect of the slope or B-value deviation on the

resistance at several temperatures can be calculated.

The fundamental equation of an NTC is:

where Rn and B are nominal values (specified values

without any tolerance).

If B is not a nominal value, it is expressed as:

where ∆RT is the absolute deviation at temperature T:

If relative deviation is applied:

RnT RrefB1T⁄1T

ref

⁄–()

nT ∆RT

+RrefeB∆B+()1T⁄1T

ref

⁄–()

∆R

ref eB∆B+()1T⁄1T

ref

⁄–()

B1T⁄1T

ref

⁄–()

–=

∆R

----------- e∆B1T⁄1T

ref

⁄–()1–

Fig.3 Effect of variations of mechanical dimensions

on the resistance as a function of temperature.

handbook, halfpage

MBD920

Fig.4 Effect of sintering on the resistance as a

function of temperature.

handbook, halfpage

MBD921

1995 Mar 31 6

Philips Components

NTC Thermistors Introduction to NTCs

Developing this equation (Taylor’s formula), the following

simplified expression can be derived:

This calculation has been performed for all major sensor

ranges to be found in this handbook, where ‘R-deviation

due to B-tolerance’ values can be found in the electrical

data tables.

If the ‘R-deviation due to B-tolerance’ is called ‘Y’ and the

tolerance at the reference temperature ‘X’, then the total

tolerance can be calculated as follows:

or, after approximation:

Z≈X+Y

If TC is the temperature coefficient and ∆T is the

temperature deviation:

EXAMPLE

At 0 °C, assume X = 5%, Y = 0.089% and TC = 5.08%/K,

then:

or (≈5.93%)

(≈1.17 K)

Hence, an NTC having a R25-value of 10 kΩ has a value

of 32.51 kΩ between −1.17 °C and +1.17 °C.

Resistance speciﬁed at more than one temperature

(2 or 3-point measurement)

Thermistors which are specified at 2 or 3 points of their

R/T-characteristic are more accurate. They have a closer

tolerance and the spread in B-value has less influence

because it is included in the tolerance at the specified

points.

The tolerances in the reference points can be expressed

either as a temperature deviation for the reference

resistance or as a resistance tolerance at the reference

temperature. This has no influence on the resulting

measuring error which is minimum in the temperature

region between the reference points, as illustrated in Fig.5.

∆RT

RnT

----------- in %()∆B

--- 1

Tref

---------

–





100

----------





100

----------





×1–100%×=

∆TZ

--------

100

----------





0.89

100

-----------





×1–{}100%×=

Z 1.05 1.0089×1–{}100% 5.9345%=×=

∆TZ

-------- 5.93

5.08

----------- 1.167 °C== =

The 2 or 3-point sensors are particularly suited for

applications with the following characteristics:

•Temperature measurement over a certain temperature

range

•High accuracy

•No further calibration for sensor tolerances in the

electrical circuitry required.

Fig.5 Temperature measurement at more

than one point.

handbook, halfpage

MBD922

25 0 25 50

T ( C)

∆ T

(K)

1995 Mar 31 7

Philips Components

NTC Thermistors Introduction to NTCs

GLOSSARY OF TERMS

Resistance

Also called nominal resistance. Formerly specified at only

one temperature, or sometimes at two or maximum three.

Now new technologies allow the specification of resistance

values on all application ranges for several types.

Tolerance on resistance

The limits of the values that the resistance can take at the

reference temperature.

B-value

The B-value may be calculated using the following

formula:

where R1 and R2 are the nominal values of resistance at

T1 and T2 respectively.

Tolerance on B-value

The limits of the value that B can take due to the process

variations.

R-tolerance due to B-deviation

Due to the tolerance on the B-value, the limits of the value

that R can take at a certain temperature increase with the

difference of that temperature to the reference

temperature.

Tolerance on R at a temperature different to Tref

The sum of the tolerances on resistance and tolerance due

to B-deviation.

α-value

Variation of resistance (in %) for small variations of

temperature around a defined temperature.

Maximum dissipation

Maximum power which could be applied without any risk of

failure.

R1R2

⁄()ln

⁄1T

⁄–

---------------------------------

HOW TO MEASURE NTC THERMISTORS

The published RT-values are measured at the

temperature T.

The published B-value at 25 °C is the result of the

measurement at 25 °C and that at 85 °C. Hence, these

values should be used when checking.

The following general precautions have to be taken when

measuring NTC thermistors:

•Never measure thermistors in air; this is quite inaccurate

and gives deviations of 1 or 2 K. For measurements at

room temperature or below, use petrol or some other

non-conductive and non-aggressive fluid. For higher

temperatures use oil, preferably silicon oil.

•Use a thermostat with an accuracy of better than 0.1 °C.

Even if the fluid is well stirred, there is still a temperature

gradient in the fluid. Measure the temperature as close

as possible to the NTC.

•After placing the NTC in the thermostat, wait until

temperature equilibrium between the NTC and the fluid

is obtained. For some types this may take more than

1 minute.

•Keep the measuring voltage as low as possible,

otherwise the NTC will be heated by the measuring

current. Miniature NTC thermistors are especially

sensitive in this respect. Measuring voltages of less than

0.5 V are recommended.

•For high temperature measurements it is recommended

that stem correction be applied to the thermometer

reading.

1995 Mar 31 8

Philips Components

NTC Thermistors Introduction to NTCs

CHOICE OF TYPE

Selection of an NTC thermistor

When selecting an NTC thermistor the following main

characteristics should be considered:

•Resistance value(s) and temperature coefficient

•Accuracy of resistance value(s)

•Power to be dissipated:

– Without perceptible change in resistance value due to

self-heating

– With maximum change in resistance value

•Permissible temperature range

•Thermal time constant, if applicable

•Types best suited to the purpose. Basic forms are chip,

disc and bead

•Protection against undesired external influences, if

necessary.

When it is impossible to find an NTC thermistor to fulfil all

requirements, it is often more economical to adapt the

values of other circuit components to the value of a

series-manufactured NTC. Sometimes, a standard NTC

can be used with simple parallel and series resistors where

otherwise a special type would have been necessary.

If no suitable combination can be found, the development

of a special type can be considered. In this event a

specification of the requirements is necessary. A

description of the circuit in which the NTC is to be used, is

most useful.

Deviating characteristics

The following example explains the resistance values

resulting from combinations of an NTC thermistor and

normal resistors.

Suppose an NTC must have a resistance of 50 Ω at 30 °C

and 10 Ω at 100 °C. A standard type having this

characteristic is not included in our programme. The

problem may, however, be solved by using a standard

NTC and two fixed resistors, e.g. an NTC disc with a cold

resistance of 130 Ω mounted in a series and parallel

arrangement with two fixed resistors of 6 Ω and 95 Ω

respectively. It should be remembered that the

temperature coefficient of the combination will always be

lower than that of the NTC thermistor alone.

Remarks on the use of NTC thermistors

Do not use unprotected thermistors in conducting fluids or

aggressive and reducing gases which may cause a

change in thermistor characteristics.

For temperature measurements do not use too high a

voltage on the NTC thermistor, as self-heating may cause

incorrect readings. The dissipation constant indicates the

maximum permissible measuring power, if an error of 1 °C

is allowed.

1995 Mar 31 9

Philips Components

NTC Thermistors Introduction to NTCs

HOW NTC TEMPERATURE SENSORS FUNCTION

NTC temperature sensors are made from pure metal

oxides. They respond quickly to temperature changes,

even small temperature increases cause their resistance

to decrease significantly, as shown in Fig.6.

So, by placing an NTC temperature sensor into one arm of

a bridge circuit, accurate temperature measurement is

possible.

The main characteristics of an NTC temperature sensor

are expressed by three parameters:

•The resistance at 25 °C (R25). Tolerances on the

R25-value are mainly caused by manufacturing and

material tolerances. By using very precise sawing,

tolerances on R25 lower than 1% (or 0.25 °C) can be

achieved.

•The material constant (B). This constant relates the rate

of change of resistance with temperature, and therefore

affects the slope of the R/T-characteristic.

where R is the resistance at absolute temperature T (in

Kelvin) and A is a first-approximation constant. In

practice, B is defined between two selected

temperatures. The B-value is very useful for comparing

sensors, but in making this comparison, care must be

taken to ensure that the same two temperatures are

used (normally 25 and 85 °C).

Tolerances on B-value are mainly caused by material

tolerances and by the effects of the sintering

temperature on the material. Our new materials have

tolerances on the B-value as low as 0.75%.

•The temperature coefficient of resistance (α), expressed

in %/K. This coefficient indicates the sensitivity of the

sensor to a change in temperature. Values of α are

given in the

“Data sheets”

in this

“Data Handbook”

For calculation purposes holds , where ∆R is

the percentage change in resistance at the required

temperature (see Fig.7), and ∆T is the temperature

deviation (T in Kelvin). So, when∆R andα are known for

any temperature, ∆T (the temperature deviation in °C)

can be calculated.

The plot of∆R as a function of T is known as the butterfly

characteristic, which really shows how good a sensor is

(see Fig.7). It shows that a typical Philips sensor is far

more accurate than a similar competitor sensor. That is

why we are renowned as world leaders in high-accuracy

sensor technology.

RAe

BT⁄

×=

α∆R

∆T

--------

Tolerance on R25 and the resistance tolerance due to

B-value combine to affect the performance of the sensor

over its operating temperature range (see Fig.8).

However, from an operational point of view, it is far

better to express sensor tolerance in terms of

temperature deviation ∆T over a temperature range.

This plot is shown in Fig.9.

Again we have shown a typical comparison with a

similar competitor sensor. Note how Philips outperforms

the competitor right across the temperature range.

Two other parameters which are important in specifying

NTC temperature sensors are the thermal time constant

and the response time:

– The thermal time constant is the time required for the

temperature of the sensor to change in air by

of the difference between its initial

and final body temperatures, when subjected to a

step function temperature change (85 °Cto25°C in

accordance with

“IEC 539”

– The response time is the time the sensor needs to

reach 63.2% of the total temperature difference when

subjected to a temperature change from 25 °Cin air

to 85 °Cin silicone oil (MS 200/50).

---

–





63.2%=

1995 Mar 31 10

Philips Components

NTC Thermistors Introduction to NTCs

Fig.6 Typical plot of resistance as a function of

temperature for an NTC temperature

sensor.

handbook, halfpage

MSB236 - 1

25 0 25 50 75 100 125

T ( C)

R25

log R (Ω)

B = 3740 K

B = 4570 K

Fig.7 Typical resistance change as a function of

temperature for a 1% Philips NTC

temperature sensor compared to a

competitor sensor.

handbook, halfpage

MLC729

∆ R

(%)

25 025 50 75 100 125

T ( C)

competitor

Philips

Fig.8 The combined effects of tolerance on R25

and resistance tolerance due to B-value.

andbook, halfpage

MSB296

25 0 25 50 75 100 125

T ( C)

R25

log R (Ω)

nominal value

total tolerance

Fig.9 Temperature deviation as a function of

temperature.

handbook, halfpage

MLC730

∆ T

(K)

25 025 50 75 100 125

T ( C)

competitor

Philips

1995 Mar 31 11

Philips Components

NTC Thermistors Introduction to NTCs

APPLICATIONS

General

Temperature is one of the variables that must be

measured most frequently. There are as many as nineteen

recognized ways of measuring it electrically, most

commonly by thermocouples, platinum-bulb thermometers

and NTC (Negative Temperature Coefficient) temperature

sensors. For general-purpose temperature measurement,

NTC temperature sensors are accurate over a wide

temperature range (−55 to +300 °C). They are stable

throughout a long lifetime, have a high impedance and are

small and inexpensive. In fact, they are the first choice for

most temperature measurements. Typically, they have a

negative temperature coefficient of approximately

−4.5%/K at room temperature (25 °C), more than ten times

the sensitivity of a platinum-bulb thermometer of the same

nominal resistance at the same temperature.

When you are aiming for accuracy, Philips has the

NTC temperature sensors to help you achieving it. We

have been making NTC temperature sensors for many

years and we have gained an enviable reputation for our

value-for-money ranges. Our component manufacturing

and marketing activities are represented in more than

60 countries. This worldwide commitment ensures

security of supply, guaranteed quality and technical

support in every major industrial market. Recent

developments in ceramics technology have allowed us to

introduce sensors with resistance tolerances lower

than 1% and B-value tolerances down to 0.75%. They add

precision to your applications and allow you to design-in

even more attractive features. And because you are

dealing with Philips, you can be sure of excellent quality,

design-in support and service.

Application areas

AUTOMOTIVE SYSTEMS

NTC temperature sensors are widely used in cars. For

example in:

•Electronic fuel injection, in which air-inlet, air/fuel

mixture and cooling water temperatures are used to

determine fuel concentration for optimum injection

•Fan motor control, based on cooling water temperature

•Oil and water temperature controls

•Climatization systems, such as air-conditioning and seat

temperature controls

•Frost sensors for outside temperature measurement

•Oil level indication

•ABS.

GENERAL INDUSTRIES

NTC temperature sensors are used in thermal switches,

measuring systems and detectors in all segments of

industry, notably the following:

•Aerospace/military

•Biomedical/health care

•Education/research

•Electronics/edp

•Energy/environmental

•Food processing

•Heating and ventilating

•Metallurgy

•Petrochemical/chemical

•Weather forecasting

•Fire and smoke detection

•Battery temperature control

•Instrumentation

•Air conditioning.

DOMESTIC APPLIANCES

NTC temperature sensors are used extensively in

domestic appliances. You will find at least one

NTC temperature sensor in just about anything in the

home that gets cold, warm or hot, such as:

•Fridges and freezers

•Cookers and microwave ovens

•Deep-fat fryers

•Coffee makers

•Food warmers and processors

•Washing machines

•Electric irons

•Dish washers

•Electric blankets

•Hair dryers

•Smoke and heat detectors

•Central heating

•Boilers

•Air conditioning

•Aquariums

•Water beds.

1995 Mar 31 12

Philips Components

NTC Thermistors Introduction to NTCs

APPLICATION GROUPING

Applications of NTCs may be classified into three main

groups depending on their physical properties:

1. Applications in which advantage is taken of the

dependence of the resistance on the temperature,

shown in the formula:

This group is split into two sub sections:

a) The temperature of the NTC thermistor is

determined only by the temperature of the ambient

medium (or by the current in a separate heater

winding).

b) The temperature of the NTC thermistor is also

determined by the dissipation in the NTC

thermistor itself.

2. Applications in which the time dependence is decisive,

when the temperature is considered as a parameter

and is written:

This group comprises all applications which make use

of the thermal inertia of NTC thermistors.

3. The third group of applications uses mainly the

property of the temperature coefficient being highly

negative:

Also in this group, applications are listed which take

advantage of the fact that the absolute value of the

temperature is so high, that a part of the V = f(I)

characteristic shows a negative slope.

The classifications given above are supported by practical

examples in Figs 10 to 23.

RfT()=

Rft()=

α0

Examples

Fig.10 Temperature measurement in industrial and

medical thermometers.

handbook, halfpage

MBD923

–

Fig.11 Car cooling water temperature

measurement with bimetal.

handbook, halfpage

bimetal

mA-meter

Vθ

MBD924

1995 Mar 31 13

Philips Components

NTC Thermistors Introduction to NTCs

Fig.12 Car cooling water temperature measurement

with differential mA-meter.

handbook, halfpage

odifferential

mA-meter

MBD925

Fig.13 Temperature measurement with a bridge

incorporating an NTC thermistor and a

relay or a static switching device.

handbook, halfpage

MBD926

–

Fig.14 Liquid level control.

handbook, halfpage

MBD927

Fig.15 Flow measurement of liquids and gases.

The temperature difference between T1 and

T0 is measured for the velocity of the fluid.

handbook, halfpage

MBD928

T1T0

NTC NTC

flow

direction

heater

1995 Mar 31 14

Philips Components

NTC Thermistors Introduction to NTCs

Fig.16 Temperature sensing bridge with op-amp

which acts as differential amplifier. The

sensitivity can be very high.

handbook, halfpage

MBD929

–

Fig.17 Basic temperature sensing configuration.

The op-amp (e.g. NE532) acts as a

Schmitt-trigger. The transfer characteristic

is shown in Fig.18.

handbook, halfpage

MBD930

–

Fig.18 Transfer characteristic of the circuit

shown in Fig.17.

andbook, halfpage

MBD931

Fig.19 Simple thermostat.

handbook, halfpage

–

relay

MBD933

1995 Mar 31 15

Philips Components

NTC Thermistors Introduction to NTCs

Fig.20 Temperature sensing bridge with 0 °C

offset and ADC. Due to RP and RS the

voltage at A varies linearly with the NTC

thermistor temperature. The voltage at B is

equal to that at A when the NTC thermistor

temperature is 0 °C. Both voltages are fed

to the comparator circuit. See also Fig.21.

handbook, halfpage

MBD935

COMP 2

COMP 1

SAWTOOTH

GENERATOR CLOCK PULSE

GENERATOR

AND GATE

Fig.21 Pulses occurring at various points in the

circuit shown in Fig.20.

handbook, halfpage

MBD936

OUTPUT PULSES

OF AND GATE

V COMP 2

V COMP 1

SAWTOOTH

TEMPERATURE

0 C REF.

Fig.22 Constant current circuit (sure start-up) for

line deflection stage.

andbook, halfpage

MBD942

line deflection

driver transistor

300 to 500 V

Fig.23 Temperature compensation in transistor

circuits. Push-pull compensation.

handbook, halfpage

–

MBD943

1995 Mar 31 16

Philips Components

NTC Thermistors Introduction to NTCs

NTC temperature sensors used as a thermal switch

A common use of an NTC temperature sensor is in one of

the bridge arms of a thermal switch circuit using an

operational amplifier such as the µA741. Figure 24 shows

a typical thermal switch circuit for a refrigerator thermostat.

The circuit consists of a 10 V (DC) zener diode stabilized

power supply, a Wheatstone Bridge (containing the NTC

temperature sensor) and an integrated comparator circuit

controlling a triac. The circuit is designed to switch a

maximum load current of 2 A off at −5°C and on at +5 °C.

TEMPERATURE SENSING IN REFRIGERATORS

Fig.24 Refrigerator thermostat using an NTC temperature sensor.

handbook, full pagewidth

MBD944

BT136-

500D

triac

green

680 Ω

182 kΩ

µA 741

10 kΩ

kΩ

R1 120

kΩ

50 µF

(16 V)

10 V

400 mW

1N4148 RdCd

390 Ω330 nF (400 V)

100 Ω

C11.5 µF (40 V)

1 MΩ

220 V

LOAD

(1)

(1) Catalogue number: 2322 593 32312.

All resistors are 0.25 W.

1995 Mar 31 17

Philips Components

NTC Thermistors Introduction to NTCs

HEAT DETECTION IN FIRE ALARMS

Fig.25 Circuit diagram of a typical heat detector using a matched pair of NTC thermistors.

handbook, full pagewidth

MBD945

R11

TH1

–

NTC1

(insulated)

TR3

TR2

–

NTC2

(exposed)

TR1

supply

17 to 28 V

R8 R2

TR4

C1 C2

Z3 Z4

R10

R9 alarm

1995 Mar 31 18

Philips Components

NTC Thermistors Introduction to NTCs

NTC temperature protection of rechargeable batteries

Figure 26 shows the circuit diagram of an ‘intelligent’

charger designed to charge, within 1 hour, a NiCd or NiMH

battery pack containing up to six AA-type cells. The

TEA110X allows any type of power regulator to be used.

In Fig.26, the unregulated 12 V (DC) supply is passed

through a linear power regulator to charge the batteries

under the control and management of the TEA110X. The

BYD13D diode inhibits further charge (and prevents

discharge) when the battery pack is full. For further

information refer to

“Application Note NTC temperature

protection of rechargeable batteries, code number

9398 082 91011”

Fig.26 ‘Intelligent’ charger based on the TEA110X with NTC battery temperature sensing.

andbook, full pagewidth

MBD946

TEA110X(T)

unregulated

MAINS

ADAPTER

BYD13D

BD132/

BDT60A

BC549/

BC635

–

1995 Mar 31 19

Philips Components

NTC Thermistors Introduction to NTCs

SELECTING A SENSOR

Use the following steps in the

specified order to select the required

sensor:

1. What temperature range is

required? Refer to Fig.27.

2. What R25 is required? Refer to

the

“Data sheets”

in this

“Data Handbook”

This

“Data Handbook”

gives the

resistance/temperature

characteristics for each sensor. In

practice, the circuit surrounding

the sensor will determine the

required resistance at room

temperature (R25). This value will

usually be between 10 and 20 kΩ

for the optimum operating

temperature range of the sensor.

Simply select the sensor having

the most suitable R25 value.

3. Are there any other important

parameters? Refer to

Tables 1 and 2.

4. Can you fulfil your need from our

standard ranges (particularly from

the Accuracy Line, see Table 1)

or do you need a special

accuracy (calculation) or

encapsulation?

Fig.27 Temperature selection chart for the standard NTC temperature

sensor ranges.

handbook, 4 columns

MLC268

55 40 0 25 85125 150 200 300

T ( C)

special

long leaded

standard

long leaded

moulded

chips and discs

miniatures

high

temperature

accuracy line

2322 641 2....,

641 3.... and 641 4....

640 90059 and 645 90028

641 6....

640 0.... and 611 9....

633 0.... 633 1.... and 633 2....

626 1.... and 626 2.... part of ranges

633 8....

633 5.... and 633 7....

640 5.... and 640 6....

1995 Mar 31 20

Philips Components

NTC Thermistors Introduction to NTCs

Table 1 Parameter selection chart

Table 2 Parameter selection chart (continued)

PARAMETER

ACCURACY LINE

2322 ... ..... HIGH-TEMPERATURE

SENSORS 2322 ... ..... MINIATURE SENSORS

2322 ... ..... UNIT

640 5.... 640 6.... 645 0.... 642 6.... 633 5.... 633 7.... 633 8.... 626 1.... 626 2.... 633 0....

633 1.... 633 2....

R25 2.2 to

470 0.47 to

470 5to

10 0.0033

to 1.5 10, 20,

30, 100 100,

220 10, 20,

30, 100 1to

1000 1to

1000 kΩ

Tolerance on R25 1, 2, 3, 5 2, 3, 5, 10 5 5, 10 5, 10 5, 10 5, 10 5, 10 5, 10 5, 10 5, 10 %

B-value 3740 to

4570 3740 to

4570 3965 2675 to

3975 3977 3977 3977 2075 to

4100 2075 to

4100 K

Tolerance on B-value 0.75 to 2.5 0.50 to 2.5 ±0.75 −1.3 1.3 1.3 5 5 5 5 %

Max. body diameter 3.4 3.3 ±0.5 3.3 ±0.5 5 ±0.3 1.7 1.85 1.85 2.5 1.6 0.7 to 1 3 mm

Lead diameter 0.4 0.6 0.6 0.6 −0.56 0.56 0.3 0.24 0.06 0.24 mm

Min. lead length 38 17 17 22 ±1−25.4 25.4 30 19 5 20 mm

PARAMETER

CHIPS AND DISCS

2322 ... .....

MOULDED

SENSORS

2322 ... .....

STANDARD LONG-LEADED

SENSORS 2322 ... ..... SPECIAL LONG-LEADED SENSORS

2322 ... ..... UNIT

640 0.... 611 9.... 641 6.... 640 90059

(insulated) 645 90028

(non-insulated) 641 2....

(epoxy) 641 3....

(water-resistant) 641 4....

(pipe)

R25 2.2 to 470 2.2 to 470 2.2 to 470 2.77 10 2.2 to 470 2.2 to 470 2.2 to 470 kΩ

Tolerance on R25 1, 2, 3, 5 −3 3.82 5 3 3 3 %

B-value 3740 to

4570 3500 to

4093 3740 to

4570 3977 3993 3740 to

4570 3740 to

4570 K

Tolerance on B-value 0.75 to 2.5 −0.75 to 2.5 −1, 2 0.75 to 2.5 0.75 to 2.5 0.75 to 2.5 %

Max. body diameter 2.7 5.2 4 ±0.2 2.5 to 2.6 2.5 to 2.6 6 6 6 mm

Lead diameter −−0.6 0.58 0.3 −− −mm

Min. lead length −−21 ±1110±5110±5 400 ±10 400 ±10 400 ±10 mm

1995 Mar 31 21

Philips Components

NTC Thermistors Introduction to NTCs

Examples

To illustrate the method of selecting an NTC temperature

sensor for your application, consider the following two

examples, applying the selection procedure.

EXAMPLE 1

A sensor is required to measure temperatures from

0 to 100 °C with an accuracy of ±3°C:

1. Step 1 (temperature range)

Figure 27 shows that all our sensors will operate over

the temperature range from 0 to 100 °C.

2. Step 2 (R25 value) and Step 3 (other important

parameters)

The sensor will normally be connected into an arm of

a bridge. The resistance of the other arms of the bridge

will determine the approximate ‘cold’ resistance value

R25. Let us assume that the resistance is 4.7 kΩ and

that there are no other critical parameters (response

time, etc.).

3. Step 4 (can you fulfil your need from our standard

ranges?)

Our range of low-cost Accuracy line sensors

2322 640 5.... and 640 6.... should be checked first,

referring to Tables 1 and 2. From the data sheets you

will find that the 2322 640 6.472 and the 640 5.472

may be suitable and that they are available with a

2%, 3%, 5% and 10% or a 1%, 2% and 3% tolerance

on R25.

From the equation you can calculate ∆T. But

since α depends on temperature, you need to know the

temperature coefficients α0 and α100.

In this

“Data Handbook”

you will find that α0=−5.08%/K

and α100 =−2.94%/K. In addition, you can find that the

resistance tolerance due to the B-value is 0.89% at 0 °C

and 2.04% at 100 °C.

So, the ±3°C accuracy on temperature imposes a

maximum allowable resistance variation at 0 °C of

∆R0=α0×∆T=−5.08 ×±3=±15.24% (of nominal

resistance at 0 °C).

Similarly, the maximum allowable resistance variation at

100 °C is α100 ×∆T=−2.94 ×±3=±8.82% (of nominal

resistance at 100 °C).

α∆R

∆T

--------

The actual resistance tolerance of the sensor is the sum of

two components:

•The tolerance on the R25-value

•The resistance tolerance due to the B-value (being zero

at 25 °C but increasing at temperatures other than

25 °C).

Considering the lower-cost 5% tolerance sensor first: at

0°C the worst case tolerance is 5% + 0.89% = 5.89%,

which is well within the 15.24% imposed by the

temperature tolerance. And at 100 °C the worst case

tolerance is 5% + 2.04% = 7.04%, which again is well

within the 8.82% requirement.

So, assuming that no special encapsulation is required,

the 2322 640 63472 sensor fulfils the requirements.

EXAMPLE 2

A sensor is required to measure temperatures from

0 to 250 °C. It must be able to measure at 25 °C with an

accuracy of ±2°C. Its R25 value must be 10 kΩ and it must

have a radial lead configuration and a minimum body

length of 25 mm for encapsulation in a special housing:

1. Step 1 (temperature range)

The high-temperature requirement (see Fig.27)

restricts the choice of leaded sensors to 2322 626 1....

or 626 2.... or 633 8.... sensors.

2. Step 2 (R25-value) and Step 3 (other important

parameters)

R25 =10kΩ is available for the 2322 626 1....,

626 2.... and 633 8.... ranges. All types have radial

lead configurations, but body length dictates the

selection of the 2322 626 1.... range, and in particular

the 2322 626 1.103 sensor.

3. Step 4 (can you fulfil your need from our standard

ranges?)

The calculation of tolerance is: ±2°C accuracy

imposes a maximum resistance tolerance at 25 °C of

α25 ×∆T.

For the 2322 626 1.103 sensor it holds that the maximum

resistance tolerance is 4.2 ×2 = 8.4% (α25 = 4.2). So the

5% sensor 2322 626 13103 will satisfy the requirement.

1995 Mar 31 22

Philips Components

NTC Thermistors Introduction to NTCs

RANGE SUMMARY

•Accuracy Line

– 2322 640 5.... and 640 6....

The flagship of our ranges. The Accuracy Line

sensors offer real value for money. They have low

tolerances (as low as ±1% on the R25-value and

±0.75% on the B-value) and an operating

temperature range from −40 to +150 °C. In addition,

they are very stable over a long life.

– 2322 645 ..... series

This range is our American standard line with an

excellent accuracy over a wide temperature range

(±0.75% on the B-value). R25-values are available

from 5 kΩ to 10 kΩ with an operating temperature

range from −40 to +150 °C.

– 2322 642 6.... series

This range is mainly used for compensation purposes

with R25-values between 3.3 Ω and 1.5 kΩ.

•High-temperature sensors

– 2322 633 5...., 633 7.... and 633 8....

This range of high-quality glass-encapsulated

NTC temperature sensors are price-competitive for

general use. Not only can these sensors be used at

up to 300 °C, but their glass encapsulation makes

them ideal for use in corrosive atmospheres and

harsh environments, even down to −40 °C. This

makes them an attractive alternative to other more

expensive sensing methods. In addition, they are

very small. Two types of tiny glass envelopes are

available: SOD27 for sensors with leads, and SOD80

(the so-called MELF execution) for leadless,

surface-mount sensors.

•Miniature sensors

– 2322 626 ..... and 633 .....

These ranges pack extremely high performance in

very small size. And they are fast and stable in the

temperature range from as low as −55 °C to as high

as +300 °C.

•Chips and discs

– 2322 640 0.... and 611 9....

When leaded components cannot be used, there is

always the possibility of mechanical fixing. For this

purpose we supply metallized square chips with

R25-values from 2.2 to 470 kΩ and five types of

circular disc sensors

•Moulded sensors

– 2322 641 6....

Designed for harsh environments, our moulded

sensors are ideal where good surface contact is

essential. The range has recently been enhanced,

and can be extended further on customer request,

based on the 2322 640 0.... series.

•Standard long-leaded sensors

– 2322 640 90059 and 645 90028

These sensors combine the features of the Accuracy

Line with long non-insulated or insulated leads for

remote sensing applications. On request these

sensors can be customized, based on the

2322 640 0.... range.

•Special long-leaded sensors

– 2322 641 2...., 3.... and 4.....

For special applications we can supply three types of

long-leaded sensors: water-resistant sensors for

permanent immersion in water, pipe sensors for use

in corrosive atmospheres and epoxy-coated sensors

for general use.

1995 Mar 31 23

Philips Components

NTC Thermistors Introduction to NTCs

PREFERRED TYPES

NTC thermistors for temperature sensing

For speciﬁc details refer to the relevant section in this data handbook.

CATALOGUE

NUMBER

2322 ... .....

R25

(kΩ)

NOMINAL

B-VALUE

(K)

2322 640 6.... 5% tolerance

63471 0.47 3560 ±0.75%

63102 1 3528 ±0.5%

63152 1.5 3528 ±0.5%

63202 2 3528 ±0.5%

63222 2.2 3977 ±0.75%

63332 3.3 3977 ±0.75%

63472 4.7 3977 ±0.75%

63682 6.8 3977 ±0.75%

63103 10 3977 ±0.75%

63153 15 3740 ±2%

63223 22 3740 ±2%

63333 33 4090 ±1.5%

63473 47 4090 ±1.5%

63683 68 4190 ±1.5%

63104 100 4190 ±1.5%

63154 150 4370 ±2.5%

63224 220 4370 ±2.5%

63474 470 4570 ±1.5%

2322 640 6.... 3% tolerance

66272 2.7 3977 ±0.75%

66472 4.7 3977 ±0.75%

66103 10 3977 ±0.75%

66473 47 4090 ±1.5%

66104 100 4190 ±1.5%

66474 470 4570 ±1.5%

2322 640 5.... 2% tolerance

54103 10 3977 ±0.75%

54473 47 4090 ±1.5%

54104 100 4190 ±1.5%

2322 640 5.... 1% tolerance

55103 10 3977 ±0.75%

55473 47 4090 ±1.5%

55104 100 4190 ±1.5%

2322 642 6.... 10% tolerance

62338 .0033 2675

62478 .0047 2750

62229 .022 3025

62339 .033 3100

62479 .047 3150

62101 .10 3300

62151 .15 3375

62221 .22 3475

2322 633 5..../7..../8.... 5% tolerance

SMD VERSION

53103 10 3977 ±1.3%

53203 20 3977 ±1.3%

53303 30 3977 ±1.3%

53104 100 3977 ±1.3%

LEADED VERSION

73104; nickel-plated 100 3977 ±1.3%

83103; tinned-coppper 10 3977 ±1.3%

83203; tinned-coppper 20 3977 ±1.3%

83303; tinned-coppper 30 3977 ±1.3%

83104; tinned-coppper 100 3977 ±1.3%

2322 641 6.... moulded

66272 2.7 kΩ±3% 3977 K ±0.75%

66123 12 kΩ±3% 3740 K ±2%

66153 15 kΩ±3% 3740 K ±2%

66223 22 kΩ±3% 3740 K ±2%

66104 100 kΩ±3% 4190 K ±1.5%

66474 470 kΩ±3% 4190 K ±1.5%

CATALOGUE

NUMBER

2322 ... .....

R25

(kΩ)

NOMINAL

B-VALUE

(K)

1995 Mar 31 24

Philips Components

NTC Thermistors Introduction to NTCs

NTC thermistors for temperature sensing (continued)

CATALOGUE NUMBER 2322 641 ..... R25

(kΩ)B25/85-VALUE

(K)

EPOXY-COATED

TYPE WATER-RESISTANT

TYPE BRASS-PIPE

TYPE

26222 36222 46222 2.2 kΩ±3% 3977 K ±0.75%

26502 36502 −5kΩ±3% 3977 K ±0.75%

26103 36103 46103 10 kΩ±3% 3977 K ±0.75%

26473 36473 −47 kΩ±3% 4090 K ±2%

26104 36104 46104 100 kΩ±3% 4190 K ±1.5%