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MFC negative temperature coefficient (NTC)

thermistors are fine ceramic semi-conductors manufactured from high purity and uniform materials to achieve a construction of near theoretical density. This ensures small size, tight resistance and B value tolerance, and fast response to temperature variations, making for a

highly sensitive and precision component. Mfc thermistors are available in a wide range of types to meet custom demands for high precision,compactness and high reliability.

Basic properties of NTC Thermistor

1.Resistance-temperature characteristics

The resistance and temperature characteristics of a thermistor can be approximated by equation 1.

(eq.1) R = R0 exp [ B(1/T-1/T0) ] R : resistance at absolute temperature T(K) R0 : resistance at absolute temperature T0(K) B : B constant T(K) = t(¡æ) + 273.15

Actually, the B value for the thermistor is constant depending on its material composition, but equation 1 may yield different results from actual values if applied over a wide temperature range. By taking the B constant in equation 1 and calculating it as a function of temperature, as shown in equation 2, the difference with actual value can be minimized. (eq.2) BT = CT2 + DT + E ,where C, D, and E are constants.

2.B constant

B constant is defined as the ratio of the resistance changes against temperature changes between two optional temperatures and is formulated by the equation 3 as below.

(eq.3) B = ( lnR1 – lnR2 ) / ( 1/T1 – 1/T2 )

where R1 : resistance value at the absolute temperature T1(K)

R2 : resistance value at the absolute temperature T2(K)

In general, B constant within the range of 2000 -- 6000K is used. The rate of resistance change by

temperature becomes large in propotion to an increase of B constant. The resistance-temperature

characteristics for different B values are shown in figure 1.

(Fig.1) Resistance – temperature characteristics

3.Resistance temperature coefficient, α

The resistance temperature coefficient( α ) is defined as the rate of change of the resistance

associated with a temperture variation of1 °Cat any given temperature. The relationship between

the resistance temperature coefficient(¦Á) and the B constant can be obtained by differentiating equation

1 described previously. (eq.4)

α= 1/R * dR/dT x 100 = - B/T2 x 100 (%/°C)

A negative value signifies that the rated zero power resistance decreases with increasing temperatures.

4.Heat dissipation constant, δ

The dissipation constant(δ) is necessary power for increasing the temperature of the thermistor

element by 1°C in state of thermal equilibrium by self heating. The self heating process is subject to the following relationship between the thermistor temperature T1,ambient temperature T2, and

consumed power P. (eq.5)

δ= P / ( T1 – T2 ) (mW/°C) ,

where P = I2R = I x V

The dissipation constants in this catalog were measured in still air at 25°Cand in their final form

before shipment. The dissipation constant will vary according to the shape and dimension of thermistor and its mounting and environmental condition.

5.Power rating

The power rating is the maximum power for a continuous load at the rated temperature. In this

catalog, the value is calculated from the following equation 6 using a rated temperature of 25°C. (eq.6)

Rated power = heat dissipation constant x

(maximum allowable working temperature – 25)

6.Maximum operating power

The maximum operating power is the power level for the maximum permissible temperature rise

through self heating when using a thermistor for temperature compensation or as a temperature

sensor. The maximum operating power, when t is the permissible temperature rise, can be calculated using equation 7.

(eq.7) Maximum operating power = t x heat

dissipation constant

7.Thermal time constant, τ

The thermal time constant is defined as the time for the temperature a thermistor, with no load applied, changes to 63.2% of the difference between its initial and final temperatures, during a sudden change in the surrounding temperature. When the surrounding temperature of the thermistor changes from T1 to T2, the relation between the elapsed time t and the thermistor temperature T can then be expressed by equation 8.

(eq.8) T = ( T1 – T2 ) exp ( - t /τ) + T2

= ( T2 – T1 ) [ 1 – exp ( - t /τ) ] + T1

The constantτis called the thermal time constant. If t =τ,

( T –T1 ) / ( T2 – T1) = 0.632.

In other words, the thermal time constant is the elapsed time it takes for the temperature of the thermistor changes by 63.2% of its initial temperature. The relationship between the changes of thermistor temperature veersus the elapsed time is shown in table 1 and figure 2

Measuring conditions for thermal time constant in this catalog are as follows: A thermistor in its final form before shipment is moved from 50¡æ to 25¡æ in a still air environment, and the time is measured until the temperature of the thermistor reaches 32.4 ¡æ. Please note, the thermal time constant will vary according to size, environmental and packaging conditions.

Tolerance,%

The rated resistance RN & B value are subject to manufacturing tolerance, Due to this tolerance of the B value,an increase in resistance spread must be expected for temperatures that lie.

Above or below the rated temperature TN

Generally,the resistance tolerance can be expressed by the following formula,

tolerance at a certain temperature is influenced by two variables: the manufacturing tolerance of the rated resistance and the variation of the B value with temperature.

Reliability Tests

1,Hot Temperature Storage

Store in hot still air at 100 °C ± 5 °Cfor 1000hrs, drift of electric characteristics should be : |ΔR/R|≤3%,| ΔB/B|≤1%;

2, Low Temperature Storage

Store in dry still air at -30°C±5°Cfor 1000hrs, drift of electric characteristics should be: | ΔR/R|≤3%,| ΔB/B|≤1%;

3,Heat cycling test

After 1000 cycles of -30°C±5°Cdry air 5mins ,room ambient temperature 1mins , 100°C±5°C

dry air 5mins , room ambient temperature 1mins.drift of electric characteristics should be: |R/R|≤3%, |B/B|≤1%;

4, High Humidity And High Temperature

Store in 95%RH, 40°Cair conditions for 1000hrs. drift of electric characteristics should be: | ΔR/R|≤3%,| ΔB/B|≤1%;

5, Resin Coating Strength

Pull 2N static weight in the direction of lead ,test sample should be no break and damage after one minutes.

6, Free Fall

After three times natural fall to a maple board from 1m high, there should be no visible damage.

7, Resistance to Soldering Heat

After lead wire of the test samples were dipped on time within 6.0mm from end of lead wire in solder bath at 260±5°C for 4s, there should be no visible damage.

P/N system

MTE2 395F 103H (Drawing No)

1 2 3 4

1, Series

M………………………MTS

T………………………thermistor

E………………………Epoxy

2, B constant

The B constant is expressed by three digit code and tolerance code (e.g) Code 395

Tolerance code

F…………………………………1

G…………………………………2

H…………………………………3

J…………………………………5

K…………………………………10

3, Nominal resistance

The resistance is expressed by three digit and in unit is OHM, the first & second digits are effective number, the third digit is number of "0" following effective number. (e.g)

Code

500……………………………50 OHM

101……………………………100 OHM

202……………………………2K OHM

303……………………………30K OHM

404……………………………400K OHM

Nominal resistance tolerance

Code

F…………………………………1

G…………………………………2

H…………………………………3

J…………………………………5

K…………………………………10

4, Drawing NO.

To temperature sensor, the P/N is expressed in series name, thermistor P/N and drawing

NO. (e.g)

MS11343F103F118-1

118-1 is the drawing No.

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