NTC thermistor having a negative temperature coefficient thermistor(Metal Oxide Varistor). Is to use a single high-purity material, having a structure close to the theoretical density, high-performance ceramic. Therefore, in the miniaturization at the same time, also has a resistance value, small fluctuations in the temperature characteristics, and fast response to the detection of high sensitivity, high precision, can be a variety of temperature changes. The company provides a variety of shapes, features a small, high-reliability products, which can meet the application requirements of our customers.
B value of the NTC thermistor constant calculation
The B value is a material constant of the thermistor, i.e. a thermistor chip (a semiconductor ceramic) is formed after high-temperature sintering, the material having a resistivity, for each formulation, and the sintering temperature is only a B value, So what kinds of material constants.
B value thermistor material constant, or called thermal index.
B value can be calculated through the measurement of the resistance value at 25 degrees Celsius and 50 degrees Celsius (85 degrees Celsius). B value, with the product temperature coefficient of resistance is a positive correlation, that is the larger the B value, the greater temperature coefficient of resistance.
The temperature coefficient means the temperature is increased by 1 degree, the rate of change of the resistance value. B can be converted into a temperature coefficient of resistance value using the following formula:
Temperature coefficient of resistance = B value / T ^ 2 (T want to convert a point of absolute temperature value)
NTC thermistor B value is generally between 2000K-6000K, can not simply say the B value is the bigger the better or the smaller the better, it is up to you to use where. In general, as temperature measurement, temperature compensation, as well as the suppression the surge resistor with the product, under the same conditions, the B value big point. Because as the temperature changes, the resistance value of the B value of the products greater changes, that is more sensitive.
These are made answer according to my own understanding, I do this, if you have any questions, you can add me as a friend, or send me a message.
NTC thermistor B value is generally between 2000K-6000K, can not simply say the B value is the bigger the better or the smaller the better, it is up to you to use where. In general, as temperature measurement, temperature compensation, as well as the suppression the surge resistor with the product, under the same conditions, the B value big point. Because as the temperature changes, the resistance value of the B value of the products greater changes, that is more sensitive.(Metal Oxide Varistor)
NTC thermistor B value formula: = the T1T2 Ln (RT1/RT2) / (T2-T1)
Where B: B value of the NTC thermistor, provided by the manufacturers;
RT1, RT2: thermistor in the temperature respectively when the resistance value of the T1, T2;
T1, T2: absolute temperature scale. v
NTC thermistor B value formula.
To correct yesterday's post, I used precision thermistor 1 [%], not [%].
B = T1T2 Ln (RT1/RT2) / (T2-T1) ---------- (1)
B: NTC thermistor B value provided by the manufacturer;
RT1, RT2: thermistor temperature for T1, T2 when the resistance value provided by the manufacturer the temperature is 298.15K (25 degrees C) resistance.
T1, T2: absolute temperature scale.
I was against the principles of yesterday Figure simple to talk about:
By equation (1) can be obtained:
B (1/T1-1/T2)
RT1/RT2 = e -------------- (2)
Take T1 = 298.15K, the resistance of the thermistor RT1 = 10K, so they chose R1 = 10K set temperature T2 when the sub-pressure value V2: V2 = RT2Vcc / (RT2 + R1) was RT2 = V2R1 / (Vcc-V2), so
RT1/RT2 = Vcc/V2-1 into (2), we have
B (1/T1-1/T2)
e = Vcc/V2-1
Have B (1/T1-1/T2) = Ln (Vcc/V2-1)
T2 = T1 / (1-T1 (Ln (Vcc/V2-1)) / B)
Set of 8-bit ADC output value of N Vcc/V2-1 = 256/N-1
So T2 = T1 (1-T1 (Ln (256/N-1)) / B)
Converted to Celsius
T = T2-273.15
You can compile a program in C or VB from N = 0 to N = 255 calculated thermometer, and N is the index look-up table directly to get the temperature. By actual test temperature values constitute temperature form, using interpolation algorithms can also get a temperature value. I here T1 = 25 degrees, you can adjust the value of T1 to test a higher or lower temperature.
NTC thermistor resistance calculation
Low-cost temperature measurement program NTC thermistor is used more, and generally use a look-up table method to obtain the temperature value, which involves correspondence between temperature and resistance. If you buy from the manufacturers NTC thermistor can be to the manufacturers temperature resistance table, but for ordinary lovers are thermistor buy from a retailer, selling components uncle and aunt will not provide you with resistance value and temperature control table. The usual method is standard thermometers, ambient temperature did not rise once measuring the resistance of the thermistor, the correspondence between resistance and temperature obtained by this method is more cumbersome, error is relatively large, poor control of other temperature changes; One way is calculated by the formula RT table, the temperature and resistance of the NTC thermistor temperature and resistance was not a linear relationship, but is calculated by the following equation can still correspondence relationship(Metal Oxide Varistor):
Rt = R * EXP (B * (1/T1-1/T2))
Of the above formula is explained as follows:
1 RT is the resistance of the thermistor in the temperature T1;
2. R is the nominal resistance of the thermistor T2 at room temperature;
The B value is the important parameters of the thermistor;
4. EXP is the n-th power of e;
Where T1 and T2 refers to the the K degree that the Kelvin temperature, K = 273.15 (absolute temperature) + degrees Celsius;
For example, I do a thermistor a MF58502F327 Model
The MF58 - Model glass encapsulation
502 - room temperature 25 degrees nominal resistance 5K
F - allowable deviation of ± 1 [%]
327 - B value 3270K NTC thermistor
That it's R = 5000, T2 = 273.15 at +25, B = 3270, RT = 5000 * EXP (3270 * (1/T1-1 / (273.15 at +25))), this time substituting T1 temperature can be obtained the corresponding temperature thermistor resistance, pay attention to the temperature unit conversion, for example, we require 10 degrees Celsius above zero resistance, so T1 (273.15 +10).
In order to facilitate the calculation we can take advantage of Excel's powerful formula to reduce the workload of manual calculations, if your software is very strong can write a small program to get.
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The choice of the size of the thermistor resistance
Such as the measurement of the temperature of 10-100 degrees
Voltage of 5,6 V with a 1000Ω the costs of electricity
If voltage is about 1V, and then a large value, will certainly affect the accuracy.
NTC thermistor characteristic parameters of basic knowledge
1.NTC negative temperature coefficient thermistor
2.PTC positive temperature coefficient thermistor
The physical characteristics of the thermistor is represented with the following parameters:
Resistance value, the value of B, dissipation factor, thermal time constant, resistance temperature coefficient.
Resistance values: R [Ω]
Approximations of the value of the resistor is expressed as: R2 = R1exp [1/T2-1/T1]
Wherein: R2: absolute temperature T2 [K] when the resistance [Ω]
R1: the absolute temperature of T1 [K] when the resistance [Ω]
B: B value [K]
B value: B [k]
The B value is a function of the resistance between the two temperature changes, the expression is:
B = InR1-InR2 = 2.3026 (1ogR1-1ogR2)
1/T1-1/T2 1/T1-1/T2
Where: B: B value [K]
R1: the absolute temperature of T1 [K] when the resistance [Ω]
R2: the absolute temperature of T2 [K] when the resistance [Ω]
Dissipation Factor: δ [mW / ° C]
Dissipation factor is the ratio of electric power consumed by the objects and the corresponding temperature rise
δ = W / T-Ta = I? R / T-Ta where:
δ: dissipation coefficient δ [mW / ℃]
W: thermistor electric power consumption [mW]
T: reach thermal equilibrium temperature value [° C]
Ta: room temperature [° C]
I: at temperature T plus the thermistor value of current [mA]
R: at the temperature T plus the thermistor on the current value [kΩ]
Measuring temperature, you should pay attention to prevent the warming caused by the thermal resistance due to heating.
Thermal time constant: τ [sec.]
Thermistor under conditions of zero energy, the effect of the step so that the temperature of the thermistor itself changed
Change, 63.2% of the time required to change between the initial and final values when the temperature coefficient of thermal time τ.
Resistance temperature coefficient: α [[%] / ℃]
α shows the temperature of the thermistor changes every 1? C, the coefficient of the degree of change of the resistance value [i.e., the rate of change] with
α = 1 / R? dR / dT, said calculation formula is:
α = 1 / R? dR / dT × 100 =-B / T? × 100
Wherein: α: Temperature coefficient of resistance [[%] / ° C]
R: the resistance value of the absolute temperature T [K] [Ω]
B: B value [K]
NTC thermistor detection method
(A) measurement of nominal resistance value Rt
With do not go directly to the contact with the thermistor or too close, to prevent damage to the thermistor.
2, if the measured αt> 0, it indicates that the thermistor is not NTC but the FTC.(Metal Oxide Varistor)
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