Negative temperature coeffcient(NTC)thermistors are manufactured from high purity and uniform materials to achieve a construction of near-perfect theoretical density. This ensures small size, tight resistance and B-value tolerances, and fast response to temperature variations, making a highly sensitive and precision component. Thermistor is available in a wide range of types to meet your demands for small size and high reliability.
■ Resistance - temperature characteristic
The resistance and temperature characteristics of a thermistor can be approximated by equation 1.
(eq1) R=Ro exp {B(I/T-I/To)}
| R |
: resistance at absolute temperature T(K) |
| Ro |
: resistance at absolute temperature To(K) |
| B |
: B value |
|
*T(K)= t(ºC)+273.15 |
The B value for the thermistor characteristics is not fixed, but can vary by as much as 5K/ºC according to the material composition. Therefore equation 1 may yield different results from actual values if applied over a wide temperature range.
By taking the B value in equation 1 as a function of temperature, as shown in equation 2, the difference with the actual value can be minimized.
(eq2) BT=CT2+DT+E
C, D, and E are constants.
The B value distribution caused by manufacturing conditions will change the constant E, but will have no effect on constants C or D. This means, when taking into account the distribution of B value, it is enough to do it with the constant E only.
Calculation for constants C, D and E
Using equations 3~6, constants C, D and E can be determined through four temperature and resistance value data points (T0, R0). (T1, R1). (T2, R2) and (T3, R3).
With equation 3, B1, B2 and B3, can be determined from the resistance values for To and T1, T2, T3 and then substituted into the equations below.
Example
Using a resistance-temperature characteristic chart, the resistance value over the range of 10ºC~30ºC is sought for a thermistor with a resistance of 5kΩ and a B value deflection of 50K at 25ºC.
Process
(1) Determine the constants C, D and E from the resistance-temperature chart.
To=25+273.15 T1=10+273.15 T2=20+273.15 T3=30+273.15
(2) BT= CT2+TD+E+50 ; substitute the value into equation and solve for BT
(3) R= 5exp {BT (I/T-I/298.15)} ; substitute the values into equation and solve for R
*T : 10+273.15~30+273.15
Results of plotting the resistance temperature ■ Characteristics are shown figure 1
■ Resistance temperature coefficient
The resistance-temperature coefficient (α) is defined as the rate of change of the zero-power resistance associated with a temperature variation of 1ºC at any given temperature.
The relationship between the resistance-temperature coefficient (α) and the B value can be obtained by differentiating equation 1 above.
A negative value signifies that the rated zero-power resistance decreases
■ Heat dissipation constant (JIS C2570-1)
The dissipation constant (δ) indicates the power necessary for increasing the temperature of the thermistor element by 1ºC through self-heating in a heat equilibrium.
Applying a voltage to a thermistor will cause an electric current to flow, leading to a temperature rise in the thermistor. This " intrinsic heating " process is subject to the following relationship among the thermistor temperature T1, ambient temperature T2, and consumed power P.
Measuring conditions for all parts in this catalog are as follows:
| (1) |
Room temp is 25ºC
|
| (2) |
Axial and radial leaded parts were measured in their shipping condition.
|
■ Maximum power dissipation (JIS C2570-1)
The power rating is the maximum power for a continuous load at the rated temperature.
For parts in this catalog, the value is calculated from the following formula using 25ºC as the ambient temperature.
(formula) Rated power=heat dissipation constant × (maximum operating temperature-25ºC)
In the detail specification, it is likely to write by "Power rating"that is a past name.
■ Permissible operating power
Definition : The power to reach the maximum operating temperature through self heating when using a thermistor for temperature compensation or as a temperature sensor. (No JIS definition exists.) The permissible operating power, when t ºC is the permissible temperature rise, can be calculated using the following formula.
Permissible operating power= t*heat dissipation constant
■ Thermal time constant by ambient temperature change (JIS C2570-1)
A constant expressed as the time for the temperature at the electrodes of a thermistor, with no load applied, to change to 63.2% of the difference between their 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 thermistors temperature T can then be expressed by the following equation.
| T= |
(T1-T2)exp(-t/τ)+T2 |
|
(T2-T1){1-exp(-t/τ)}+T1 |
The constant t is called the heat dissipation constant.
If t= τ, the equation becomes : (T-T1) / (T2-T1) = 0.632
In other words, the above definition states that the thermal time constant is the time it takes for the temperature of the thermistor to change by 63.2% of its initial temperatrue difference.
The rate of change of the thermistor temperature versus time is shown in table 1.
Measuring conditions for parts in this catalog are as follows:
| (1) |
Part is moved from a 50ºC envirconment to a still air 25ºC environment until the temperature of the thermistor reaches 34.2ºC. |
| (2) |
Axial and radial leaded parts are measured in their shipping form. |
Please note, the thermal dissipation constant and thermal time constant will
vary acccrding to environment and mounting conditions.
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