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Although thermistors are basically resistors, they possess a few additional specification beyond those of normal resistors. In addition to this it is necessary to choose the correct type when looking for one.
To select a thermistor for any given application it is necessary to understand the specifications and parameters that are used.
Basic thermistor specifications
Some of the more important thermistor specifications are summarised below:
- Thermistor type: The first decision to be made when choosing any thermistor is to ensure that the correct type of thermistor is selected. Not only are there positive and negative temperature coefficient types, but also other forms of thermistor including switching, silistor and others.
- Resistance: The thermistor base resistance is naturally one of the key parameters. Thermistors can be obtained with a variety of resistance figures ranging from Ω up to many kΩ.
Naturally as the resistance varies with temperature, it is necessary to state the temperature at which the component has the required resistance. Normally a temperature of 25°C is used and this may be stated as the as the R25 value. For more specialist applications other temperatures may be used. Also note that sometimes temperatures may be quoted in absolute temperatures, i.e. °K.
- Tolerance on resistance value: As with any resistor, there is a tolerance of the standard resistance. This is taken as the R25 value, or the value at the temperature for which the resistance is given. Values of ±2%, ±3% and ±5% are normally available.
- Β value / constant : Also referred to as the β value, this thermistor specification is a simple approximation for the relationship between the resistance and temperature for an NTC thermistor. To obtain the value, two temperatures are used to obtain the value for β. This is a very useful parameter where relatively small temperature differences are likely to be encountered. The two temperature values are added to the B value, as they are an integral part of the specification. This assumes a nominally linear relationship, which is generally true for most practical applications.
- Tolerance on Β value / constant : As the name indicates this is the tolerance on the value of β.
- Time constant: The thermistor time constant is important for any applications where a swift response is needed - for example when protecting against overloads, etc. No body can instantly raise its temperature from one value to another. It follows and asymptotic curve. Also the larger the body, the longer it takes for the temperature to rise. Accordingly the time constant of the device is an important thermistor specification for some applications.
The thermal time constant parameter designated the Greek letter τ and it is defined as the time required for the thermistor to change to 63.2% (i.e. 1 : 1/e) of the difference between the initial temperature (t1) and aiming temperature (t2) when no power is being dissipated in the thermistor and the temperature difference is applied as a step change.
For measurement purposes the temperature required for the τ measurement; i.e. the time to reach the resistance for 63.2% of the temperature difference is:
- Thermal dissipation factor δ : This is an important feature of the thermistor because all thermistors need to pass some current for the operation of the circuit in which they are included. This causes self-heating of the thermistor.
This thermistor specification defines the relationship between the applied wattage and the thermistor self-heating. If too much current is passed through the thermistor, then it will offset the operation of the thermistor. Accordingly this specification governs the current that can be passed through the device. The dissipation factor, δ is expressed in terms of mW/°C.Where
P = power dissipated in watts
ΔT = the rise in temperature in °C
A particular value of δ will correspond to the level of power needed to raise the thermistor temperature by 1°C. The dissipation factor depends upon a number of factors and as a result the thermistor specification for dissipation factor, δ is really only useful as a guide rather than an exact figure.
- Operating temperature range: This is the temperature range for which the thermistor is designed to operate. Materials, construction and other similar factors limit the range over which the device can operate. Accordingly, for reliability as well as performance, the thermistor should not be operated outside its specified temperature range.
- Maximum power dissipation: For sensing applications, the power dissipation is kept low to prevent self-heating, but under some circumstances there may be reasons to dissipate more power. The specification for maximum power dissipation should not be exceeded if damage is not to result. For greatest reliability the device should be operated well inside its maximum power dissipation - often only 50 - 66% of the specification.
It is possible that some additional specifications of parameters may be used for specialist thermistor applications, but these are some of the main ones normally seen.