NTC thermistor application - temperature measurement

Thermistor belongs to the category of sensitive components . According to the temperature coefficient, it can be divided into positive temperature coefficient thermistors （PTC） And negative temperature coefficient thermistor (NTC). At different temperatures , Behave differently . among PTC Thermistor , As the temperature rises , The resistance increases . and NTC Thermistor , As the temperature rises , The resistance becomes smaller .

However, today's Chapter , The main point is NTC( In terms of the cases in the practice of the small series project ).

The selection
With smooth collaterals NCP03XH103F05RL Model number NTC For example , Observe it datesheet, Many parameters are given , However, when selecting models , We still need to select the type with the following parameter values .

1. Nominal resistance
   The nominal resistance is NTC Resistance value of thermistor design , It is often marked on the surface of thermistor . The nominal resistance value means that the reference temperature is 25℃ Zero power resistance value .

2. Rated power
   The rated power refers to the temperature of the thermistor at ambient 25℃、 The relative temperature is 45%～80% And the atmospheric pressure is 0.87～1.07Pa Under atmospheric conditions , Allowable dissipation power of long-term continuous load .

3. B value
   B Range of values (K) Is the thermal index of the negative temperature coefficient thermistor .

4. Rated current value
   That is to say 25℃ The nominal value of the current passing through the environment .

5. Dissipation coefficient
   Dissipation coefficient refers to the ratio of the power consumed by the thermistor to the change of ambient temperature .
   

This is this type of model NTC Screenshot of resistance and similar specifications .
We can see from the picture NCP03XH103F05RL This model NTC Thermistor , Its resistance value at room temperature is 10K, The rated current is 0.31A, The rated power is 100mW, The dissipation coefficient is 1 mW/℃. Observe carefully B value , We will find that ,B Value is not fixed , It is in different temperature ranges ,B The value is different .

effect
NTC Thermistor has many functions . Temperature measurement , Temperature compensation , Overheating protection , Anti surge is useful NTC Thermistor .

Today I'm going to talk about NTC Application of temperature measurement , Also in our project NTC Thermistor plays the most commonly used role .

Measuring temperature

On the cell phone , Tablet and some consumer electronics products , Will be useful NTC Carry out temperature detection .

This diagram is the simplest , It's also the most commonly used NTC Circuit diagram of temperature detection .

among
R1 For accuracy 1%, The resistance value is 390K The resistance of .
RT1 It's our NTC Thermistor .
C1 by 1nF The capacitance of , Yes, but not . If you add it, you can filter out the high-frequency interference signals from the power supply or the circuit board . In some practical projects , Mostly to save costs , Will choose not to add .

To know the temperature , We have to know NTC The resistance of . But we can't get it directly , But we can use it NTC Partial pressure obtained , Measure the voltage value , Thus, we can further understand its resistance value .

According to Ohm's law ,RT1 Branch , The current value is I=Vcc/(R1+RT1),RT1 The voltage value of is U=IR=VCCRT1/(RT1+R1).

At the same time, suppose MCU It's using 12 Bit ADC sampling
Then input. ADC Of AD The value is ：
AD=4096U/UADC= 4096VCCRT1/[(RT1+R1) UADC].
If VCC And MCU If the supply voltage of is the same , Is more convenient, that is
AD=4096*RT1/RT1+R1.
according to AD Value can then be understood RT1 Partial pressure of , Know its resistance , And then understand its temperature .