Application of NTC Thermistors

release time:2018-5-30  Views:782

1 Introduction to NTC Thermistors Semiconductor thermistors are temperature-sensitive components made of semiconducting materials whose resistivity varies with temperature. They are divided into positive temperature coefficient (PTC) thermistors and negative temperature coefficients according to their temperature characteristics. (NTC) Thermistors and Critical Temperature Coefficient (CTR) thermistors, etc. NTC is an abbreviation for Negative Temperature Coefficient. It is a negative temperature coefficient meaning. It is a high-tech semiconductor functional ceramic material mainly composed of barium titanate. The main characteristic is that the resistance increases with temperature in the operating temperature range. reduce. The NTC series thermistors have been widely used in household appliances to achieve automatic gain adjustment, overload protection, temperature control, temperature compensation, voltage stabilization, etc., using its basic resistance (temperature characteristics, voltage) current characteristics. .

1 Resistance-temperature characteristics
Figure 1 shows the temperature characteristics of NTC-type thermistors. NTC-type thermistors used for measurement have a temperature dependence of resistance within a relatively small temperature range: RT = R0eB1T-1T0 where: resistance at RT, R0-temperature T, T0; T-thermodynamics Temperature; B-Thermistor material constant, generally taken from 2000 to 6000K, can be expressed by

Temperature coefficient of resistance
If B=4000K, T=323.15K (50°C), then a=-3.8%/°C. Visible a is an important parameter to characterize the performance of the thermistor material.

1.2 Volt-ampere characteristics
The relationship between the terminal voltage on the thermistor and the current through the thermistor in the static condition is called the volt-ampere characteristic. It is an important characteristic of thermistor, as shown in Figure 2.
The thermistor is only proportional to the terminal voltage and current in the small current range, because the current is also small when the voltage is low, the temperature does not increase significantly, and its current and voltage relationships are in accordance with Ohm's law. However, when the current increases to a certain value, the resistance of the element decreases due to the temperature increase, so the voltage drops instead. Therefore, according to the allowable power dissipation of the thermistor to determine the current, the current can not be selected too high in the temperature measurement.

2 NTC thermistor application example analysis

2.1 The application of fan temperature control circuit

Electric fan temperature control switch circuit, the bridge rectifier circuit consisting of VD1~VD4 and the single-phase thyristor VS constitute the main loop, the operational amplifier IC2 acts as a comparator of the temperature control circuit, and the input terminal resistance R1~R4 of IC2 constitutes a single Arm bridge. The gate of VS is connected to the output of IC2 and is controlled by the output of IC2. The step-down resistor R0 and the power supply integrated block IC1 form a simple power supply to provide the DC voltage for the bridge and the comparator. The bridge arms R2 to R4 have a fixed and equal resistance, and the bridge arm R1 has a variable resistance, which is composed of a potentiometer RP and an NTC thermistor RT having a negative temperature coefficient. RT is a temperature sensing element of a temperature control circuit, and its resistance value changes with the ambient temperature. RP is a temperature presetting element, and adjusting RP can preset the temperature at a certain temperature state. When the ambient temperature is lower than the preset temperature, the resistance of R1 is relatively large, so that the voltage of the non-inverting input terminal of the comparator IC2 is lower than the voltage of the inverting input terminal, that is, UR2UR4, the IC2 output high level triggers the VS conduction, and is serially connected to the main circuit. The electric fan in the room is connected to the mains and starts operation. The switch S in the circuit is a temperature control selection switch. If the S is closed, the temperature control circuit will not work. Only when the S is turned off, the temperature control circuit will work, so that it can be freely chosen by people.

2.2 Application in RC Oscillator
The RC bridge oscillator, also known as the Venturi bridge oscillator, consists of a non-inverting amplifier and an RC series parallel feedback network with frequency selective effects.

The integrated operational amplifier LM741 forms the in-phase amplifying circuit, the signal of the output frequency f0 of the 6 feet is feedbacked to the input terminal 3 of the amplifier through the RC series parallel network. Because the feedback coefficient of the RC frequency selective network is F=1/3, so long as the amplifier's amplification factor Auf=3, the amplitude balance condition can be satisfied; because the phase difference between the input signal and output signal of the non-inverting amplifier is 0b, the RC string The phase shift of the parallel frequency selection network is also 0b, so the total phase shift of the signal satisfies the phase balance condition and is positive feedback. Therefore, the circuit can generate self-oscillation for the component with the frequency f0 in the signal, and other frequency components do not generate self-oscillation due to the effect of the frequency selection network, the feedback voltage is low, and the phase shift is not zero.

The oscillation frequency of the RC bridge oscillation circuit depends on the R1, R2, C1, and C2 parameters of the RC frequency selection loop. Normally, R1=R2=R, C1=C2=C, and the oscillation frequency is f0=12PRC.

In the RC bridge oscillator circuit, the feedback resistor Rf often uses an NTC thermistor with a negative temperature coefficient in order to smoothly start. When the output amplitude of the oscillator increases, the current flowing through Rf increases, the resistance decreases as the temperature of the thermistor increases, the gain of the amplifier decreases, and the oscillating output signal automatically stabilizes.

2.3 Temperature compensation

Most of the parts usually used in the instrument are made of metal wire, such as coils, wire wound resistors, and the like. Metals generally have a positive temperature coefficient. The use of a thermistor with a negative temperature coefficient compensates for the errors due to temperature changes. In practical applications, the negative temperature coefficient thermistor is connected in parallel with the manganese-copper wire resistance and is then connected in series with the compensated element, as shown in FIG. 5 .

2.4 thermistor temperature measurement circuit