Application Design of Periodic Conversion Circuit by Using Thermistor

“Designers most often use thermistors rather than temperature sensors because of their higher sensitivity, small size, economy, and small time constant. However, the resistance-temperature characteristics of most thermistors are highly nonlinear and must be corrected for applications requiring linear response. Figure 1 is a simple Circuit using a thermistor as a sensor, whose time period varies linearly with temperature, with a nonlinearity error of less than 0.1K up to 30K. A frequency counter can be used to convert this period to a digital output.There is an approximation according to Poisson’s law for the calculation of the resistance value of the thermistor

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Designers most often use thermistors rather than temperature sensors because of their higher sensitivity, small size, economy, and small time constant. However, the resistance-temperature characteristics of most thermistors are highly nonlinear and must be corrected for applications requiring linear response. Figure 1 is a simple circuit using a thermistor as a sensor, whose time period varies linearly with temperature, with a nonlinearity error of less than 0.1K up to 30K. A frequency counter can be used to convert this period to a digital output. There is an approximate formula for calculating the resistance value of the thermistor according to Poisson’s law, that is, the resistance value RT of the thermistor is a function of temperature q, RT=AB-q. Over a narrow temperature range, this relationship can be approximately described as the behavior of a real thermistor.

A resistor RP with an appropriate resistance value can be connected in parallel with the thermistor to obtain an effective resistance close to 30KΩ. In Figure 1, the network connected between terminals A and B provides an effective resistance RAB of AB-q. JFET Q1 and resistor RS form a current regulator that provides a constant current source IS between terminals D and E.

The voltage on R4 excites the RC circuit composed of R1 and C1 in series through the buffer amplifier IC1. When R2 is greater than RAB, an exponentially decaying voltage is generated on R1. When the voltage on R1 drops below the voltage of the thermistor RT, the output state of the comparator IC2 changes, the circuit oscillates, and a voltage waveform is generated at the output end of IC2 in Figure 2. Oscillation period T = 2R1C1ln(R2/RAB)≈2R1C1[ln(R2/A)+qlnB]. This formula shows that the period T changes linearly with the temperature q of the thermistor.

By changing the value of resistor R1, the switching sensitivity ΔT/Δq can be easily changed. The current source formed by Q1 and R1 determines the output period T, which is very insensitive to changes in supply voltage and output load. The period T can be changed without affecting the switching sensitivity by changing R2. When the temperature range qL to qH is given, the conversion sensitivity is SC, and the circuit can be designed as follows: let qC represent the center temperature of the temperature range. Measure the resistance of the thermistor at temperatures qL, qC and qH, and use three resistance values ​​RL, RC and RH to determine RP, and RAB at qC represents the geometric mean of RAB at qL and qH. For this value of RP, the RAB at the three temperature points (qL, qC, and qH) can be made exactly equal to AB-q.

When the temperature range is 30K or below, most thermistors will produce a non-linear error of significantly less than 0.1K for RAB deviating from AB-q for other temperatures in this region. The RP value can be calculated by the following formula: RP=RC[RC(RL+RH)-2RLRH]/(RLRH-RC2). Since the sensitivity SC of the temperature-cycle conversion is 2R1C1lnb, the following formula can be established when R1 and C1 are selected: R1C1=SC[qH-qC]/ln (RAB at qL/RAB at qH) to obtain the required SC value. To obtain a specific output period TL at a low temperature qL, R2 should be equal to (RAB at qL) eY, where Y represents (TL/2R1C1). In practice, the value of R2 is relatively low, because the non-zero response delay of IC2 will increase the output period. Next, set the potentiometers R1 and R2 values ​​close to the calculated values. After adjusting R1 to get the correct SC, adjust R2 so that T is equal to TL at temperature qL. The two voltage divider resistors, R3 and R4, should be equal in value with similar tolerances. As an example, a standard thermistor can be used, such as the Yellow Springs Instruments 46004, to convert the temperature range of 20°C to 50°C into a period of 5ms to 20ms. The RL, RC, and RH resistance values ​​of this thermistor are 2814Ω, 1471Ω, and 811.3Ω, corresponding to the low, mid, and high temperatures, respectively.The design parameters are also

Since the current IS is only partially through the thermistor, IS should be low enough to avoid self-heating effects. This design uses an IS of about 0.48 mA, and the self-heating error is less than 0.03K when the dissipation constant of the thermistor is 10 mW/K. Figure 1 shows the values ​​of the components used in the example. All resistors are 1% toleranced, rated at 0.25W, and C1 is a polycarbonate dielectric capacitor.

Replacing with a standard 2814Ω to 811.3Ω, 0.01% tolerance thermistor can simulate a wide range of temperatures between 20°C and 50°C, resulting in a T value of 5ms to 20ms, with a maximum deviation of less than 32ms for correct readings, The maximum temperature deviation of the response is less than 0.07K. If a thermistor with a dissipation constant not greater than 10 mW/K is used, the maximum error in practical applications is less than 0.1K.