# So that the linear response of the thermistor temperature - Cycle Conversion Circuit

Designers most often use the thermistor temperature sensor, not because thermistors have higher sensitivity, and compact, economic and small time constants. However, most of the thermistor resistance - temperature characteristics are highly non-linear, linear response to requests for applications must be corrected. Figure 1 is a thermistor as the sensor with a simple circuit, it's time period changes linearly with temperature, up to 30K in the context of nonlinear error is less than 0.1K. Can use a frequency counter the cycle is converted to digital output. Resistance of the thermistor has a calculated approximation by Poisson law, that is thermistor resistance RT as a function of temperature q, RT = AB-q. In the narrow temperature range, the relationship can be approximately described as an actual thermistor behavior.

So that the linear response of the thermistor temperature - Cycle Conversion Circuit

The thermistor resistance can be an appropriate resistor in parallel RP, get a close to the effective resistance of 30KΩ. In Figure 1, the endpoints A and B provide a network connection between the AB-q effective resistance RAB. JFET Q1 and resistor RS constitute a current regulator, in the end-point between D and E to provide a constant current source IS.

R4 The voltage buffer amplifier IC1 through incentives from the R1 and C1 form the RC series circuit, when R2 is greater than when the RAB, R1 to generate an exponential decay of the voltage. When the voltage falls below R1 thermistor RT instantaneous voltage comparator IC2's output state change, oscillation circuit, Figure 2, the output of IC2 to generate a voltage waveform. Oscillation period T = 2R1C1ln (R2/RAB) ≈ 2R1C1 [ln (R2 / A) + qlnB]. That period T of the type with thermistor temperature q is linear.

So that the linear response of the thermistor temperature - Cycle Conversion Circuit

By changing the value of resistor R1, you can easily change the conversion sensitivity of ΔT / Δq. Q1 and R1 form a current source, determine the output cycle T, its supply voltage and output load change is not sensitive. Can change R2, in the case does not affect the conversion sensitivity to change cycle T. Temperature range between qL qH to time, change the sensitivity of SC, to design the circuit as follows: to qC, said temperature range of core temperature. Measuring thermistor temperature qL, qC and qH Department resistance, with three resistance RL, RC and RH to determine RP, qC Department qL and qH Department RAB RAB said the geometric mean. Value of this RP, you can make the three temperature points (qL, qC and qH) of RAB precision equal to AB-q.

When the temperature range of 30K to 30K or below, most of the thermistor, the other in this region the temperature deviation from the AB-q of the RAB will have a significantly lower than 0.1K error of the non-sex line. RP values can be calculated: RP = RC [RC (RL + RH)-2RLRH] / (RLRH-RC2). As the temperature - the sensitivity of SC conversion cycle for the 2R1C1lnb, so choose R1 and C1 can be set to the following formula: R1C1 = SC [qH-qC] / ln (qL Department RAB / qH Department RAB), to obtain the required SC value. To get a specific output of the low temperature qL cycle TL, R2 should be equal to (qL Department RAB) eY, where Y indicated (TL/2R1C1). Practical applications, R2 low electoral value because IC2's non-zero response delay will increase the output cycle.

Next, the potentiometer value is set to R1 and R2 close to the calculated values. SC in the regulation of R1 correctly, the adjusted R2, so that when T is equal to the temperature qL TL. Two divider resistors R3 and R4 resistance should be equal, and similar tolerances. As an example, the standard thermistor can be used such as the Yellow Springs Instruments 46004, will be 20 ℃ to 50 ℃ temperature range is converted to 5ms to 20 ms cycle. The thermistor RL, RC and RH, respectively resistance 2814Ω, 1471Ω and 811.3Ω, corresponding to low, midpoint, and high temperature. The design parameters are SC = 0.5 ms / K, qL = 20 ℃, qH = 50 ℃, qC = 35 ℃, and TL = 5 ms.

Because only a portion of the current IS through the thermistor, IS should be low to avoid self-heating effect. The design uses the IS of about 0.48 mA, when the thermistor dissipation constant of 10 mW / K, the self-heating error of less than 0.03K. Figure 1 is an example used in the component value. All resistors are 1% tolerance, rated at 0.25W, C1 is a polycarbonate dielectric capacitor.

Using standard 2814Ω to 811.3Ω, 0.01% tolerance thermistor for replacement, can simulate between 20 ℃ to 50 ℃ temperature range of the resulting T value of 5ms to 20 ms, the correct reading of the maximum deviation is less than 32ms in response to the maximum temperature deviation of less than 0.07K. If you use a dissipation constant of less than 10 mW / K of the thermistor, the practical application of the maximum error is less than 0.1K.