Linear Temperature Sensor Design

The final design can be divided into the following three modules :

1. The sensor contains the temperature detector and outputs an analog signal which is linearly proportional to the Kelvin value of the measured temperature. The design aim was for 0.5mV = 1K



2. The scaler converts this output to one which is linearly proportional to the Centigrade value of the measured temperature. The requirement is that 0.1V = 1 °C.



3. The hysteresis unit determines if the temperature is in a safe range. It will output a high logic signal if this is the case. When the temperature rises above 90°C (9V), the signal will go low and will remain that way until the temperature drops below 80°C (8V).

Since we are assuming ideal op-amps, each module can be dealt with individually without having to consider interconnection problems such as loading effects.

Sensor Unit

+V = 15V R3 = 470K + 22K = 492K Rin = 1K // 1K = 500W Vin = 5V

The above circuit is the logarithmic converter presented in lectures, but with a fixed input voltage and no scaling correction.

The temperature sensor design makes use of the non-ideal property of the scaling factor V_T changing linearly with temperature, but corrects the problem of the offset I_E0.

Hence, from the lecture notes,

The result is an output which changes linearly with temperature, at a rate of 0.5mV / K.

Scaling Unit

This is a non-inverting amplifier, where we need

Let R₄= 1K

R₃ = 32.67K

R₁ = 193.09K

V_- = V₊ = V_in for linear operation

So,

For V_in = 297*0.0005 (0°C), V_out = 29.7 - 29.7 = 0V

For V_in = 397*0.0005 (100°C), V_out = 39.7 - 29.7 = 10V

This verifies that the amplifier is correctly configured.

Hysteresis Unit

R1 = 10K R2 = 200K R3 = 10K R4 = 15K + 2.7K = 17.7K R5 = 18 + 2.7 = 20.7K R6 = 100K VZ = 10V

For positive saturation, . For this to be valid, we need

 

For negative saturation, . For this to be valid, we need .

 

This circuit has negative feedback and hence is unstable. Therefore, the op-amp output will always be in a saturated state. It goes into positive saturation (turning on the transistor and pulling the output low) whenever it is already in that state with V_in > 8V. If V_in drops below 8V, the circuit will revert to the other unstable state, negative saturation.

When in negative saturation, the op-amp switches off the transistor and the logic is pulled high by R₆. Similarly, it will remain in this state until V_in rises above 9V.

Hence, it can be seen that this module functions as required.