Location : Home >> Technology Notes >>Temperature Sensor

# Linear Temperature Sensor Design

By Geoff Knagge

This note is from a 3rd year theoretical project in a subject that studied the use of transistor technology. There are obviously better ways to do it, but the aim here was to demonstrate an understanding of the temperature effects on transistors.

The basic requirement of this project was to design a circuit which would :
• Use the transistor junction characteristics to measure a change in temperature
• Represent the actual temperature on a scale 0 to 10V, corresponding to 0 to 100°C.
• Provide a logic signal which goes low if the temperature exceeds 90°C, and returns to high once the temperature falls back below 80°C.

This assignment scored 99%... I lost the mark because it would be better to have added an additional resistor to the hysteresis output stage.

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 VT changing linearly with temperature, but corrects the problem of the offset IE0.

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+ = Vin for linear operation

So,

For Vin = 297*0.0005 (0°C), Vout = 29.7 - 29.7 = 0V

For Vin = 397*0.0005 (100°C), Vout = 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 Vin > 8V. If Vin 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 Vin rises above 9V.

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

(C)opyright, Geoff Knagge.