Lesson 12 Temperature sensor and Operation Amplifier

Thermocouple: Connect two different metal wires, there will be a small voltage on the conjunction point. So we can measure the temperature by this voltage. WE use K type thermocouple here. The color of outer skin is blue. There are 2 wires in the thermocouple, one is red, the other is black. The red side have positive voltage. The voltage increases as the temperature increases.

Thermocouple can measure temperature precisely. Thermocouple is used in refrigerator, air-conditioner, temperature controller.

Thermister: The resistance of material very with temperature. So we can measure temperature by the resistance. The thermister we use here is negative coefficient. That means that the resistance decreases as the temperature increases.

The resistance of non-metal material decrease as the temperature increase. Because the electron get more energy in higher temperature and that is helpful for electron to move.

But the resistance of metal material increase as the temperature increase because the electron in metal have enough freedom to move already. More energy has counter-effect.

The code of the thermister we use here is 102. That means that the resistance is 1000Ω at 25℃. 102 means 10 add 2 zeros.

Thermister is widely used to prevent equipment from being too hot. Like computer, electronic oven,….

Operational amplifier (OP) OP have many variety of functions. OP can do addition, multiplication, integration, differential, comparison……

In the thermocouple experiment, we use OP to amplify the small voltage.

In the thermister experiment, we use OP to compare two voltages. There are many kind of OP IC, We use LM324 here. One LM324 include 4 OP’s.

OP is widely used in measurement equipment, control circuit….

If the voltage of inverting input is lower than non-inverting input, The voltage of output will increase until this two voltages become equal.

Experiment:

(1) Measure the resistance of thermister. Heat it and try again.

(2) Measure the voltage of thermocouple. Heat it and try again.

(3) Heat the thermocouple with hair dryer, then the LED on.

We use OP to amplify the small voltage of thermocouple.

The amplify rate is (R1+R2)/R1. You may try different R1 and R2.

(4) If voltage 2 is smaller than voltage 1, then LED ON.

The resistance of thermister become smaller, so the voltage at point 2 become smaller. When the voltage 2 is smaller than voltage 1, then the LED ON. You may try different R1,R2 and R3.

Connect 2 300Ω resister in series makes a 600Ω resister.