Team:Grenoble-Alpes/Temperature Control
T°C Control
It was at Amont à Aussoi that we had met that night to sleep by the fireside. Credits: Estelle Vincent
How to control the temperature within the kit ?
As biological material is used along the analysis, maintaining a specific temperature within the kit is essential. In order to control the temperature, an electrical circuit was designed using an Arduino card. This system has to first perform an enzymatic cleavage at 37°C .Then, DNA target denaturation at 73°C is realized, and finally, a heat shock occurs which is needed for the bacterial transformation. Whenever the system controlling the temperature must not perform special operations, it keeps the temperature around 37°C.
The idea we had was to design an electronic card with a resistance thermometer which would measure the temperature. With a direct current source, the resistance variation is transformed into a voltage variation. The circuit also required the design of a voltage divider and as well as an instrumentation amplifier to adjust 0V at 0°C. An Arduino card was also essential to compare the tension relative to the current temperature with the tension relative to the temperature wanted. Eventually, the output tension was sent to a power converter, then to the Peltier device to cold or heat within the kit.
Temperature acquisition
The first step was to find a way to detect temperature variations in order to adjust the temperature inside the kit. A specific type of resistance thermometer was used: the Pt100 sensor. In fact, its internal variable resistance changes linearly with the temperature variation. At 0°C, the value of Pt100’s resistance is equal to 100 Ω. But, the problem is that working with resistance variations is not that easy. This is why a continuous current source was added thanks to the LM334 component so as to work with voltage variations.
The current passing through the Pt100 is set with the LM334, so the resistance variation is converted into a voltage variation. To choose that current, it is recommended to fix it under 1mA, so as to avoid burn it. This is why a resistance Rset was added, which value is given by the following equation.
For a 100 µA current, Rset must be equal to 640 Ω.
Offset regulation
Now, even though the system was designed to have 0°C at 0V, it was not the case, an offset was observed. To compensate this offset, a voltage divider is needed. A reference tension is created, from which the tension related to the temperature is subtracted.
At 0°C, the Pt100 resistance should be equal to 100 Ω and the current which passes through this component is 100µA. Thanks to the Ohm’s relation, the tension can be deduced and it must be equal to 10 mV. The following equation gives the relation between R1 , R2 and VR2.
With two unknown parameters, infinite values of R1 and R2 were possible. We chose R1= 120 MΩ and R2= 100 kΩ, thus VR2 is equal to 9,9 mV.
Now, as explained before, the reference tension and the tension related to the actual temperature have to be subtracted. The difference between these two tensions is in the millivolts range which is a little bit too small to be used: the difference was therefore amplified with the AD8223 instrumentation amplifier. This device subtracts the two inputs and amplifies the result by a gain which is defined by the following relation.
where
