Temperature Control System
Temperature is important for growing seeds because it can significantly affect the germination and growth of plants. Different types of plants have different temperature requirements for seed germination and growth, and if the temperature is not suitable for a particular plant species, the seeds may not germinate at all, or may germinate slowly and produce weak, stunted plants.
System Overview
Driving a Low Power Heater
The heating element included in this project is a polyimide film heater, also known as PI heater. It is made from a thin, flexible polymer film coated with metallic conductor that generates heat when current passes through it.
To drive a load that consumes a few Watts of power, using MOSFET switch is a simple and cost-effective solution. The MOSFET gate voltage is maintained at around 4V by connecting a 3.3V Zener diode and a general purpose diode with forward voltage drop at around 0.65V. This structure gives a relatively stable reference voltage even if the control signal V(in) accidentally jumps to a higher voltage, this gives an extra layer of circuit protection since we are dealing with heating elements.
While this circuit is constantly operating, you may also notice that the MOSFET is getting warm. In later design we need to use a signal to control the ON/OFF of the heater.
Temperature Control
To design a circuit that possesses the capability to sense the surrounding environment and deactivate the heater when the temperature rises beyond a certain threshold, a circuit should monitor and regulate a process or event, which the fundamental requirements to be classified as a control system.
The block diagram represents a conventional feedback structure that is widely employed in contemporary control systems. This closed-loop configuration involves continuous monitoring and adjustment of the output temperature until the desired outcome is attained.
To implement a temperature control system, it is necessary to incorporate a thermal sensor that enables the electronic system to determine the current temperature needs to be measured. One simple solution for temperature sensing is to use a thermistor.
Thermistors are a special type of resistor where its resistance changes non-linearly with temperature. Thermistors are categorized as PTC and NTC, which stands for Positive Temperature Coefficient and Negative Temperature Coefficient respectively. PTC thermistor increases its resistance as temperature rises whereas NTC thermistor is doing the opposite.
Again, the solution to convert thermistor resistance into voltage signal is to use a voltage divider. The value of the series resistor should be equal to the thermistor resistance at the midpoint of the temperature range of interest, so we choose a 10kΞ© here.
The next step is to design a controller that performance corresponding reactions based on the input signals. To start off from the simple concept, a controller is simple a device that takes inputs, process them and generates outputs to adjust or maintain the systemβs behaviour. To switch ON/OFF the heater based on the temperature of the object, we can a simple comparator to serve the purpose. A basic comparator circuit using only an Op-amp.
In Chapter 4 we built a voltage buffer using the negative feedback structure of an Op-amp. A comparator circuit does not have negative feedback therefore the virtual short effect is no longer appliable. Regardless of the feedback structure, one fundamental property of Op-amps that always hold is:
where,
A(OP) is the open-loop gain, which is usually very large (+β)
The formula indicates that any imbalance between the two input voltages will lead to output voltage saturate to either side of the rail.
To generate control signal, a potentiometer is used to enable the user to manually set the threshold for switching the MOSFET that operates the heater. If the potentiometer voltage is higher than the reference voltage, a high level output signal will be generated by the Op-amp and therefore enables MOSFET to activate the thermal film.
Complete Circuit
When you build this circuit on a breadboard, ensure that the thermistor is in direct contact with the thermal film so that it can detect the surface temperature and update the reference voltage that is sent to the comparator circuit. You should also keep in mind that this wattage of this thermal film is far from enough to heat up the space inside greenhouse, therefore the circuit is mainly for learning practices.
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