Now if the thermistor is showing low temperature, then I want to increase my temperature supply voltage thus providing more temperature! But if the resistance of the thermistor is close to the correct value, I want to decrease the temperature supply or stop it!
Rather than say "increase my temperature supply voltage thus providing more temperature" we say "increase the heater power to increase temperature".
The way we do that is to use proportional control. We switch the heater on and off periodically.
Let's say that your heater takes 20 s to rise 1°C at full power. If we switch the relay on every 5 s we should be able to control within about 0.25°C.
0 5 10 15 20 25 s
| | | | | |
__ ____ ______ ________ ___________
on time _| |________| |_______| |_____| |__| |____
power 20% 40% 60% 80% 100%
Now, to get 20% power we switch on for 1 s out of 5. For 40% power we switch on for 2 s out of 5, etc. This is time proportional power or very slow pulse-width modulation.
The big advantage of this control is that only one power supply, one relay and one heater is required so it is simple and low cost.
Note that you need to make the time period short enough that the temperature doesn't rise or fall too much during one cycle but long enough that you don't wear out your relay by switching too often. Solid state relays are often chosen for temperature control as they have no moving parts.