The circuit has been designed to run off 12v DC, it uses a 12v relay to control the 240v AC pump.
BE CAREFUL with mains wiring - if you are unsure get it checked by someone who knows what they are doing BEFORE you plug it in !!!
The temperature is measured by a Thermistor. This thermistor can be attached to almost anything..... to control the solar hot water pump, I embedded it inside the the solar panel, and thermally attached it to the outlet pipe.
Thermistors are often called NTC resistors, they cost only a few cents and are a very cheap way to measure temperature.
Thermistors unfortunately don't have a linear change in resistance with respect to temperature. Thus they are often overlooked in design these days, with people using lots of high tech, and expensive components instead. However by adding a single resistor in series, (R1 in this circuit) it is easy to achieve an almost linear response over a temperature range of, say, 30 - 40 degrees.
This trick does not work over the entire range of the thermistor. But for this application, where we know we want to switch the pump on and off in the range of 40 to 60 degrees, it's perfect.
I have a dail on the front of the control box, and simply set it to the temperature I want the pump to turn on at.
When the temperature of the water in the solar heat collector is above this preset temperature, the pump will circulate water. If a cloud comes, or when the sun sets, pump turns off.
The temperature controller uses cheap readily available components. Available either from an electronics store,
or perhaps you can get creative and salvage most of the components from old electronics equipment :)
Sensor = 47k NTC thermistor
R1 = 15k
R3 = 47k
RV1 = 100K
D1 = IN4001 or similar diode
IC1 = LM393 Comparitor
FET can be any small switching FET with high enough Ids (current handling) to hold the relay.
Any comparitor could work for this circuit, I choose the LM 393 for a few reasons
- it was what I had in the garage :)
- it operates from a single voltage supply from 2-36 V DC
- the input offset voltage of 2-5 mV is enough to give a little bit of hysterisis, this keeps the circuit stable and thus it doesn't oscillate on and off.
These components are good for a temp range of 20 to 80 degrees and an operating voltage of 12v.
However it should run off anything from 5v up to 30v.
Simple Temperature Controller circuit description:
Assuming that the circuit is controlling a circulation pump for a solar hot water heat collector, and the morning sun is warming the panel from cold.
As the temperature rises the resistance of the thermistor decreases, and thus the voltage at point A rises.
When the voltage at point A exceeds the voltage at point B by 5mV the output of the comparator gets pulled HIGH by R2. The FET turns ON, and energises the relay, turning the pump ON.
If a cloud blocks the sun, the pump continues to circulate the water and the temperature decreases. The voltage at A decreases until it is 5mV below point B, then the comparator output goes LOW, turning OFF the FET, and thus the relay and pump also turn OFF.
NB This circuit uses a "pullup" resistor, R2 on the output of the compatator. (Almost all comparators have an "open collector" output, and therefore require an external pull up resistor.)
NB The IN4001 diode is a "flyback diode"
needed to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.