Temperature Control Circuit

[brianmew]

Hi all,
Please could someone explain the circuit diagram below, ie what each component actually is and their function.
My Electronics is not very good and I am trying to design a circuit that keeps my conservatory between 20-25 degrees C. I will be using an electric heater with this circuit.
http://electronicsproject.org/temperature-control/
Thanks in advance.

 

[ghostman11]

Velleman do something similar in kit for very cheaply, it might be worth looking at that.

 

[brianmew]

Velleman do something similar in kit for very cheaply, it might be worth looking at that.

Thank you ghostman11, I have had a look and found the circuit I think you were talking about but it still does not explain what the components are actually doing. I would like to understand what they actually do.

 

[ghostman11]

i will ask LG to explain it when he gets i, he is better at electronics than i am

 

[MikeMl]

What a horrible circuit. You can do what this circuit does (thermostat, switches relay with temperature change) with about 1/4 to 1/10 of the parts. I wouldn't bother with any circuit that uses an obsolete 741 opamp as comparitor.

 

[Mike odom]

X1 reduces the 240V input down to 18V. D5 and D6 form a half wave bridge. The schematic says +/-24V, but you won't get that from a Half Wave bridge. You'll only get about half of that, so I would say, +/-12V. If you don't draw a lot of current, it may go higher, but I doubt it. D3 and D4, along with R7&8, form a +/- 15V? regulated supply for the opamp circuit. R5 and D1 form a 5.1V reference supply for the inputs. Note that the schematic says -5V. This is incorrect, it's +5V.

R1 and R3 supply a reference voltage, your temperature set point. R2 and R4 (R4 being the temperature sensor) are the input you are measuring. Thermistors are NTC (negative temperature coefficient), meaning as they warm up, they have less resistance. Colder = higher resistance.
Control input = (-) input. Measurement input = (+) input... therefore, when your temperature voltage goes above your control voltage (COLDER than the set point), the opamp output sources current, and turns on the transistor T1 (through the un-needed R6), which turns on the Relay RL1 (R1 in the parts list). When the temperature voltage goes below the control voltage, then the opamp ouput sinks current and turns the T1 transistor off, turning the relay off.

I say R6 is un-needed, as opamps are current output devices, not voltage ouput devices, therefore, the voltage floats as necessary. Transistors are current input devices, only needing base resistors when driven by a voltage output device.

I'm not as critical as some about the use of an (obsolete) opamp, as you can replace it with just about any general purpose opamp off the shelf. They use an opamp for the voltage comparator because it saves the biasing resistor to turn the transistor on (most comparators have an open collector output and pull to ground or open up). What I don't like about this circuit is that they have no hysteresis built in. Imagine your control voltage being right at your input voltage... the smallest amount of noise will cause your output to continually switch on and off, on and off.... garbage design. Also, why do they need +/- supplies? If they're worried about the temperature input voltage going too close to ground (common mode on most opamps is +7v and -5V on the inputs) then they could add a resistor to ground under the thermistor to raise it up.

 

[MrAl]

Hi all,
Please could someone explain the circuit diagram below, ie what each component actually is and their function.
My Electronics is not very good and I am trying to design a circuit that keeps my conservatory between 20-25 degrees C. I will be using an electric heater with this circuit.
http://electronicsproject.org/temperature-control/
Thanks in advance.

Hi,

Do you know what an op amp is and perhaps how it works?
That's probably the most complicated part in the circuit.

 

[alec_t]

Wouldn't it be a lot simpler, safer and cheaper just to use a standard room thermostat?

 

[ghostman11]

i found it after all! here is the schematic

 

[Mike odom]

Note that Ghostman's schematic has hysteresis built in on the opamp stage used for the comparator, IC1D (comparator because there's no feedback into the negative input, hys = feedback into the positive input).

 

[NorthGuy]

If you have an old wall-wart power supply, you can use it instead of building your own power supply. The comaprator circuit can work with variety of voltages (and you don't need a negative rail), so nearly any wall-wart will work for you. You only need to match the relay coil voltage to the voltage of your power supply and also choose an op amp/comparator which will work with this voltage.

This takes off half of the circuit shown in the OP.

 

[brianmew]

Thank you all very much for the in depth and quick responses. I would like to put all this into practice and simulate my own circuit on my laptop. Is there any software I can download that would let me test this?

Thanks again guys for your time.

 

[ghostman11]

legally your best option is LT spice (just google it) it's free and used extensively on this and many other sites, you will need a hand building a sim to start with as it takes some getting used to. There are 'OTHER' options but we dont go there and to be honest most the other sims i have seen are not as accurate.
if you are wanting coloured lines and leds lighting up then you are going to have to shell out a huge amount of cash! so best stay legal and take the time to learn a good tool like LT spice

whatever you decide one tip. DO NOT POST ASKING FOR CRACKS OR SERIALS, this will kill off any chance you have of getting help! it is very much frowned upon here! not saying you would but keep it in mind

EDIT 2

i forgot to add that if you use a thermistor then check out Eric Gibbs tool on this site for thermistors, it could save you alot of calculating!

 

[ghostman11]

didnt get chance to read it yet as its my last day off! but take a look at this one as well it might help and seems like you can connect it to pc (not sure because i didnt read that far) http://www.velleman.co.uk/manuals/k8067.pdf

 

[brianmew]

Thanks again Ghostman and everyone else. I am going to try this with LTspice now I have a bit of time on my hands.