differential thermostat circuit

[chrissmiff]

I'm installing a hot water solar heating sytem and need a differential thermostat. The company supplying the panels want £70 ($130) for this thermostat. Can any one help me to make a thermostat. Either a circuit diagram or purchase a kit. I live in the UK.

Many thanks.

 

[-=GST=- Nemisis (cs/cz)]

whats a thermostat and/or what do u want it to do?

 

[chrissmiff]

The differential thermostat monitors the temperatures of the solar panel and the hot water cylinder. If the solar panel is hotter than the hot water cylinder the circulation pump is turned on. When the hot water cylinder is hotter than the solar panel the circulation pump is turned off. I hope this answers your question.

 

[stevez]

There are several parts to this as I see it. You need to measure or sense the temperature then make a comparison of the two temperatures.

There are numerous sensor types. Among them are thermistors, RTDs, thermocouples. A thermistor is probably lowest in cost however you'll have to figure out how you'll get it to "see" the temperature of the fluid you'll be measuring. Clamping it to a metal pipe (without crushing it) might suffice.

The thermistor changes resistance with temperature - increasing or decreasing depending on the coefficient. With the proper electronics, which could be as simple as some resistors, you can end up with a voltage that varies with the temperature. You would need to do this for each temperature that you are measuring. I do think that the current flowing thru a thermistor should be very low though I have no advice to offer on how much.

You could then compare the two voltages with a comparator. An LM339 is common and low cost. I understand that an op amp, like a 741, can be configured to perform that function. The output of the comparator will either be low or high depending on the difference between the values. You might want to add some hysteresis (see applications literature) to keep the circuit from toggling.


What I've done is described to parts - the temperature sensing portion and the comparator portion. I'd seach for info on each piece - or maybe others can provide info - then essentially put them together if someone hasn't already done that and made the information available on the web.

 

[chrissmiff]

Hi Stevez

Thanks for your great detailed explanation, but as you rightly say, I now need a circuit diagram on how to connect all these parts together. If you know of a website with this information I would be eternally grateful.

 

[csaba911]

Maybe you should look for cheaper unit or start to learn more about electronics

Here is very basic and simple comparator, low_on pot set the threshold witch turn the output on and keep it on until the temperature is bellow this level.
You have to figure out the best value for the thermistor. I don't know the temperatures your tank working on !
The best set up for this type of app is to make a window comparator, where you can set low and high limit. Like turn on @70C and of @50C.

I hope this helps.
STEVE

 

[stevez]

I'd be willing to bet that the two most critical items are circuits you'd find in basic electronics books such as Forrest Mimms (Radio Shack). One would be the thermistor connected to an op amp and the other is the comparator circuit. Sorry I can't point you directly to something that describes the whole thing. You'd have to do rest yourself or ask for help in doing that.

Realize that if your skills and experience are such that anything other than a complete kit would be a challenge and time consuming you might find that the $130 unit starts looking like a pretty good deal.

Another approach might be to search under PICs. If someone has described a way to use a thermistor as an input you might find that creating this thermostat is more of a programming challenge - and maybe you know someone nearby who can help - or you can use Nigel's tutorial's to get your thermostat and end up with some valuable skills.

Best of luck to you.

 

[samual]

I need a circuit for a differential thermostat that can maintain a temperature differential of 3-5 degrees C at normal atmospheric temps.

 

[Diver300]

The simplest solution would undoubtably be a thermistor on each point, two fixed resistor and an op amp.

However, I would suggest that you go digital, so that you can have a display of each temperature. It would also allow you to have any differential that you want.

Digital temperature sensors are very cheap and there are a wide range available. You will need a microcontroller to drive the sensors.