Hi,

I am looking for a solution that can control the heating of my domestic hot water by measuring the temperature in the middle of the water. When the temperature reaches eg. 50 degrees (celcius) the circuit switches on a relay opening a thermoelectric actuator (24V) allowing hot water from another heating source to warm up the water (coil) to eg. 60 degrees which makes the circuit to switch the relay off again.

As you can see on the attachment I need a circuit with 2 trim pots to control the "min" and "max" temperature, 1 sensor (which I already have) and a relay (24V) to control the actuator letting the external hot water flow into the coil and warm up the water. I just have no idea how to construct the circuit solution...

I have been searching for a solution that can handle this and found a couple but they are controlled by a PC interface and quite expensive. I really just need a simple solution that allows me to keep the temperature of my domestic hot water between eg. 50 and 60 degrees. I have built small circuits before and would be able to build something myself if needed.

I found a circuit that partly can do what I need, but it has no way of using an interval temperature:

http://www.uoguelph.ca/~antoon/circ/heatsens.html

As you can see a second trimmer pot (+logic) needs to be added to the circuit in order to provide "max" and the power to be used should be 24V instead of 12V.

Any help would be greatly appreciated!

Thanks and regards,
Lars

 

What is the heat sensor that you already have? Is it a 10K NTC thermister as shown in the schematic you found, or is it something else? Designing a working circuit requires knowing the constraints, and your heat sensor is one of the constraints.

Also, I assume your "24 volt relay" has contacts rated for 24 volts. Do you know what voltage is required to energise the coil?

One last thing... (Most folk here won't need this, but I need to be straight in my own head.) You want the relay to come on when the water cools below 50C and to go off when the water heats above 60C?

 

Hi, sorry for not making myself clear!

The idea is that hot water (appr. 65-70 degrees) used for heating the floors in my house is also used to heat the domestic water, by opening/closing a valve controlled the circuit/relay. But only when the domestic water temperature drops below eg. 50 degrees and until heated until eg. 60 degrees.

You can see the 24V actuator that controls the valve here:

http://varmtvandfrasolen.dk/assets/pdf/m100-pd-en0h1512ge23r0203.pdf

So the relay is needed for switching the actuator (24V) on when water temperature drops below 50 degrees and off again when it reaches 60 degrees.

So I somehow need to provide 24V to the actuator eventhough the circuit is only powered by 12V. Can this be done easily?

The temperature sensor to be used could be any external sensor that can measure temperatures in the mentioned interval, eg. KTY/PT1000.

I only use electricity to heat the domestic water in the summer-period when there is no need of heating the floors in the house.

Thanks for your help!

- Lars

 

Let's back up a little. As you've already said, the circuit from you opening post will not work. Forget all about that circuit and the 12Vdc supply it requires.

We need to look more closely at that sensor which you say you already have. I cannot find Enlish refernce to a KTY/PT1000. Would you be able to translate some of the datasheet if I told you what to look for? We need to know what kind of sensor, whether thermistor or thermostat or thermopile or something else. We need to know it's characterstics at the two key temperatures. Example: if it is a thermistor, we need to know resistance at 50C and 60C - if it is a thermopile, we need to know voltage at 50C and 60C.

Once we know your sensor's characeristics, we may use a couple dollars worth of components to provide 24Vdc to your actuator. Which components exactly, and where they go, depends on the sensor characteristics.

Edit: In both of the following schematics, I've got the op amps feeding the wrong pins of the flip-flop. Gate A should route to Clear, and gate B should route to Preset.

Example One: Input from PTC thermistor


Example Two: Input from type K thermocouple


In these examples, the LM358 runs from a single supply - this could be derived from a "wall wart."




One other consideration: Is there an existing means of preventing overfill, or will our circuit need input from a float switch as well?

 

Ok, I see what you mean

I checked the type of sensors I have once more and it turns out the type is called KTY (KTY 16-6 specifically)

You can see the KTY values here (page 108):

http://www.ta.co.at/files/UVR61/Manual_UVR61_V2_5_engl.pdf

So it seems to me that the resistance at 50 degrees is 2417 Ohm and 2597 Ohm at 60 degrees. But it should be possible to adjust the interval from a minimum of 30 to a maximum of 70 (or even 0 to 100) degrees by using the trimmer pots? This way the interval can be adjusted depending on summer/vinter, vacation etc.

It is not possible to overfill the system as it is a closed system so no need for any switch controlling this. Basicly there is a flow of water in the system when the relay turns on the actuator or no flow when the relay switches off.

Thanks and regards,
Lars

 

Looks like the KTY sensors are PTC Thermistors. We can begin with the first scematic in my post above. (We can forget about the second one.)

Warning: I'm still running a fever, so the following may have errors. (That is, it may have even more errors than I am usually prone to.)

If we choose a value of 1.6Kohm for R1, then voltage input to the LM358s will be roughly 1/3 of V+ for 0C and a smidge above 2/3 of V+ for 100C.

I suggest using panel pots instead of trim pots to make seasonal adjustment more convenient. Add limiting resistors in series on each side of each pot. A 20K resistor on either side of a 10K pot will allow adjustment from 25C to just over 80C.

Note commentary added by edit in my previous post. Connect pin 1 of IC1 to pin 3 of IC2. Connect pin 7 of IC1 to pin 2 of IC2.

IC1B is for turning the actuator on when the measured temperature is too cold. If sensor resistance is too low, then voltage at pin 5 of the LM358 will be lower than pin 6 - this will force pin 7 to zero volts. The 7476 JKFlipFlop will reset, and Q will be low - turning Q1 off.

IC1A is for turning the actuator off when the measured tempereture is too hot. If sensor resistance is too high, then voltage at pin 2 of theLM358 will be higher than pin 3 - this will force pin 1 to zero volts. The 7476 JKFlipFlop will set, and Q will be hi - turning Q1 on.


I don't know if you've got the M100-AG, or the M100-AGX, so let's be on the safe side and pick a part for Q1 that can handle 600milliamps. Since the thing might get warmer than 25C when operating (it gets strapped to a heating system, yes?) let's pick one that can handle 600mA (or more) at 100C.

One choice is the IRFD014, carried by both Digikey and Newark-In-One.




It has taken me more than an hour and a half to compose this post. Fever and drug side effects are conspiring to shut me down. I'll do the updated schematic and PCB layout another time, if you please.

 

Thanks a lot, your help is really appreciated!

I am looking forward to see the updated schematic and PCB layout so I can start building the solution

Also hope you got rid of that nasty fever. If not then get well soon!

Regards,
Lars

 

I apologise.

I'm getting "Upload of file failed" when trying to post either the schematic or the board layouts. I've either exceeded file size or bit proportions or alloted storage space or am making some other booboo.

It will be Monday before I can sort out my error(s) and get these posted.

I apologise.