Going insane with thermostat design

[Hero999]

What are you using as a temperature sensor?

At what temperature do you need it to cool and what temperature do you need it to heat?

The effect of feedback voltages can be minimised by putting the thermistor output on the - inputs and add an inverter to one of the outputs or by ensuring that the feedback resistors are of a high value relative to the source impedance.

 

[namezero111111]

Where's your reference voltage?? It looks like you are taking it from a resistor voltage divider off your main power rails. Is that 12v regulated? If not, it needs to be. And you need some caps on the + and - inputs of your comparators.

Well, there is a capacitor across the power rails after the voltage regulator, close where the chip takes its power from. I used a big one, I don't really understand capacitors too well. I mean I know they "store" electricity, but I don't know what size to use, and what different types do (like Ceramic vs. Electrolytic).

 

[namezero111111]

What are you using as a temperature sensor?

At what temperature do you need it to cool and what temperature do you need it to heat?

The effect of feedback voltages can be minimised by putting the thermistor output on the - inputs and add an inverter to one of the outputs or by ensuring that the feedback resistors are of a high value relative to the source impedance.

Well with the rheostat I set the temperature (which would be set somewhere between 70* and 80* and then it should keep that temperature within a degree, so if it's set to 75*, it should turn cooling on at 76* at the latest and heating at 74* at the latest.
This kind of off period I have in there by having a PCB mounted rheostat that is connected between one of the + inputs (the upper one in the terrible diagram I drew )

EDIT: Sry I forgot to tell you, I am using a thermistor (50K) as a temperature sensor.

 

[Hero999]

Here's how I would do it.

Obviously I'd change the resistors to suit the resistance of the thermistor.

 

[Andrew Leigh]

Hi

I had relay chatter problems on my thermostat circuit, it was all due to a poor supply. A nice big cap sorted it out, a 4700uF after the bridge rectifier.

My heating thermostat uses a comparitor and noticed that it is critical to match the resistor used in the comparitor to the thermistor resistance when at desired temperature. It is fairly critical that you know what the thermistors resistance is at the desired temperature, my thermistor needs to operate at 3degC so I selected a thermistor value of of 4k7 to match. At your temperature a 10k thermistor would be ideal.

See Craig's Thermostat Circuits

I based mine on this, if you want to see my schematic then I will post it, it may get you thinking on other stuff. It works well, no relay chatter and it has a hysteresis well better than I need. In fact too good so I have set it to about ± 1degC.

The bigger the tank the less the temperature swings become, where do you live where a tank needs cooling? What I did was to put in long 12V wire element under the sand, it was brilliant for plant growth, the mulch breaks down much quicker. It also provided much more uniform heating and improved natural flow in the tank.

Cheers
Andrew

Edit: Its actually not +- 1 degC, mine is for a fridge element. The fridge drifts up to say 4 degC, the heater then activates and drops the temperature to 3 degC whereafter it drifts back up to the 4 degC.

 

[namezero111111]

Thank you both of you!

That's where I based my circuit off off, only I put in 2 comparators, Andrew. And yes, in Florida fish tank cooling is needed, especially for the kind of turtles in there that I got.

Hero, I appreciate the diagram you drew, and am trying to understand it thoroughly. Why are there two rheostats in there? And also, where do you get +14 and -2V from? I mean how would I generate that off a 12V power supply?

 

[richard.c]

Hi Andy,

Had the same problem as you a few years ago, so the pdf might give you some ideas - it can be built as a simple two relay heat and cool unit, but as you can see things got carried away at bit.

 

[namezero111111]

Soo guys I ran one more test. It turns out that this severe "humming" of the relays goes away if I connect the +12V source of the heavy 30A automotive relays via BEOFRE the voltage regulator and filter capacitor (180uf).... Could that have been the real and only source of the trouble???

 

[namezero111111]

Thank you everyone! All your advice has helped me! The info on the comparators, Hero's new circuit diagram from which I took info, the links, and all the suggestions everybody gave.
As a little gratitude I attached two pictures of the project.
One showing the whole project with labels and the other one the circuit board in detail (it has a lot of connections and some resistors on the bottom too which can't be seen from above)

Let me know what you think!

Thank you so much again!

EDIT: Now I just have to figure out how to keep the peltier from condensing because in heat mode when the heatsink gets too cold it "shortens out" and creates weird electronic humming again and when I dry it off that stops, but I'm sure that's something I'll figure out. Researching right now : )

 

[Mr RB]

It depends on the temperature sensor, if it's a thermistor it doesn't matter since the potential divider ratios will remain constant regardless or the power supply voltage.

Yeah I saw that, but it's still really nasty to have no caps on either comparator input, AND badly filtered main supply with inductive loads. The whole thing becomes one big supply noise amplifier which is kinda the symptom he was getting.

 

[Hero999]

Hero, I appreciate the diagram you drew, and am trying to understand it thoroughly. Why are there two rheostats in there?

One adjusts the high temperature and the other adjusts the low temperature.

And also, where do you get +14 and -2V from? I mean how would I generate that off a 12V power supply?

Just ignore it.

I use some kid's simulation software (Crocodile Clips) for simple circuits like this and the only model it has is for the 741 which has a saturation loss of 2V on either supply. To get round this, I simply add 2V on to the + rail and take 2V off the - rail. I normally change the rails back to 0V and 12V before posting but this time I forgot.

 

[namezero111111]

Haha no problem, that's what I did. I just modified the circuit a little to use one rheostat and then a second one to set the "off window" (just a PCB mounted one) so once it's all installed and I figure out the best setting I can superglue it shut!

Thanks again for all your help, the whole thing is working perfectly right now. On to the waterproofing the peltier against condensation so it doesn't short out again : )

 

[blueroomelectronics]

Are their one or two peltiers in the design? Heat / Cool

 

[namezero111111]

Are their one or two peltiers in the design? Heat / Cool

Just one, plus two relays connected to the heat side of the thermostat that reverse the voltage to the peltier.

 

[zipdogso]

Just out of interest what pressure have you got the TEC/peltier clamped under ?

TEC's naturally vibrate in use... failure to stop this has a detrimental effect on the longetivity of the life of the TEC. TEC manufacturers recommend a pressure of between 150-300 psi. I usually get this using two copper plates at least 3mm thick. Quashing the vibration also has a marked effect on the heat transfer it is much better.
How are you effecting the transfer of heat between the peltier and the water ?

 

[namezero111111]

Well, it looks like I can't use a heat exchange since copper is detrimental to fish. I'm thinking about using an aluminum heatsink, or maybe coat it in some sort of epoxy with a thin layer, that should'nt affect the heat transfer much but might be safer.

I haven't come up with a way to clamp it down yet, but I was originally just thinking about cable ties around the two heatsinks or epoxying it, too.

 

[zipdogso]

Well, it looks like I can't use a heat exchange since copper is detrimental to fish. I'm thinking about using an aluminum heatsink, or maybe coat it in some sort of epoxy with a thin layer, that should'nt affect the heat transfer much but might be safer.

I haven't come up with a way to clamp it down yet, but I was originally just thinking about cable ties around the two heatsinks or epoxying it, too.

You can use aluminium it's just copper transfers heat better but alu is fine. Just clamp the TEC between your heatsink and another alu plate and just bolt down with stainless steel screws to the right pressure. You might need to use 5mm plate or even thicker as it might bend at the pressure of 150-300 psi. Make sure your bolt holes are half way along the sides of the TEC NOT at the corners. The corners are weak and may break.

I bucked out of building a thermostat and bought one that does exactly what your looking for for GBP £30. Hardly seemed worth the effort to build one.