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Temperature control circuit for a complete beginner...

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I've added positive feedback
You don't need the diodes in the feedback path, but you may need a ~1k resistor between the non-inverting input of each amp and its respective 470k feedback resistor (otherwise the smoothing caps will slow the feedback action).
I've dropped in the 1μF capacitor around R7 but I don't have a clue whether that's appropriate?
Should be fine.
Would you mind clarifying for me what the interference is that I'm trying to smooth out here and where it comes from?
Inter-component wiring acts as an antenna and can pick up all sorts of radio-frequency garbage, e.g. from fluorescent lights nearby, or from fridges etc switching on and off. There may also be residual ripple on the power supply.
I'm also wondering if I'll need to do the same around R14?
R14 is not needed (it will reduce the sensitivity of the circuit to temperature change). I assumed it was just part of your simulation (as, presumably, are the 470Ω resistors near the Peltier).
unless these chips are likely to give me problems?
They'll likely do. Depends on their output swing capability and the pull-in/drop-out currents of the relays.
 
unless these chips are likely to give me problems?
They'll likely do. Depends on their output swing capability and the pull-in/drop-out currents of the relays.

I think I'm okay with everything else but I'm struggling with the opamp/relay details.

The opamps I have are LM741CN, and the datasheet I found gave the output swing as follows...
View attachment 66648

The relay specs from the Maplin website are...
View attachment 66649

Unfortunately, with the given info I'm still not sure if the 741 will work, or what to go for if not?
 
The spec info is on the thin side, so I ran a sim with a 741 model. Looks as though the opamp output swing with a single-ended 12V supply (as distinct from a +-15V supply which the specs relate to) is ~ 1V to 10.5V, leading to a coil current of 20mA to 75mA when using the components in your schematic. The relay will probably drop out at 20mA, but to be on the safe side you could reduce R10,R11 to 3k3. That will pull the current down to ~ 4mA and ensure drop-out.
 
Wow! That's incredibly good of you to go to so much trouble Alec, thank you!

I'm going to be busy for the next week with visitors but, as soon as I can get back to it, I think it's time to start putting all this theory into practice and do some breadboard testing :)
 
After a week of being distracted from my little project I finally got round to creating a test circuit. As I expected it didn't work too well to start with but, after some playing about with various resister combinations, it's actually beginning to look pretty close...

View attachment 66841

View attachment 66840

At my current room temperature neither relay is activated but raising or lowering the temperature of the thermistor by a few degrees will trigger one or the other, just as I wanted.

The only issues I have are that the relay on the cooling side gets a bit jittery as it switches and it's a very narrow range between switching a relay on and off again. Ideally I would like the cooler circuit to lower the cabinet temperature by a few degrees before being turned off and similarly for the heater circuit it should keep the warm air flowing until the temperature has been sufficiently raised.

I'll keep playing around with it but any deas on how to solve these problems, or any other circuit improvements, would be very much appreciated, cheers.
 
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You have very little positive feedback (hence the jitter) because R3 and R9 are connected to ground. They should instead be in series with the non-inverting inputs to the opamps as per post #21; i.e. R3 should be between R2 and IC1 '+', R9 should be between the top end of R13 and IC2 '+'.
The ratio of R4 to R3 (likewise R10 to R9) determines the amount of feedback.
 
maybe I missed something but why are you using the two op ams in this circuit?
Assuming the Pelitier is for cooling?
have a look at this schematic
http://www.solorb.com/elect/constemp/
it has a stabilized input voltage. your circuit could go awray if the input voltage is off.
 
maybe I missed something but why are you using the two op ams in this circuit?
Assuming the Pelitier is for cooling?
have a look at this schematic
http://www.solorb.com/elect/constemp/
it has a stabilized input voltage. your circuit could go awray if the input voltage is off.

The peltier is for both cooling and heating.

The circuit is designed to switch the polarity of the current through the peltier depending on whether the temperature is above or below the thresholds controlled by P1 for the low temp and P2 for the high temp. From the schematic you linked I don't see how that could be used in this case, but please correct me if I'm wrong.

What I have so far does the job but I'm certainly open to any ideas for a simpler and/or more robust solution.
 
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One relay KISS circuit

KISS = keep it simple stupid.
No your not stupid its just an abbreviated term used.
Here is the schematic simulation using TINA. I changed the feedback resistance to reduce relay chatter. You need to compute the base resistance according to your relay coil (86 ohms aprox for 12 v coil?)
Included attachment for using only one relay.
I found TINA is really great for simulating most circuits, especially if there is a pot involved. LTSPICE is kinda the cream of the crop but using a pot ??
 
Hi MrDeb, much as I appreciate your efforts, I think you're missing the point of me wanting a temperature range between which the heating/cooling is off, because I only need active temperature control either side of an 'acceptable' range.

With your single relay circuit the peltier will always be in either heating or cooling mode, which doesn't match my requirements.
 
Although a single relay can switch the Peltier polarity, there is then something (another relay :)) needed to switch the Peltier off altogether, since with a well-insulated enclosure it shouldn't be active continuously once the enclosure is at the desired temperature.

Edit: Agent57 beat me to the post!
 
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Well, it might not be the most awe-inspiring piece of electrical engineering ever, but after a few hours of soldering, de-soldering (de-soldering braid is may new favourite accessory) and replacing a few burnt out components following an accidental short-circuit... The finished article actually works!

View attachment 66933

Many thanks to everyone for their input, especially Alec!

Now I'm just waiting for the arrival of my peltier to get started on phase 2 :)
 
Nice to see a working project. Good luck with phase 2.
 
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