Wine heater

[Allycat]

Im trying to build a circuit to switch a 40w light bulb on and off to keep fermenting wine at a fairly constant temprature. Ive tried a circuit but cannot get it to work. The problem ive got is I can follow a circuit and solder but cant do the maths to select correct componants. Can anyone help me design the circuit. Any help greatly appreciated. Here is the circuit so far. The relay is a RUDH-SS-112D
https://www.dropbox.com/lightbox/home/Share

 

[Arouse1973]

Your drop box needs a email and pass word. You can post pictures less than 100k on here. Go to the Go advanced tab and select manage attachments.
Adam

 

[Harald Kapp]

Have a look here.

 

[Allycat]

Your drop box needs a email and pass word. You can post pictures less than 100k on here. Go to the Go advanced tab and select manage attachments.
Adam

Sorry, here it is.

 

[Merlin3189]

In some ways I prefer this circuit to your 741 circuit, but I still wonder about the negative temperature coefficient thermistor.
If I am right about it, then you just need to swap the thermistor with the adjustable resistor, so that it switches off as it warms up.

The other point in both circuits, is the positive feedback that someone else mentioned with the 741. If you want to add that to this circuit (I think you do) you may want to use a Schmidt trigger circuit. I can't do that of the top of my head, but if you want to persevere with this line, I'll look up the details later (or maybe someone else will chip in with it.)

 

[Allycat]

In some ways I prefer this circuit to your 741 circuit, but I still wonder about the negative temperature coefficient thermistor.
If I am right about it, then you just need to swap the thermistor with the adjustable resistor, so that it switches off as it warms up.

The other point in both circuits, is the positive feedback that someone else mentioned with the 741. If you want to add that to this circuit (I think you do) you may want to use a Schmidt trigger circuit. I can't do that of the top of my head, but if you want to persevere with this line, I'll look up the details later (or maybe someone else will chip in with it.)

Thanks Merlin, I would be interested in any alternatives as i am still a novice and learning from everyone's contributions.

 

[Harald Kapp]

The NTC is correct here. Note the position of the relay. It is normally closed. It will be activated -> open if the temperature gets too high.
Of course, that is an unfortunate setup. While it will work as long as the battery voltage is o.k, the lamp will be on without battery or too low a battery voltage.

either use a PTC or swap the resistors. And also swap the relay contacts so you use the normally open contact. It will be closed when the temperature is too low, open, if the temperature is too high.
It will also be open if the battery volateg is too low or no battery is present at all. This deems to be the safe mode.

 

[Allycat]

The NTC is correct here. Note the position of the relay. It is normally closed. It will be activated -> open if the temperature gets too high.
Of course, that is an unfortunate setup. While it will work as long as the battery voltage is o.k, the lamp will be on without battery or too low a battery voltage.

either use a PTC or swap the resistors. And also swap the relay contacts so you use the normally open contact. It will be closed when the temperature is too low, open, if the temperature is too high.
It will also be open if the battery volateg is too low or no battery is present at all. This deems to be the safe mode.

Good advice,Thanks

 

[Merlin3189]

Thanks Harald. I had not spotted that.

 

[Jagtech]

Not meaning to put you off from your original project idea, but I use a heating pad that easily wraps around the wine fermenter / carboy and is thermostatically contolled. The lowest heat setting is just right, and is very consistent.
Good luck with your wine, and your project!! Save me a bottle

Heating pad: http://www.amazon.com/Sunbeam-732-5...ef=sr_1_1?s=hpc&ie=UTF8&qid=1393209116&sr=1-1