Due to popular demand, this is a follow up post outlining the electrical layout of the temperature control circuit for the fermentinator.

OK, so top eft we have the power supply. Peltier devices more commonly run off 15v and not 12v. Due to this it is difficult to find a suitably large enough power supply, however I was able to find a laptop supply on ebay.

You can use 12v however the system is not was efficient, meaning longer heating or cooling times.

The negative supply goes to a panel mount amp meter. I use this to judge how hard the system is working.
From the amp meter and the positive supply both go into a terminal block for distribution.
The positive goes to the common switching pole on the temp controller. How the controller works is quite simple.
There are 3 poles. One common, one that is closed with common for cooling and one that is closed with common for heating.

I have used a single peltier device and relied on the fact that one side gets hot while the other cold, and by reversing the polarity you get the opposite. Due to this I have had to use two additional relays. I have used double throw double pole relays so that I can feed the relay a negative and positive then when it latches these two get passed through to the output poles of the relay. This allows for the swapping of polatiry.

Note that the top relay has the polarity swapped. The left hand side is the power input, the right hand side the power output, the bottom the switching signal.

Because I used relays there is no feedback of the reverse voltage because the opposite relay is not latched.
Due to the design of the temperature controller, it is impossible for both relays to be latched at the same time, which is  a very good thing

Now, the peltier works best when the heat and cold is removed from the heat sinks so I needed fans on the inside and outside. The issue is that you dont want them running unless the peltier is running as they are loud and the peltier is using a variable polarity depending on heating or cooling , so I used a bridge rectifier in the fan circuit to maintain the same polarity no matter what the input polarity is.

And that is that.

There are a couple of other bells and whistles that I have added but not included in the diagram.
1. power LED for when the system is on. (doesnt have to be running, just powered up)
2. blue LED that is on when the system is running in cooling mode
3. red LED that is on when the system is running in heating mode
4. an extra fan to ensure that the power supply is cool (as it is a fanless supply)

The red and blue LED’s are handy as you can see at an instant that the system is doing something and what exactly it is doing.

I am happy with both the mechanical and electrical systems now and wont be making any modifications to them now, I do however need to finish off the door and build an enclosure for the electronics. Then it needs a lick of paint and is done.

I will post some finished pics when that is done.