Hi MrDEB,
Thanks for your comments, they're very much appreciated.
The schematic you linked was actually my starting point. I've expanded on that to allow independent control of the temperature settings for when cooling and heating are switched on as I only need heating if the temp drops bellow about 25°C and cooling if it rises above 30°C. Between that range I'll just have the fans circulating the outside air through my cabinet at whatever the ambient temperature is and the peltier can have a bit of time off.
I've made some adjustments to my
circuit design, adding a couple of resistors (R8 & R9) to limit the upper and lower temp settings on either side to prevent any overlap and the risk of powering on both relays simultaneously, which would obviously be bad news! I've also popped in a couple of clamping diodes across the relays to prevent EMF as the relays switch off, which was something I read about somewhere along the way. If there are any other alterations I should do for circuit protection, simplification or whatever else then I'd appreciate any more feedback I can get.
This is the layout as it is right now...
View attachment 66559
As far as specific details about the components go, I have a box of bits with most of what I think I need, but I don't really have much confidence in 'what I think'. If any of these parts are blatantly not suitable for what I'm doing then please let me know.
* The sensor is a 4K7 bead thermistor with a B(°K) of 3977, which gave me the values for R8 & R9
* The OpAmps are LM741 chips
* I have a couple of DPDT relays (N04AW 1A/12Vdc)
* The transistors I already have available are BC108 and BC548. Are either of these good for what I need or should I go and get the ULN2803 as suggested, and what's the difference?
* The peltier I was looking at is
**broken link removed** or maybe the 100W version
**broken link removed**
The other thing I'm not certain about yet is power supply. I have plenty of old 12Vdc PSUs laying about and was hoping one of them could be recycled. Is there anything I need to be careful of when selecting a suitable unit?
Sorry if these seem like silly questions but thanks again for all the help!