So I had this idea to make refrigerator/heater to make kefir in a consistent way. I'll use a peltier element sandwiched between two stock AMD heatsink/fans and I built a board to provide power and switch hot and cold sides to the peltier element with two DPDT relays (one should be enough, the second one is just to prevent a short from happening).
What I didn't think of was a better way to limit current to the peltier than using a big ass resistor in series with it. I have a 5a 12v PSU and I expect the control circuit to use 200ma at most and would like the peltier to use only 3a rather than the full 5A it's rated for. What would be a more efficient way of doing that without using a huge 4 ohm resistor ?
>>900997
Peltier elements are so weak, why would you want to make one weaker?
Also, even at 3A, your peltier will eventually cost you more in electricity than buying a proper mini-fridge/AC/or whatever you are building.
>>901006
The cheapest incubator I can find is of a shitty DIY design and will cost more than one year of electricity, the proper ones are expensive lab equipment.
A peltier element will maintain a temperature difference between the two sides, since I don't want more than 30º of difference it's better than a "buying a mini-fridge". If I wanted to buy one rather than making one I'd post in /g/, not /diy/.
>>900997
find the voltage where it draws 3A, adjust the feedback voltage divider in the power supply so that's what it will output.
if it draws 5A at 12.6v or whatever 3A will be somewhere at 11? somewhere close to that.
>>901013
That sounds like a good idea, I'll try it later today
what you would like to do is either buy a constant current buck converter or build one your self..
the only thing you have to be sure about is a margin for the input to output voltage. usually a margin of 2 volts is enough..
what you could also do, is to create a feedbackloop that monitors the actual temperature with the desired, and then regulate the current based on the temperature difference. this can be applied both to the homemade buck or the bought one..
Well, guess what? I didn't even need a resistor to make the psu not blow up. The lower heatsink gets ice cold and the PSU warms up a bit, but since I had designed the PCB with two extra fan headers I have got that covered as well. Now I only have to tape and screw everything together in order to have a functional incubator.
Cold side gets pretty cool, cool enough for condensation to build on it, but it's having a hard time actually cooling the air inside the box.
Could it be because of poor thermal interface between the peltier element and the heatsinks? What if I drilled holes in them to screw them together?
Man, fridge design is really fucking hard.
>>901146
What are you using as thermal compound between the peltier module and the heatsinks?
>>901149
Nothing, should I use thermal grease or toothpast or something like that? I thought pressure was good enough, since one is getting hot and the other one ice cold.
>>901151
I know from a reliable source that automotive oil can be a good replacement if you find yourself short of anything else, however if you do not have it at hand, I'd suggest using toothpaste to test out whether it would make any difference. At the same time I'd like to remind you that toothpaste will no doubt dry out over time and because of that it's only good as a short-term solution.
>>901153
I have some leftover cooler masteer thermal grease, that should do the trick
But I guess I'll have to come up with a better way to attach one heatsink to another
>>901154
Please come back with the results. Of course making sure the heatsink is firmly attached to the module is also important. While it may not seem professional nor long-term using zip ties is alway an option. Good luck!
>>901157
Results are in: thermal grease made no difference. I'll try a 300ww ATX psu later today.
>>901198
That's not good to hear. Leave it running for a night and see what happens. Keep in mind that these things have _terrible_ efficiency (IIRC ~5%)
>>901200
I did that
Nothing happened
The air inside the box didn't get any cooler, even though the cold heatsink is pretty cold
I think my problem is actually making that heatsink absorb the heat in the air inside the box.
This should be working, for fuck sake.
>>901362
no it shouldn't.
these elements have a horrible efficiency and you probably have about the same amount of heat flowing into your enclosure than that your are pumping out.
terminology is incorrect but lets not complicate this story by using the proper physics terms
>>901375
addition to that post:
I see that you used a 300 watt powersupply, say that you use all 300 watts and the efficiency of your peltier element is 5%
that means that you have 15 watts being used to cool the air in there, that is nothing
you should have just used a mini fridge like >>901006
said
also cheaper on electricity
>>901377
>you should have just used a mini fridge
See this post: >>901011
>If I wanted to buy one rather than making one I'd post in /g/, not /diy/.
I'm not using all of the 300watts.
I built a circuit to cut power to the peltier element once the temperature gets to 10ºC.
God damn, it's like you people don't bother reading the fucking thread and keep posting stuff like herp derp buy a mini fridge. If I wanted a fucking mini fridge I'd buy one, period.
That doesn't help me identify and correct problems with my design (for example, no one pointed out that I'm using relays rated for 1A to drive a 5A load).
>>901395
Except for welding the contacts together if you are unlucky that wont matter much.
Over here >>901377 I showed you a quick and dirty calculation to show that you do not have enough cooling power even if you used all 300 watts.
The efficiency of those peltier elements is horrible, as has been stated earlier, and most like likely there is more heat leaching into your box than that you can pump out with the very limited cooling power that your peltier has.
This is also the reason why a normal fridge doesn't use peltier elements.
In other words, get over yourself and buy a second hand mini fridge and hack the thermostat; easier and more efficient
>>901407
People have done it before and it works.
There are commercial peltier mini-fridges that work.
See, that means there is a sequence of actions that can be taken in order to make a peltier element cool a certain volume of air. It may involve using another heatsink or engineering the air flow inside the box. It does not involve "buying a second hand mini fridge and hacking the thermostat". I've fixed my actual fridge, and it wasn't fun at all. I want nothing to do with freon-compression refrigerators.
>>901428
so did you get it to work?
your box might be too big, shitty insulation, your room temperature might be too high, i don't know much about this kefir but the reactions going on in there might create more heat that that your peltier element can remove, etc
all these tiny things contribute a lot if your cooling capacity is only a few watts. back in the days when I worked at a retails tore in the weekends we sold actively cooled refrigeration boxes. they did use a peltier but consumed 275 watts or something when hooked up to mains and 120 watt when hooked up in your car.
even on mains they took quite a bit of time to get stuff at room temperature to 5 degrees C, in your car it could only keep it cool but was incapable of cooling it further than when you plugged it in to the sigarette lighter.
in other words; you need more POWAH!!!
keep in mind that this is going to cost you a lot in electricity costs if you want to run this rig often
>>900997
If you want cold go phase change, phase change cooling can move beyond 2 watts of heat per watt of compressor draw (cos cold isn't energy it does not defy the laws of thermodynamics) Peltiers move .02 0.10 watts of heat per watt you give it, no competition. A single compressor setup running propane as refrigerant could get you in the -30 to -40 range if you wanted. Sure theres heaps of brazing, expensive parts and a short but steep learning curve, but at the end you will know enough to become a hvac tech if you wanted, diversify those skills yo
>>901428
You might try modifying the power supply, then running several te coolers in series. Hell, don't even mod the ps, just run a couple of your coolers in parallel. It sounded like you had an abundance of amps to start with, and throwing in a second cooler in parallel would be a good way of efficiently using those amps. Plus, more coolers means more surface area, which is one of the things the cooling effect is dependent on.
>>902544
He can just get a mini fridge and modify that
