I'm trying to cool my computer's CPU by sticking a peltier between the CPU and the heatsink, however I don't want to deal with preparing my motherboard and for condensation. Therefore I want to keep the cold side temperature to 50/70*F. How do I regulate the cold side temperature? Is it by limiting voltage or current?
Peltier aren't that good at carrying a lot of heat transfer.
Yes peltier can get pretty cold, but you cannot load much energy onto it, hence you won't be able to cool down your CPU much once it is running.
It is even reported that simple air cooling with the traditional radiator is more effective than using a peltier, reason being is that the power you use to drive the peltier is also generating heat on the system by simple laws of physics..
Would a Peltier plate be an efficient CPU/GPU cooler ?
Not really. All it would do is move heat from the cold side of the Peltier to the hot side. You still have to remove the heat to the environment somehow.
Further, Peltier cells are not 100% efficient, they take power to work, which ADDS to the head on the hot side. Moving 100 watts of heat might take 50 watts of extra input to the Peltier, giving you 150 watts of heat to remove from the hot side.
It is substantially more efficient to have a big heat sink and fan, or a heat pipe arrangement to move the heat out to where you have space for a big heat sink and fan.
What you want to do is a bad idea, at least the way you want to do it is.
If you want to cool your CPU to a lower temperatur than a high-quality air to finned heat tube style cooler as are the ones Noctua makes, you should use a liquid cooling system, if you want more cooling than a normal liquid system will give you then I would suggest cooling the liquid using Peltier modules.
Just put automobile antifreeze in the liquid and seal the CPU off around the edges with high temp silicone caulk. You can get the fluid down to below freezing if you want, but as you mentioned, condensation is not at all desirable. You can use more modules to run like a dehumidifier for your case air and they will cool it at the same time allowing you to use cooler liquid. If you just put it between the CPU and the Cooler and it overheats and possibly starts to melt some of itself, the chances of you burning up your computer are pretty good. I have cooled computer using Peltier Modules quite a few times and it is a lot more work than you might think. In order to make using a Peltier Module worthwhile, you need to present temps. to the CPU that is well below a normal liquid cooling system. If you are really determined to do it that way, I hope you talk to at least a few people who have done it and see what they say.
A 30mm square module will draw up to about 30-35 Watts at around 15 V. and 3-4 Amps and will have a Delta Temp. of about 65-almost 70 degrees C.The hot side could get as hot as 220+ deg. C if it goes bad your computer doesn't stand a chance and at full power, with adequate cooling on the hot side it will go well below freezing. If you have any moisture it will condense, once it thaws...poof. Be careful with moisture in the case. If you live in a damp place I wouldn't bother cooling the CPU with anything below45 or 50 deg. F. In most cases, the little bit of extra CPU speed you could get isn't worth it. Others may have very different opinions. I have actually used them and cooled my CPU with a liquid system pumping fluid that was near 0 deg. F.
Hmmm why not just put some stuff here about peltiers?
In cooling mode the peltier behaves more or less linearly. Current is proportional to voltage, so you regulate current by regulating voltage. Peltiers respond best to constant current/voltage rather than PWM, so you'll have to embed a temperature sensor (or use the one built into the cpu) and regulate the voltage provided to the peltier accordingly.
One thing to note about peltiers, the hot side cooler must dissipate all heat the peltier pumps, plus its own electrical losses, so you can achieve a lower cold side temperature, but you must have a larger overall cooling apparatus. Regular thermal conduction and convection also apply to the peltier, transferring heat back from hot to cold side proportional to the temperature difference
Peltiers as a result perform better at low load, and abysmally if they are not powerful enough. This means if you're controlling it and reducing the applied voltage/current as well as the temperature gradient in your efforts to avoid condensation, it's an all around win. The thing you really want to avoid is an inadequately sized peltier or cooler, as that will create waste heat from overdriving the peltier, or a large temperature differential and therefore waste heat(bleedback) from an inadequate cooler. It's important to size the peltier based on the full output power of the CPU plus the thermal conduction losses of the peltier and to size the cooler based on this plus the full waste heat created by the peltier.
If everything is sized correctly, you should be able to simply achieve constant temperature under heavy load. Run the peltier based on cold side or cpu temp and run the fan/water pump based on hot side temperature.
You'll have to convert this to American somehow, but I have a somewhat older 140W CPU in my system running with a factory made watercooling loop, bought on sale about the price of a good air cooler, and it hovers about 35-39\$^\circ\$C. It really is a special use case to use a peltier if you're not looking for sub ambient temps. Not necessarily a bad idea though, hope it works for you.