How does a Peltier work?

[gclg2000]

Can someone break down how peltier's work, how cold they generally get, the different power ratings, etc....how good of a water cool do you need for them?

 

[RS3RS]

-- A Guide To Peltiers
-- 1.13.2001
-- By: Winterstick
Warning: Using a peltier can be dangerous if done incorrectly, and I will not be held responsible if you damage anything in your computer.

Before we show you how to add one of these bad boys into your system, we are going to educate you about peltiers. How they work, how to choose the right size, and what you need to have to have one running on your system, along with some other goodies…

The Theory Behind Peltiers:

French physicist Jean-Charles-Athanase Peltier first discovered the peltier effect in 1834, when he connect two pieces of copper wire to a length of bismuth wire to a battery. Peltier noticed were the current passed from the copper to the bismuth the temperature increased, and were the current passed from the bismuth to the copper the temperature decreased. This effect is called the Peltier effect and is the principle that all peltiers work off of… This effect is amplified when two different semi-conductors are used.

How a peltier works:

It is simply a “heat pump” that pumps heat from one side to the other when an electrical current is applied to the peltier. A peltier itself is made up of paired up p- and n- type semi-conductors wired in series that are sandwiched between two ceramic plates. As an electrical current is applied to the peltier one side gets very cold while the other side gets extremely hot. It is not uncommon that there will be a 65ºC difference between the hot side and the cold side of any peltier without a load.


So what does this mean for us overclockers? Well it means you can finally go after that gigahertz machine that you always wanted to have. It also means a lower operating temp, which also increases your CPU’s life span so you can have your gigahertz machine longer.

How to Power a Peltier:

Not only will your chip now consume more power when you overclock it but now you have a peltier leaching off your power supply as well. Chances are that your peltier will consume 60+ watts (depending on how big your peltier is, take the wattage rating of the peltier is how much power you peltier consumes.) out of you power supply so if you have a 300 watt power supply, your out of luck as soon as you kick on your peltier you will probably experiencing brown outs. “So no problem” you say “I’ll just slap in an old AT power supply that I just have lying around”. Sorry that won’t work either. Peltiers require a power supply that can supply at least 14 amps at 12 volts to work efficiently; most AT power supplies won’t supply the required 14 amps at 12 volts. So what do you have to do now? Get another power supply, there a lot for sale on the internet that will provide the need 14+ amps at 12 volts, hell I’ve seen an AT power supply that can supply a whopping 24 amps at 12 volts!!!

Note: I am currently powering a 72 watt peltier with my 400 watt power supply that produces 15 amps @ 12volts with no problems what so ever.

-- So how big of a peltier do I need?
Well it all depends on how much overclocking your going to do. First off we need to figure out how many watts of heat your processor produces. To find out what your processor produces use the chart below:

Intel Users

PIII 500-1 GHz users click here (480k) page 47.
Celeron users click here (1.49 megs) page 56 and 57.
AMD Users

Athlon users click here (1.41) page 21.
Duron users click here (1.43 megs) page 19.

Ok now that you have the maximum heat output of your chip, use the formula (taken from overclockers.com) to figure out what your overclocked CPU will produce:

Pnew = Pspec * (Fnew/Fspec) * (Vnew/Vspec)^2

Pnew = the new heat output in wattage

Pspec = the heat output specified by the manufacturer

Fnew = the new CPU Frequency

Fspec = the default CPU Frequency

Vnew = the new voltage setting

Vspec = the default voltage setting

So with that said lets say, I want to overclock a Celeron 600 to 900 at 1.75v:

32.58 = 15.8 x (900/600) x (1.75/1.5)^2

That means when I overclock a Celeron from 600 to 900 we increase the amount of heat produced from 15.8 watts to 32.58 watts, which is 106.2% increase in the amount of heat produced. Note that this is only an estimate the true heat output could be higher or lower.

Now to get any sort of benefit of using a peltier over an air-cooled system (cooling below the ambient temperature) you have to double or more the amount of watts that the CPU outputs. In other words take the calculated Pnew and multiply it by two or more. So I would need a 66+ watt peltier to cool the chip below the ambient temperature.
Note: You won’t always get the maximum wattage out of you peltier so it is better to allow for more, plus you may always want to overclock even more that what you though you wanted to...

So what does this mean for the CPU temperature? Well here is a handy formula from Toby at BxBoards to see how many ºC a peltier will remove.

Delta T = (1 - (heat load/max cooling power))*max temp difference
Heat load = the heat output of the CPU in watts
Max cooling power = the max heat rating your peltier can remove in watts
Max temp difference = the temperature difference of the hot side of the peltier and the cold side in ºC

So lets say that our 66-watt peltier has a max temp difference of 70 ºC. So Delta T = (1-(32/66))* 70. Which give us a change of 36.0ºC, that means this peltier will remove 36ºC from my chip!!! Ahh I can see it in your eyes you just can’t wait to slap that bad boy on your chip can you? Well hold on, there is another area we have to cover. With all this cooling your peltier generates a lot of heat, I mean a lot of heat! Lets to a look at what you have to do to cool this bad boy…

-- Cooling a Peltier
Ok so how much heat are we talking about having to dissipate here? Well our chip produces 32.58 watts of heat, and our peltier produces 66 watts of heat, for a grand total of 98.58 watts of heat. That is a lot of heat!!!



So what happens if your heat sink can’t cool your peltier setup well enough? Well the heat sink will continue to get hotter and hotter, and pretty soon all of that heat will start traveling through the peltier and into your chip, heating up the chip and either causing the computer to lock or in extreme case frying the chip entirely. Since we don’t want this to happen we have to choose a really good heat sink.

So what exactly makes a heat sink “good” well there are a couple of things, a good strong fan moving lots of air, a large surface area, and the color of the heat sink (yes the color of the heat sink). So how do we choose a good heat sink well there is one good way and that is to look at the C/W rating of the heat sink.

The C/W stands for Celsius per watt, or how many degrees Celsius the heat sink will raise per watt of heat applied to the heat sink. The C/W rating is the efficiency rating of a heat sink. The C/W ratings of heat sinks can usually be found on any heat sink manufactures web page. For my example I will be using an Alpha pep66.


Now the Alpha has a C/W ratting of .35 which means for ever watt absorbed it will increase the heat sink temperature .35ºC that’s with a 60 mm fan moving 19 CFM, which is very good for a heat sink, compared that to the C/W rating for the popular Golden Orb which is .98 not bad but not good enough to have a peltier put on it…

So how hot is the heat sink going to get? Well here is another formula for you:

Theatsink = Tambient + ((C/W)(Wtotal))

Tambient = the ambient temperature in ºC

C/W = C/W rating of the heat sink

Wtotal = the total heat output of the peltier and the CPU in watts

Theatsink = 25 + ((.35)(98.52))
Theatsink = 25 + 34.5
Theatsink = 59.5ºC

According to this formula the heat sink will get up to 59.5ºC, which is fairly hot… well at least hotter than what I would like. So what is my solution well this heat sinks efficiency rose from .50 C/W with a 9 CFM fan blowing on it to .35 with a 19 CFM fan blowing on it so what do I propose? Stick a 38 CFM fan on it, which should theoretically raise the efficiency to .23 C/W, which in turn should lower the heat sink temperature to 47ºC, still hot but a lot cooler than 59.5ºC.

Note: I don’t now for certain if sticking a higher volume fan will increase the efficiency of the heat sink by as much as I say, but the more air you have moving across the surface of the heat sink the more heat can be removed…

The other problem that you will run into using an air-cooled setup is that the heat sink is putting a lot more heat into your case (usually double the amount of heat that would be put into your case without a peltier on it) which in turn will increase the ambient temperature which in turn will decrease the ability of the peltier to cool which in turn will raise your cpu temperature.

So what do you do about that? Well one make sure you have good air flow in the case, to get rid of all of the hot air and bring in fresh cool air. Since you are constantly supplying fresh cool air into your case, the ambient temperature should raise much in your case, and your CPU will stay cooler.

Or two, you could go all out and put in a water cooled system that way you can go way past the limits of air coolers. If you’re hoping to run below 0ºC at a load then you have to go this way. There just aren’t enough cooling capabilities in an air-cooled system to get your system running under a load below 0ºC.

Ok so now that we know how, or at least know what we need to cool the peltier unit what is next? Well now we need to seal the processor to stop condensation.

-- Getting Rid of Condensation
Whenever you cool something below the ambient temperature, you always have to worry about condensation, and since your using a peltier you have to worry even more since your CPU can go below 0ºC. Since we don’t wont to invite water into our computers to fry our components there are a few simple steps that we can take to avoid, or at least fight off condensation.

One, seal your peltier unit and copper cold plate (I’ll explain what this is later in the article) in CLOSED cell foam or a neoprene. The closed cell foam or neoprene seals your processor and peltier from outside air that way you don’t have condensation next or close to the cpu.



Now there are other places that we have to stop condensation from forming and that is on the backside of the CPU, and the backside of the motherboard or slocket.

To try and stop outside air from reaching the back of the CPU, we want to take a silicon-based rubber sealer (the more silicon the better). And place a bead of silicon around the outside of the socket and around the inside. This will stop the outside air from getting next to the backside of the CPU and will help prevent condensation.



Also take a piece of closed cell foam or a neoprene and put in the middle of the socket, this insulates the backside of the cpu from out side air that may get in and condensate on the backside of the cpu.



Ok so with the CPU pretty much insulated now we have to focus on the back of the slocket or motherboard. This is a very important step. You need to either take a neoprene patch (I would only use a piece of neoprene patch, I wouldn't use closed cell foam.) bigger than the area of CPU and glue it to the back of the motherboard or slocket. This would insulate the back of the motherboard or slocket from condensation, because it gets quite cool as well… Or you can get a can of "conformal coating silicone spray" and spray the back of motherboard or slocket heavily. This will have the same effect of insulating the backside of the slocket or motherboard and preventing condensation.



Also to help keep your CPU warm boot into the bios and turn off ACPI, or at least set the throttle/duty cycle to the max it will let you, on the BE6-2 the max is 75%. This will keep you computer from going into power saving mode and prevent your CPU from going below 0.

Ok early I mentioned a copper cold plate, so what is it? Well you never want to have direct contact between you CPU and your peltier for a couple of reasons. One Peltiers fluctuate in the amount of heat they can remove, so you need a buffer zone to keep cooling your CPU in your peltiers weaker moments, which is exactly what the copper cold plate does. The second reason why you use a copper cold plate is you need to spread the heat load from the relatively small area of the CPU die, to the whole peltier for max cooling efficiency.



Pic ripped from 2CoolTek.com

Now that we have covered all of the hardware things you can do to prevent condensation, lets look at the software things to stop condensation.

You don't want to tempt fate and let your CPU run at idle with your peltier running this will cause your CPU temp to drop down below 0ºC, which could allow condensation to form. So what is the solution to this? Well there are many programs on the net that will allow you to run your CPU at 100% CPU load one of these is the distributed.net client for RC-5. This little baby tries to break RC-5 64 bit encryption by brute force. It will eat up all of your spare CPU clock cycles keeping your chip warmer. Which lessens the chance of condensation forming… While you're at it why not sign up for the gideontech.com RC-5 team?

Another program worth taking a look at is Mother Board Monitor (MBM). This little gem is one of the best monitoring programs around and it also has a handy function built into it its called CPU Heat up, what does it do? Well if your processor falls below a certain temperature that you can set, it creates a thread in the cpu which use 100% of the operations which causes you cpu temp to warm up and after it reaches a temperature that you can set, its stops the thread and everything goes back to normal, that way if the program that you are using to keep your cpu temp up for some unknown reason crashes, well you won't end up risking frying your chip… MBM also has a shutdown function that will turn off your computer automatically if your CPU gets to hot, so lets say that your peltier fan dies, and as you CPU temp slowly rises, MBM can turn off your computer before it gets to hot to fry your cpu.

Note: MBM 5.x doesn't contain the CPU heat up function to my knowledge, only MBM 4.18 and lower contains it and you have to read the readme on how to enable it…

Ok now that you learned something about peltiers (hopefully) lets put it to good use and do some overclocking and extreme cooling.

Taken from a website explaining pelters, if you want more info, just do a Google search.