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?'s: peltier + PWM. (pulse width modulation)

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Squija

New Member
Joined
Jan 16, 2005
This is my first post here, so I'll start by saying hello to all.

So I've been searching and searching and I can't seem to find sufficient documentation on peltiers. What I mean is, I haven't found anything that references the heat transfer-ability of the peltier, when no power is applied to the N and P junctions.

What I'm thinking about doing is having a peltier turn on when the cpu reaches a temperature, after which have a PWM device controlling the % of the time the peltier is powered, until the CPU reaches a low enough temperature, at which point I'd have the peltier completely turn off. (effectively completely avoiding the possibility of any condensation)

I do not plan to overclock the system too much, I merely want to keep the processor temp around room temp. I also don't want to make an electricity hog.

My main concern is, will the cpu still be cooled by the heat sink with a non powered peltier between the two? In other words, is a peltier a heat insulator or a heat conductor, when it's not actively transferring heat?
What is the "Pepi" thermal switch?
Also, is it possible to tap into the CPU temperature reading onboard the motherboard? (to stick an analog or digital temperature reading into my circuit I would design to control the on off functions of the PWM, as well as control the PWM's output to the peltier)
Another ?: Would a PWM to power the peltier be better than varying the voltage to the peltier? (either with a voltage regulator or an op-amp)
Any comments/suggestions?

I'm thinking of making this for a P4 3.6 775 processor, with the Arctic Cooling Freezer 7 from newegg N82E16835185116 .
As of now, the only part I have is the barebones CPU. I have the knowledge to design the control circuit.
Now the search continues for a motherboard...

Thanks,
Squija
 
In other words, is a peltier a heat insulator or a heat conductor, when it's not actively transferring heat?

It is a heat insulator.

Also, is it possible to tap into the CPU temperature reading onboard the motherboard? (to stick an analog or digital temperature reading into my circuit I would design to control the on off functions of the PWM, as well as control the PWM's output to the peltier)

Why not use your own temp sensor? You can buy a very tiny one with much better accuracy than is provided on our mobos. You seem to have the knowledge to wire it up. With your own sensor you can control placement of it, and also buy one that will work with your circuit, instead of adapting a sensor not made for external use.

What is a Pepi switch?

Essentially just a switch that turns on or off when it reaches a set temperature. You would want a temperature differential in your application of probably 3F-5F. That way you wouldnt be cycling the TEC on and off.
Heres one.
http://www.master-instruments.com.au/Data Sheets/Thermal Cutout Devices/Pepi Switches.pdf

Would a PWM to power the peltier be better than varying the voltage to the peltier? (either with a voltage regulator or an op-amp)

No. voltage would be better. Ill leave you to think about that one :santa:
 
Mysubaruimp is right. When the peltier is not acting as an active thermal pump it does nothing more then insulate.

Here's the problem. Most factory peltiers use a silicon material between the actuall bromium and aluminum (metals differ by who makes the peltier. These are the most popular to use because of the vast perdical difference.) Otherwords you have this silicon plating on either sde of the conductors. The problem is silicon doesnt actually transfer heat. Because of the make up of silicon it doesnt excite as easily as most metals do. So the electrons don't jump around fast enough to move the energy or the heat.

When it is turned on it acts more like a energy vacume. That's why one side get's hot and the other side gets cold. It's technically a heat pump. Energy is traditionally attracted from a less excited electron to a more excited electron, or from cold to hot. There for when the pump is active it activily pulls the heat from the processor and transfers it somewhere else entirly. It works on the same principals as a heat pipe just with a completly different mechanical theory.

Sorry it's hard for me to explain. Technically when a peltier is turned on or powered it doesnt absorb heat the same way a heatsink or water block would. It simply picks it up and moves it to something else that does absorb it. That's why when it's not powered it doesnt actualy cool and just insulates, it cant absorb anything.

Im guessing you've read a lot about peltiers and understand how they are built and how they work though. And Im also guessing your very good with electronics. Peltiers traditionally only have an on and off state and nothing in between. Technically they are nothing more then a resistor. Because of this the best way to power them would be to simply supply a constant voltage and not modulate it at all. What you want to do could easily be done by using an external tempature probe and hooking it up to an and/or gate and an electronic on off switch to the power supply for the peltier. Basically you need the hardware to say if it's below this tempature stay off and if it's above this tempature stay on. You just need to cycle the power and simply turn it on or off as needed and not modulate it at all.

Here's another problem though. If your using a standard heatsink you almost dont need this circuit though as most heatsinks cant exchange the heat fast enough to worry about condensation. The way traditional heatsinks are designed they simply cant handle the amount of heat a peltier is able to throw at it.

A traditional heatsink simply absorbs the heat energy and disperses it in a wide area. A fan is used to transfer that energy from the heatsink metal to the air through conduction. The problem is though that air is also one of the worls greatest insulators. Scientifically no one knows of a better insulator with a higher r-value then dead air.

Peltiers are traditionally used with a water cooling setup too. A water cooling setup works much better for a peltier because it is able to use a traditional heat exchanger (or a heatsink) with a transfering material that does not insulate. Traditionally a water rig transfers the heat from the block to the water and the water transfers the heat energy to the air but means of a much larger surface area and much more air then a traditional heatsink can use.

Sorry for all the mummbo jumbo. Im gonna wrap this up quick because my fiance is yelling at me to come to bed.

Id have to say because of these reasons and your reason for using a peltier just to reach room tempature and not below I would highly reccommend looking into a water cooling setup. Some people are scared of mixing water with their computer but has come a long way and is very safe to do as long as you have common sense which it sounds like you do. Also a water cooling rig is much quiter and much cheaper to run then a peltier. The only downside is the inital cost is more.

Theres also the thought of just buying a swiftech heatsinks also. $50 dollars but no worrying of using water and a very very long tradition of excellent quality and outstanding tempatures with a minimal cost os daily usage.
 
Thank you both for the info, it makes perfect sense now. I definetely do not want a peltier directly on my processor. If power goes off on the peltier any reason, it's just gonna insulate the CPU.

Those swiftech heatsinks look pretty decent, though spendy, and newegg dosn't have the 775 version. I'll look into watercooling prices, but I think I'll have just use the arctic freezer 7 alone. Looks like if I were to get the swiftech, I'd get like 4 degrees lower during load, which isn't worth the price difference, IMO.

I am considering eventually sticking a peltier on the top of the heatsink itself, for when load temps kick in, to cool the heatsink down, which will cool the processor, and eliminate the possibility of the peltier killing the processor if it's not on. Maybe fashion some metal pipes to connect from the peltier to the case, and use the case itself to dissipate the heat moved by the peltier from the heatsink. Just an off the wall idea that popped into my head. :)
I'll see what kind of overclockability I have with the processor and just a heatsink.

Thanks agian for the info, very useful.

Peace,
Squija
 
I have the swiftech and it works very well. I am using the thermaltake ducting mod as well, I highly reccommend the combo.

I also recommend a 92mm fan with adapter if you want a little quieter setup with amazing cooling capacities.
 
I wouldnt suggest putting a peltier on top of your heatsink either I've tried it for ****s and giggles and it doesnt work to well. There isnt enough surface area that will touch the heatsink to effectivly cool it well. It will basically act as if your using your heatsink without a fan.

Im not too familiar with the heatsink you have. My friend has an older swiftech on his Pentium 4 2.8 and it keeps it at room tempature consistantly too. He doesnt use a high powered fan either. Just a smaller 80mm panaflow turned down to about 800 rpm's. Granted he has an Antec Sonata with 120mm fans in the front and the back, but still there only running at 1500 rpms. The swiftech's are very very good performing heatsinks. They really blow away any other heatsink to market including those new fancy heatpipe tower heatsinks. They come pre lapped and with the pin design and a high powered fan they work almost as well as a good higher grade water cooling kit. Granted you will spend about $60 dollars for the heatsink itself and a good fan I would highly suggest getting one if your looking for good overclockability, dont want to use water cooling, and dont have the money for anything really high end.
 
The heatsink I was looking at is the Arctic Cooling Freezer 7 from neewegg.com, better pictures available in the product link. If I'm looking at the pictures correctly, it looks like there's ample space on the top fin to fit a little peltier, I'm not thinking of sticking a 100+ watter on, if I were to do it setup this way. Anything that'll cool below room temperature would be a complete waste of electricity and counter productive. Akin to using a dehumidifier in a house in with only screen windows. My thinking was just to use something to keep the temps from soaring above 60c. Plus, I'd do it for sh*ts and giggles too :) A little peltier is $10.

Though watercooling is looking more and more appealing... I'm eyeballing the "Thermaltake CL-W0005 BigWater 12CM Liquid Cooling System" for $141.99 +free shipping.

ATM, I'm waiting to see what nvidia has in store for nforce and the 775 processors, cuase I'm not ready to buy into the PCIe x16 just yet. So right now it's all talk and thinking, and no buying nor building.

-Squija
 
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