Some more thoughts on iMac cooling

The easiest way to keep your iMac's processor running cool is probably to install a larger heatsink & fan combination, with the fan reversed to pull air up off the heatsink; that's what I did, and it works well In point of fact, my processor is running cooler now than it did from the factory! This involves removing, and relocating, the iMac's case fan in order to accomodate the extra height of a different heatsink & fan.

The specific product I used was an AOC heatsink & fan, item # EC-K6-5B. I had to replace the included 50mm fan shortly after I installed it into my iMac. I also had to cut part of the heatsink away to get it to fit.

I tried a number of different options after clock-chipping my iMac; I thought I would share what I tried and why I think those solutions didn't work.

• My initial thought was to install two tiny (25mm x 25mm x 10mm) fans directly on top of the stock heatsink. This wouldn't work; even a 10mm thick fan is too tall to fit between the heatsink & the case fan. I got it all together, but realized that if I tightened the screws for the chassis, my two small fans would be interfering with the case fan. They probably wouldn't have been able to move enough air anyway, since they would have been competing with the case fan directly above them.
• Unattaching the case fan and turning it over so that it was blowing air right onto the heatsink seemed promising, but a little forethought would have told me that this would fail because the fan was pulling hot air from the monitor area and warming up the heatsink with it.
• Relocating the case fan to the top of the case and installing a 486-size fan on the stock heatsink. This suffered from the same conceptual flaw as the idea above.
• Installing a modified Pentium heatsink with a 50mm x 50mm fan blowing down onto it; again, it was getting its air from the monitor area (maybe I'm a slow learner...).

After trying all of these, and considering the final solution I used, I think it's safe to say that those who choose to go to 300 MHz will most likely have to discard the stock heatsink in favor of something more aggressive. 266 MHz seems to be borderline for the A & B models; some people need extra cooling & some don't.

I do think that a speed even higher than 300 MHz may be possible, say 333 MHz or perhaps even a bleeding-edge 366 MHz (though that is 116 MHz / 46% faster than the chip is rated!), but I doubt that even a large heatsink & fan will be able to compensate for the extra heat generated. I think in this instance that it might be best to borrow a page from the world of PeeCee overclocking and employ a device known as a Peltier junction -- also commonly called a chip cooler or thermoelectric (TE) cooler.

Peltier devices come in varying sizes, but for personal computer applications they generally range from 25 mm square to 50 mm square, and are several millimeters thick. A Peltier device uses a trick of electricity to actually make one side cold while transferring the heat to the other (plus a lot extra, I'm told, as it's not an efficient process). So you put the cold side onto the chip, and then stack a heatsink and fan onto the hot side of the Peltier.

I am no expert on these devices (I've never even seen one), but they sound as though they would fit the iMac cooling bill fairly well. Using one of these would necessitate removing the case fan and using a larger heatsink & fan combo; I have no doubt, from what I have read, that you couldn't simply stack the stock heatsink on top of one of these.

I have searched the Web over (using Sherlock; I love OS 8.5) for information on these devices, and I have tried to contact several PC cooling kit manufacturers for information on their devices, but have only heard back from one (and they said they no longer carry the one I was interested in).

From my own observations & measurements, here's what I think one would need to make such a device work for the iMac:

• Peltier device between 26 mm and 29 mm square; smaller won't cover the G3 chip and larger may leave too much extra surface area exposed; condensation could form.
• Control board for Peltier which only turns on above a certain temperature; the most common I've seen is about 115 F. Will also turn the Peltier off below a certain temperature, as well as sounding alarms for fan failure or overheating.
• Pentium or K6 heatsink modified to fit into iMac's available space.
• 50mm x 50mm Pentium fan, installed to pull air up off the heatsink
• A separate, external power supply. Peltier devices draw a lot of power, up to about 2 Amps, which is more than you probably want to burden the iMac power supply with. You could use a power supply from an LC-style Mac and run the wires in through the iMac expansion port hole. These power supplies usually run about $20.

It is possible to use Peltier devices without a control board, but from what I've read, not recommended. In this case, a control board would be ideal, as it would automatically turn off the Peltier shortly after you turned off your iMac; no need to worry about turning off the extra power supply and then remembering to turn it back on again.

I wish I had more information to offer on these things, like a part number and a phone number, but this is an area where you'll have to forge ahead on your own if you really want to try it. If you have any luck finding parts suitable for the iMac, drop me a line at danb@thelittlemacshop.com.

Here's a link to a good page with information on heatsinks in general, The Heatsink Guide. There's also a page on the same site with lots of links to heatsink & Peltier manufacturers; click here.

You might also find some useful information at the CPU Overclocking Guide, even if it is somewhat PeeCee-centric.

A reader informed that Shreve Systems is selling Peltier junctions, complete with small heatsinks, at 3 for $25. These were designed for the PowerPC 601 chips, and may or may not be workable for iMac processor cooling ... but they would at least be a fun toy! I'm going to order a set just to play with. Here's the link. Thanks Jeff.