Fanless Power Supply: Marko's Homebrew

Do-It-Yourself Systems | Power
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Testing

Before going any further, I wanted to test my wiring to make sure the PSU was still working. I placed the PSU on the wood floor and connected some hard drives I had lying around plus the original fan (just in case it needed to be cooled).

Before plugging the PSU into the wall, I hid myself behind something, so if it exploded or something, I wouldnÂ’t get hurt! I did the same thing when I powered it on. To do this, connect wires from a switch to the green wire contact and any black wire contact on the 20-pin plug that goes to your motherboard. (Editor's Note: If the wires are long enough, you can run to your underground bunker before flipping the switch.)

The first time I turned the power on, it worked!!!! That was a great moment! Then I checked the 5 and 12 volts rails, and they were OK.

Mounting to Case

After being assured that the electronics were still working, I went on to figure out the new heatsinks. (Should have done this at the beginning.) I calculated that the heatsink should not cover the cutout of the motherboard tray, and at least cover the hole for the PSU. I went to a electronics shop nearby with my measurements. I wanted a large heatsink that had all fins going into one direction, but they only had such heatsinks with really short fins, 2 cm, so their cooling performance wasn't that good.

I got two heatsinks that had perfect width and the right combine height when stacked on top of each other. They were rated for 1 C/W each. That means the temperature of the heatsink will rise 1C per 1 Watt of heat applied. Obviously a lower number is better. Because the heat of the PSU would be split between the 2 heatsinks, it would be like having one HS with a 0.5 C/W.

(Editor's Note: If 50C is the target maximum safe temperature of the HS, for an ambient temperature of 25C, the heatsinks would be able to dissipate 50W before hitting 50C. Assuming an AC/DC conversion efficiency of 65%, the PSU would be able to deliver ~93W in DC voltage to the system at that temperature. That is probably adequate for Marko's Katmai-core P3-500 MHz system, although if all the drives were working at the same time with a heavy CPU load, it would be close.

Mind you, it is not that the PSU would not deliver higher power. It would simply try to deliver whatever current was being demanded, and the temperature of the PSU would continue to rise. But because the HS is exposed, there is more of a fire hazard in case flammable material with a low ignition temperature comes in contact. So it is important not to load the PSU to the point where the HS gets much hotter than 50C.

It is a known fact that PSU efficiency tends to drop as thermal limits are reached, often dramatically. This means that as the PSU gets hotter, it becomes less efficient and produces more heat, which makes it work harder, and so on, in a heat-inducing spiral that can become something called thermal runaway. Like Chernobyl. Not a nice thing to have happen in your PSU or heatsink.)

At home, I held the heatsinks on the back panel and marked on them the PSU-mounting holes. I hammered slightly with a nail on these markings on the heatsinks so the drill won't slip, drilled the holes and tapped them. The heatsinks now could be screwed from the inside. Then, I drilled 4 holes on each heatsink, on a place where I could easily screw the MOSFETs to once mounted on the case. I drilled and tapped 4 holes on each heatsink for a total of 8. I only needed to mount 6 MOSFETs, but wanted to be able to move the MOSFETs around in case one of the heatsinks got much hotter than the other. This didn't happen in the end, but it was good insurance planning.

Find a place in the case where you can mount the PSU and have good convection airflow, because the other components still have to be cooled, not only the MOSFETs! In my case, this was no problem, I just put it where the PSU was before, and there was also metal behind it, so I could directly mount it there.

To mount the PCB directly to the case, I marked on the sheet metal through the holes on the PCB, and drilled the holes, a bit larger than the screw hole of the spacer I would place between that metal and the PCB. Screwed everything together, finished! (Editor's note: To be on the safe side, you may want to place a piece of that stiff insulating plastic sheeting between the PCB and the case.)



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