Fanless Power Supply: Marko's Homebrew

Do-It-Yourself Systems | Power
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April 13, 2003 by Marko Djokic (quix) with Mike Chin

Marko's how-to article is not for the faint of heart or for the soldering-challenged. He describes the steps he took to replace the heatsinks in a conventional ATX power supply with a massive heatsink mounted on the back of the PC case, allowing fanless operation. It assumes a certain level of electronics and PC hardware savvy. It is, as with many SPCR reader submissions, a testament to DIY ingenuity, sheer will power and courage -- to try, to risk failure.

Marko's willingness to write an article in a language not his mother tongue is another measure of his courage! (Some might say his willingness to expose a socked foot on the web is one too. :D) I spent extra time carefully going over edits and proofs with Marko to ensure that the correct information is conveyed, and to smooth out issues with language. I also added more photos to better illustrate some of Marko's text. If there is any confusion, by all means, email the author (or me) with questions. Modding a power supply is intrinsically much more dangerous than something like wiring up a fan for 5V operation or rigging up a hard drive suspension.

Marko is not an electronics technician, and this article does not delve deeply into the technical background of the tasks performed. Obviously, if you try something similar, it is at your own risk. My main caveat: It has not been that long since Marko finished this project and got his fanless PSU up and running, so while I have no specific reason to question its longevity, it has not withstood the test of time. Enjoy, and be careful! - Mike Chin, Editor.

Introduction

Hello! I'll briefly introduce myself here. My name's Marko Djokic, but you'll often find me as quix on the net. I live in Switzerland, I'm 20 years old and studying computer science. I'm mostly coding on the PC for fun, 3D engines and such.

The PC I'm modding was my main computer until 6 months ago when I got a laptop (Dell Inspiron 8200, P4-Mobile 1.7 GHz, 768 MB DDRRam, 60 gig HD, ATI Radeon 9000 Mobile with 64 MB, UXGA 15" Display). Now, it's just a file-server / leech-machine, and silencing it is mostly a challenge. I'm trying to keep my mother from noticing that it's on 24/7, she would kill me if she knew that it has been using up electricity continuously for over a year now ;-). But I would really like to have it in my room and be able to sleep next to it without hearing it!

Components

  • Asus P2B Motherboard
  • Katmai-core P3 500 MHz
  • 384 MB (256 + 128) PC100 SDRAM
  • 2x Maxtor 120 GB 7200 rpm Plus9 hard drives (FDB Motors :-D)
  • 1x Maxtor 80 GB 7200 rpm 740DX Hard drive
  • 1x Western Digital 30 GB 5400 rpm Hard drive
  • Elsa Erazor III (NVidia Riva TNT 2) with 32 MB RAM
  • SB Live! Value
  • Realtek-NIC
  • 5-Volted Coolermaster TF8-25IM for HD's
  • 5-Volted Coolermaster TLF-R82 for the CPU
  • 1 CDRW drive which currently isn't connected (until I get a controller)

The PC is installed in an old (now heavily modded) Viper Case, with the original PSU. I started the heavy modding after I got the laptop, because, if I blew this system up, I had a backup now! Here is the label from the PSU:

(Editor's Note: As you can see, it is a standard 250W rated for only 8A on the 12V line. It is perfectly suitable for Marko's system, assuming honest ratings, but not for XP or P4 systems.)

I changed the PSU fan, and 5-volted it. This airflow was also enough (when idle or downloading at least) to passively cool the CPU. The other Coolermaster fan is cooling the two 120G HDDs. The hard drives turn off after 10 minutes, so usually, only the system hard drive (one of the 120G HDDs) is running, and it's very quiet! Also it didn't need too much cooling as it's running only one hard drive and the CPU is idle most of the time. The 5-volted fan of the PSU was the loudest, then the fan for the HDDs, then the hard drives themselves, and finally the coil buzzing from the PSU.

My Fanless Idea

Since the power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are the only parts that are attached to heatsinks, I figured they are the only components that need to be cooled. My idea was to extend the leads of the MOSFETs so that they could be attached to a very large heatsink mounted outside the case. The heatsink would have to be big and efficient allow the MOSFETs to be cooled with just passive convection airflow. Then the loud fan in my PSU would be completely unnecessary.

I had this idea for quite some time, but didn’t have the courage to try it. I don’t know much about electronics and whether it would really work. The problem is that the wires would introduce resistance, inductance, impedance, and who knows what else that might interfere with proper operation of the PSU.

Inspiration

Then, one day, as I went through the links on this great site (SPCR), I stumbled across Zero Fan Zone by BladeRunner. There, he wrote on how he extended the leads on the MOSFETs to mount them on a waterblock. Since it worked very well for him, I decided to do it too! I had a spare PSU lying around, so it would be no problem if I blew it up.


WARNING!

If you also want to do this mod, be very careful! Some experience with electronics work and soldering is helpful. It can be dangerous, the voltages can kill you. I’m only summing up how I did it, I’m not responsible if you blow up something or hurt yourself! Also, I read that a modded PSU could invalidate your home fire insurance, so if your PSU catches fire, and your home burns up, if the insurance company finds out that your modded PSU was the cause, they may not pay anything.


Getting Started

You should start this by first planning how and where you want to mount the printed circuit board (PCB) and the new heatsink(s), so you don’t use wires that are too long, like I did. I wasn't sure if the PSU would work at all, so I started with the soldering!

The PCB should be vertically mounted to have some convection airflow, because other components still have to be cooled, not only the MOSFETs! You should leave some place above/next to the PCB, so you can mount additional heatsinks if needed.

The main heatsink for the MOSFETs should be mounted with the fins running vertically for good convection airflow. I mounted the heatsink on the back because looked nicest and the least obtrusive. I've also chosen to mount it where the PSU usually would be, so I can use its mounting holes, and also have a place inside where I can mount the PCB.

To the PSU

Before doing anything inside the PSU, you should wait some time after unplugging it, so the capacitors can unload and you won't get a shock when working on it. I’m not sure how much is enough, some PSU have resistors that unload the caps, so an hour may be enough, but a few days should be enough on any PSU. (Editor's note: A thread in the SPCR PSU Forum called PSU modding dangerous? discusses this issue. If unsure, take the safe route and wait at least a day.)

The first task is to examine:

  • the heatsinks,
  • the way the heatsinks are attached to the PCB,
  • the way the MOSFETs are mounted on the HS, and
  • whether there is any electrical connection between the HS and the MOSFETS or between the HS and the PCB.

LIVE?!

You must check whether the heatsinks are live -- this means carry any voltage that could cause a short when touching something else in the PC or cause a shock if you touch it. Normally, there are at least 2 lugs or anchors that secure the heatsink to the PCB. The photo below (of another PSU) shows a HS anchor.

The anchors are usually soldered to a heavy trace (metal contact path bonded to the board) on the bottom side of the PCB. If other components are soldered to that same trace, the heatsink is probably live!

If the other anchor also is on a trace with components soldered to it, then the HS is also acting as a signal path between those two traces. This is the case with anchors #1 and #2 in the photo above. You should check to see if there's some other trace that directly connects the two anchor points. If not, you'll have to solder a wire between them when the MOSFETs and heatsink are moved off the PCB.

The anchor #5 on the PSU PCB above is an example of an anchor that is NOT connected to any components. The trace is separate, sort of like an island on the PCB. It is acting purely as a mechanical support. Note that this HS has 3 anchors, which means you have to examine any trace contacts among the 3 anchor points and record that info in case you need to wire in one or more jumper wires.

Mica-shims should be used to isolate the MOSFETs when they are mounted to the new heatsink. Otherwise, the heatsinks and the case would be live -- a very dangerous thing for components as well as for anyone touching the case. You should check if the MOSFETs have a direct electrical connection to the heatsink. This happens when there isn't a shim, and the MOSFET has a metal back. Usually all the MOSFETs in a PSU have a mica-shim.

If you don't understand what I've explained so far, you shouldn't do the mod. Make sure you understand fully all of the above or get someone more knowledgeable to help you before starting to do any modification in the PSU.

In my PSU, the smaller heatsink was soldered to a live trace on one anchor, but not on the other anchor, so the heatsink was not acting as a signal path. The two MOSFETs on this HS had a completely plastic case and didn't make a connection through their case. The MOSFETs could thus be removed and mounted on a new HS without worrying about the role of the HS in the circuit. Mica shims were not even needed because of the insulated plastic casing of the MOSFETs.



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