PC PSU fan


2005-09-17 1:24 pm

is it necessary to have the fan hooked up with a computer PSU?? i want to use the power supply in a box but i don't really want to cut a big hole in it for the fan. i am going to use the power supply to run a car radio so it will be drawing quite a bit of current.

also, do these computer power supplies have any protection circuits?? ie: short-circuit, overcurrent, thermal overload?


2007-09-21 6:28 am
Yes, the primary transistors will get too hot without a fan and will probably experience secondary breakdown fairly quickly. If the secondary rectifiers are significantly over-rated, then it may be possible for them to withstand breakdown even without a fan - however, if they get above 150c, there's a possibility that they'll either desolder themselves or damage the nearby electrolytics. The fan is imperative, and it has to be a high-quality ball-bearing fan - NMB, Panaflo, Pabst or Nidec is recommended.

They have short-circuit, over-voltage and cycle-by-cycle over-current sensing, but depending on the manufacturer, these may never have been populated or tested to specs on the cheaper units - YMMV.
I'm doing this

Hi, I'm running a car amp for my sub, off a PC psu. Noisy fan does my nut, but, you could reduce the speed a little.

Looks pretty good. Aparently there is a mod, whereby the 2 live wires power the fan, (yellow plus red minus), but talk on various forums suggests its bad. I think a simple resistor divider is called for. I'm not using anything at the mo. My setup is temporary, I can put up for now.

Cheers iUSERTLO72p:)


2005-09-17 1:24 pm
AndrewT said:
there is no isolating transformer inside the computer PSU.
The maximum voltage in there is the peak mains voltage (~300V for a 220Vac supply).
Do not consider modifying any part of it (removing the cover) without considering the safety implications.

good point - i never really thought of that before:bigeyes:
Can use the 5v

Since you need to have something occupying that 5v rail, you can use the fan on 5v and it will run very quietly.

There's also quieter fans made by Zalman, Panasonic, and Cooler-Master. Those able to run 5v, need to be a bit faster speed so that they don't stall.

Generally, PC power supplies with 120mm fans make much less noise than those with 80mm fans.

Be careful of the power supply. Generally one should pull the power cord while the power supply is under a load (running something), before attempting to work on it.

That will not remove the potential for shock, but it will certainly reduce it.
Next, (now that its disconnected) wait about 2 minutes, then use its power-switch "ON" contacts a few times. You'll see the fan either start for a moment or jiggle. Now, most of the energy is drained out of the power supply.

Anyway, fan on 5v is a favorite trick of PC builders to quiet a loud fan.


2005-09-17 1:24 pm
sss said:
car radio wont make it hot , it can run with no fan for car radio.
car amp is a different story.

Can u run a car amp off that PSU????

i have a sony 250W amp that i want to use - i just haven't got a power supply for it. i guessed the computer PSU wouldn't deliver enough current, 'cos with the car radio the display dims when the bass is heavy - and thats only 25W x 4. i also don't really want to blow the psu.
It sounds like. . .

It sounds like that you probably shouldn't run the 250 watt amp from the computer power supply. I think it would blow.

Class A-B amps consume almost twice what they put out (more or less) and computer power supplies are rated while they're new and inside a fridge. So, a sturdy 500 watt top-line power supply (Enermax, Fortron/sparke, Thermaltake/Enlite, Artec), stands a chance of running that.

Howabout more efficient speakers instead?
Over at parts express are DC160S-4, a 6" woofer with over 90db that plays as low as a jukebox, and the 98db Dayton 12" DVC subwoofer (28hz-500hz) which is currently on sale. There's the 90+db Pioneer 5" Cup midrange, and quite a few tweeters do play in the 90+db range.

It depends on the space you have to play with. One cool project might be to stretch the 12" (it just takes one of them) to meet a pair of the 5" midrange (somewhere about 400 to 500hz) and build the mids+tweet+car radio into a small open back enclosure that matches up with and sits on top of a 3.5 cubic foot (seperate or divider panel) ported enclosure for the 12" sheilded Dual Voice Coil woofer. That would make it a 1 piece radio looks like from the 40's except that it would make sound like from the 70's (or your nearest discotech). ;)

Hey! and their Dayton 3-way crossovers are on sale ($39 and it takes two). They have a 500hz+4000hz unit, but ask at the forum (you'll get ideas, all of which both work and conflict). I think that works, but you might need to bypass the coil that blocks the low end of the tweeter (just use the cap) because the 5" cup doesn't quite make 4k (almost does). So, that's 98db - 3db, is 95db (per each channel of stereo) and bust down the sheetrock long before the car radio makes its full 25 watts. ;)

Anyway, I think I'd turn that car radio into something cool, like that big "unified" project. Sure, its big, but do you have 19 inches space to spare (about that width)?

If that above project is too big and just too much, then those above mentioned 6" drivers need only 0.43 cubic foot ported enclosures (see WinISD for reference), a 2-way crossover (somewhat 2300hz) and a tweeter that can play that low (2k regions) and loud. Except for lumber, the price isn't much different, but the little ones are also useful for HT mains, center, or surround. ;) That's about 90db per each (93db, 55hz bass in ported enclosures, 90db at 52hz), and they can sit on bookshelves or desk (blow everything right off the desk). ;)

Anyway, 25 watts *can* be more than enough for party loudness levels. That depends on your speakers. So, if you were going to invest in *more*, I'd rather see efficient speakers for *more* beautiful sound instead of mucking a nice radio with an in-line amp (more music--much louder, not just more watts).

There's also, Tang Band 6-1/2 (15 watt 8 ohm amp or 30 watt car amp--doesn't need a woofer crossover), 92db rumble. . . and Dayton 6" PA (prosound), a 92db mid-bass (largest size midrange).

There's a lot of false advertising on efficiency, but these 4 (above) have all won awards, so here's a small selection (there is only a small selection) of 90+DB efficient speakers.

Car radio makes 25 watts.
Speaker of 83db, with car radio at full, maxes out 96db
Speaker of 83db, with 170 watt amp, maxes out 106db
Speaker of 92db, with car radio at full, maxes out 106db

*Calculations between 4 and 8 ohms were ignored--possible 3db variance in figures; however, amps clip sooner at 4 ohms than 8 ohms so the mathmatical difference may not apply during actual use. **Possible slight variance in figures due to waking up at 4 a.m. to build a new amplifier inside an old amplifier--no coffee yet!!! ;) ***In the above maximum db output figures, the efficient speakers will play both more quietly and more loudly (more dynamic) which makes them more fun (more live).