changing computer PSU fan

[legodude]

Hi,

My PSU in my computer is still good, but the fan isn't. It is really noisy because I think the bearings wore out. I opened it up and saw that it is replaceable. I already know not to touch anything like the capacitors and stuff. But I have a question. The new fan I bought is a Vantec Stealth 80MM rated at 12V 0.10A. The old one is rated at 12V 0.19A. What would happen if I put it in? Would it go faster or would it fry the motor on it or would it just run fine?

I'm going to measure the current coming from the PSU with a meter soon to see what the current really is. If it is too high for the new fan, how would i go about reducing it to the rated level?

 

[SnooP_WiggleS]

No need to worry about current etc. by ohms law:

current = voltage / resistance

since voltage is known (12V) and resistance of fan is fixed, current is already determined. Basically The fan draws the current it needs. It will be fine.

 

[mono]

Given that it's getting noisey it's likely a sleeve-bearing fan. A temporary fix is to put a drop of heavyweight oil into the bearing. Sometimes that even works long term, depending on how worn (and filled with debris) the bearing is.

There is no point in measuring current from the power supply, as it is not relevant to the tasks at hand. Voltage makes a fan go faster, current is the result of the fan's circuitry and the voltage applied to it. Having a current reading would not benefit you.

The fan (presuming the new and old were both spec'd for 12V, as "almost" all ATX power supply fans are (check the label, it should tell you)) is set by manufacturer to operate at a certain RPM @ a nominal voltage, which in this case is 12V.

Roughly speaking, fans of same basic design will have similar RPM and flow rate with same current. That's not 100% true but close enough for practical purposes. So with the new fan having significantly lower current spec, it can be expected to spin slower and provide less airflow. If your power supply had a lot of margain, ie- it didn't need as much airflow as it was getting to stay cool enough to promote long-life, you can substitute a lower RPM fan, up to the limits of the acceptible heat-retention due to this lower flow, which also varies per system load and ambient temp.

TO compound this, many averag-or-better PSU now have a thermal control circuit which varies fan speed based on a simple thermal adjustment, so in such cases the lower speed fan would simply be spinning at a higher percentage of it's max (per 12V) speed. The exact speed can vary, there are a lot of variables involved here and it wasn't meant to be an overall tutorial on Power/fan/case/cooling, and yet in the end all these things have to be considered to a certain extent.

The easiest answer is, yes you can put the fan in, IT will be safe, but if it doesn't move enough air (exhaust feels too hot, something easier to judge from experience, I don't have a rate or degree figure in mind) then you'll need find a higher RPM fan. Genearlly speaking, I found such ~ 2200 RPM fans to be sufficient for a P3 era system but with a modern Athlon or P4, especially with a moderately hot running video card, you might find a power supply with the thermal control circuit is better off with a .14A or higher spec'd fan.

The alternative is that you can CHOOSE to have a quieter system at the expense of power supply longevity, but considering that the original fan wasn't very good, the power supply itself probably isn't either, making it even more subject to heat-induced failures per same typical system load.

 

[Garbz]

since no one has mentioned it take care when opening. There's some nasty 200v capacitors in there!

 

[DaKi][er]

Quote:

Originally Posted by Garbz since no one has mentioned it take care when opening. There's some nasty 200v capacitors in there!

which should have resistors across them that will discharge them in the time it takes to open the case up (few minutes)

 

[diredesire]

It's not really an issue if you're careful...

As far as the Vantec Stealth, it's not very powerful, you should probably look into a 30-35cfm or higher fan. The Panasonic Panaflo line is highly recommended for noise:air ratio, the M1a or H1a would probably suit you best, other recommendations are the Coolermaster LED fans, there are also the normal non-lighted variety, although the bearings aren't rated for as long an operation they push a decent amount of air per noise... Any standard 80mm fan should probably do you fine, the stealth is a little on the lower side for CFM, and it's also a little clicky in operation, not really as quiet as they'd lead you to believe.

Your powersupply may have a temperature control circuit where if it gets too hot it will actually run the fan at full speed, or overvolt it in some cases, this won't really be an issue unless you have an under powered fan, which i suspect you might...

 

[mono]

- dp

 

[mono]

The high-side caps will be bled off by resistors and bled off by the 5VSB subcircuit on any ATX supply. There's no need to wait even a moment when opening it- just be use it's unplugged from AC.

Panaflo H1 speed is probably too loud, unless the PSU has an atypically aggressive thermal throttling circuit in front of it. In most uses an H1 would easily be the loudest part of an (otherwise) quiet system. HOwever, the Panaflos are one of the few good noise/flow ratio fans for this use. It would be a bad idea to use any other sleeve-bearing fan for the exhaust except maybe a Papst or (supplimentally-lubed) Sunon. PSU intake isn't exposed to as much heat as the exhaust, a sleeve-bearing fan might work better there except for it being horizontally mounted.

 

[legodude]

Hi again,

Thanks for all the help guys. I probably should of mentioned that the PSU, 235W, is from a really old p3 500mhz slot 1 computer that recently turned into a dedicated firewall for my home network. I assume you guys are talking about a beefier PSU (350-550 range). Smoothwall.org they call it. Only 0.02 or somewhere around that of the CPU is being used. I imagine that dosen't use that much electricity and thus not making the PSU work that hard. Also, I unplugged what i didn't need like the floppy, cd drive, removed sound & vid cards. The only things that the PSU connect to are the mobo, hard drive, and 1 vantec stealth fan going in the front.

Once again, thanks for all the help.

 

[Garbz]

I think it was Oli or someone who actually desoldered all of the chips and mounted them to a giant heatsink at the top of the case, and used cables to get back to the PSU. Fanless in the end

 

[grandenigma1]

Be extreamely careful when you open up a PSU... they can retain a charge for a very long time... while in most cases not enough to seriously hurt you you can still get quite the shock!

 

[jerb]

if its a newer PSU there are bleed ressitors on all the caps (meaning they'll ddischarge in seconds, while i wouldnt go touching anything i wouldnt be too worried) also make sure that your fan will fit, most PSU's use a slimline fan and an ordinary 80mm fan will not fit unless its externally mounted.

like i said if its a newer PSU i wouldnt wory about It (BTU DONT GO TRYING TO PROVE ME WRONG!) also make sure you'r new fan will fit before tossing the old one.

 

[mono]

MOst power supplies do not use a slimline fan, they use a standard 80 x 25mm (thick) rear exhaust fan.

 

[Garbz]

May I just point out that the PSU is from an p3 500. Who knows how old it is.

So far any psu i've ripped open which was old enough to have a fan bearing go, was old enough to use standard 80mm sized fans. How old is your new ATX psu? While they may have bleeder resistors in them, don't take the chance.

So for fan size just check and double check, and treat the psu with the common curtosy of not licking your fingers and "Testing" the capacitors. Infact by changing the fan you shouldn't need to remove the PCB at all.

 

[mono]

Thainking about age and role of the system - needing constant uptime, it may be as well to just replace whole power supply. Since it's older you could probably get away with using something in the 250W range which are now considered mostly surplus in full PS2 sizes so they're pretty cheap.