Is a fan really needed?

[Logistics]

Power supplies are of course rated or I should say advertised by their maximum output, correct? But if you use a power supply to power nothing but a CD-ROM for instance, does the fan really need to run or could it be removed?

[archae86]

Re: Is a fan really needed?

Power supplies are of course rated or I should say advertised by their maximum output, correct? But if you use a power supply to power nothing but a CD-ROM for instance, does the fan really need to run or could it be removed?

Some supplies control the speed of their own fan to reduce acoustic noise. For at least some for of them, this goes all the way down to zero speed.

Using a supply like that would be more prudent than just clipping out the fan and hoping all is well. But if it stays cool, I doubt you have done harm.

Also, be sure you honor the minimum load requirements for the particular supply. Less load is not always better, in this case.

[dood]

Re: Is a fan really needed?

I used to run an old, large AT supply out of something like a Tandy 3000, as a benchtop power source. Just for testing 12v devices and the like. While I didn't use it for very long stretches of time, I did use it without a fan. I forget why, I think maybe it died, and was an odd size fan.

[Fizzycapola]

Re: Is a fan really needed?

Removing a fan removes a very large amount of cooling. Even multiplying heat sink mass and thermal conductivity over components I feel cannot compensate for lack of air flow, hot air simply be trapped and components heat quicker than they can cool, with no air stream for the hot air to attach itself to.

Whether the PSU being under load or not I'm not sure how relevant this is. PSU efficiency at low load may be as low as 50% maybe.

If the PSU makes such an audible noise that it annoys enough to warrant removing the fan, I think replacing the PSU totally would be the wisest decision.

There's a wide range of genuinely near silent fan cooled PSU on the market also as well as fan less PSU, but as long as a PSU is truly audibly silent (i.e under ~1200RPM fan with quiet blades, bearings and quiet lubricant) I don't see why one would choose fan less given the exponential thermal problem this causes.

Reference (but not to be narrowed to this, these sites are selling/reviewing a small sample of just what their wholesaler sells them):
http://www.silentpcreview.com/
http://www.quietpc.com/

[999999999]

Re: Is a fan really needed?

With any switching supply there is a certain output power it could support with passive cooling, but units designed to be passively cooled may be laid out differently, different heatsinks, and more open / perforated casing. So due to original design intentions, it will be capable of less load without the fan than it could've been, but still some.

Since the unit is not rated for passive operation, we cannot know the max load it could support nor to what extent the inevitable temp rise at a significant load, would reduce it's lifespan. We can instead just be very conservative and consider that even with lower efficiency at very light load, it's still only a tiny output power.

HOWEVER, powering only one CDROM drive is not a significant load. You can be sure it is capable of that without the fan running at all. I wrote "one CDROM drive". Add another and another powered device and as you increase the load it becomes less and less reasonable. Having the PSU and powered devices running, you should monitor the PSU temp for awhile (at this slight load it need not be anything elaborate, just feel the metal casing to see if it's more than barely warm).

The one other issue that can be significant depends on the layout. If it has integral load resistors on any rails and they're situated on the PCB very close to capacitors, it may make the caps run on the warm side. Sometimes it is only a matter of bending the resistor away some or if it's cemented to an adjacent cap, cutting away that cement and them fixing it again (or putting heatshrink tubing (ideally silicone formulation) over it to isolate it instead).

So there are a few minor concerns but in general it should work without the fan. You might drill a few holes in the casing, in strategic locations so there are intakes (or exhausts, depending on which orientation you mount this in) low nearer the PCB, and then holes in the top lid, so there is a flow-by path for air. Doing this you should be able to get at least a couple amps out of the 3.3/5/12V rails without problems. Probably more than a couple amps but to be conservative, and we don't know how the environment it's placed in will effect it's passive cooling. Put that PSU inside another wood or metal box and it's worse than sitting alone with nothing around it, but to power only 1 CDROM, it would still handle that no matter what unless you deliberately boxed it into a highly insulative enclosure or had extreme ambient, room temps.