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Power supply : difference between small and big fan?

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February 7, 2009 8:07:33 PM

I see that there are two type of fans on power supplies... Is there one better than the other and why?
a b ) Power supply
February 7, 2009 8:22:23 PM

Larger fans usually are less noisy because they don't have to spin as fast to provide the same airflow.
February 7, 2009 9:02:53 PM

Larger fans tend to be quieter, but because they tend to be on the top they also create "dead spots" which hinders cooling. Small fans tend to cool slightly better because of the ideal placement at the back of the PSU, but are also generally louder. There could also be an argument that a good PSU should not need much cooling, but some of the best PSUs utilize both 1 small and 1 large fans. There really is no true correlation between size of the fans and PSU quality though so worry about 12v stress and ripple results, not fans.
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a b ) Power supply
February 7, 2009 9:35:34 PM

The_Blood_Raven said:
Larger fans tend to be quieter, but because they tend to be on the top they also create "dead spots" which hinders cooling. Small fans tend to cool slightly better because of the ideal placement at the back of the PSU, but are also generally louder. There could also be an argument that a good PSU should not need much cooling, but some of the best PSUs utilize both 1 small and 1 large fans. There really is no true correlation between size of the fans and PSU quality though so worry about 12v stress and ripple results, not fans.


Large fans are normally on the bottom, of course it doesn't really matter if you flip the PSU over. The only difference is that the back fan will pull air through the PSU while the bottom fan will push air through. There will be dead spots with either, unless the PSU casing is empty.
February 8, 2009 2:29:32 AM

theAnimal said:
Large fans are normally on the bottom, of course it doesn't really matter if you flip the PSU over. The only difference is that the back fan will pull air through the PSU while the bottom fan will push air through. There will be dead spots with either, unless the PSU casing is empty.

He is talking about dead spots in the case, not the PSU.

Heat rises so if you have a top mounted PSU and a fan in the back it will pull any heat generated from the front portion of your case out the back of your case. It all depends on how your case is set up and the air flow pattern that was designed into it. If you have a bottom mounted PSU it will be different and if you have a top mounted fan it will be different.
a b ) Power supply
February 8, 2009 3:12:52 AM

ausch30 said:
He is talking about dead spots in the case, not the PSU.

Heat rises so if you have a top mounted PSU and a fan in the back it will pull any heat generated from the front portion of your case out the back of your case. It all depends on how your case is set up and the air flow pattern that was designed into it. If you have a bottom mounted PSU it will be different and if you have a top mounted fan it will be different.


A top mounted PSU with a fan on the bottom also pushes hot air out the back of the case. The location of the fan in the PSU does not change the airflow within the PSU at all.
a c 83 ) Power supply
February 8, 2009 12:38:12 PM

120mm fans are quiet(lower speed with more air), but they do also limit the size of the heatsinks that can be used within the psu. So in some cases they will have to spin faster to cool those smaller heatsinks.

With all the push for 80+ psus, smaller heatsinks are needed and help to offset this.

As long as the psu is well designed it should not matter if it has a 80mm or 120mm fan(the company should optimize component layout for the fan size being used anyway). Always make sure you have a rear case fan since you do not want the psu sucking too much hot air from your system.

Clearly this one is optimized for its 80mm fan. PC Power and Cooling 750 watt

Image from http://www.dvhardware.net/review119_3_silencer750.html

This one folds over the heatsink fins to allow the 140mm fan to send air through it. Corsair TX750

image from http://www.hardwarecanucks.com/forum/hardware-canucks-r...

Both are good units with a different design.
a c 248 ) Power supply
February 8, 2009 12:50:50 PM

Nukemaster - thanks for posting the photos. It's rare to see the internal components of a psu at this forum.
February 8, 2009 3:19:00 PM

theAnimal said:
Large fans are normally on the bottom, of course it doesn't really matter if you flip the PSU over. The only difference is that the back fan will pull air through the PSU while the bottom fan will push air through. There will be dead spots with either, unless the PSU casing is empty.


Yes you are right there will be dead spots inside the PSU either way, but there will be more with a larger fan on the top of the PSU (the correct orientation for most large fan PSUs). The reasn is because the small fan at the back of the PSU will be exhausting air that is coming from multiple openings in the PSU casing which will increase amount of angles the intaking air can cover. A large fan on the top will instake air into the PSU casing from 1 single angle which will create a lot of dead spots around and through the various PSU components.

There is not much difference in cooling, but a small fan is generally better cooling wise and a large fan is generally better noise wise. You could always replace the fan inside a PSU like I did with my old PC Power and Cooling 750w, and get the better CFM for less DBA.
a b ) Power supply
February 8, 2009 7:20:13 PM

The_Blood_Raven said:
a larger fan on the top of the PSU (the correct orientation for most large fan PSUs).


I don't think I've ever seen a PSU with a large fan on the top. If you're referring to the pictures in PSU reviews, they are photographed upside down.
February 8, 2009 7:53:18 PM

theAnimal said:
I don't think I've ever seen a PSU with a large fan on the top. If you're referring to the pictures in PSU reviews, they are photographed upside down.

You are correct that they are upside down but some cases (like my Stacker 832) are designed to have the fan facing up.
a c 243 ) Power supply
February 8, 2009 9:33:27 PM

ausch30 said:
You are correct that they are upside down but some cases (like my Stacker 832) are designed to have the fan facing up.

Huh?
The Stacker 832 mounts the psu at the top of the case, and has enough mounting holes to allow the psu to be mounted in either direction depending on wether you're using the case as ATX or have flipped the motherboard to use it as a reverse ATX case, this is in order to keep the wire bundle at what would be considered the back when looking in from the open side.
Obviously this is only possible with a flow thru psu ( 80mm on the back )
If you have a psu with a large "bottom" fan and have it mounted upside down, it's a personal problem, not a case that was designed to choke the airflow to the psu.
February 8, 2009 11:49:56 PM



Sorry, that's my PSU so I did not know that the fan is usually on the bottom of some.
a c 243 ) Power supply
February 9, 2009 12:33:08 AM

Thermaltake, Antec, Corsair, OCZ and some Coolermasters among others are just the opposite,when looking at them with the fan up the name would be upsidedown, but basing top or bottom on the way the name is set on the psu case means nothing, the pattern of the mounting holes is what matters.
Check out the hole pattern on most case's without a removable psu mounting plate, you'll see that you would have to turn the Galaxy over in order for the holes to line up.
Many are now adding extra mounting holes ( Like the Stacker ) so that such power supplies can be used by turning them over so that they are not choked for air by having that large fan so close to the case wall.
a c 248 ) Power supply
February 9, 2009 2:54:04 AM

I have a Corsair power supply with one 120mm fan. My current pc case is a CoolerMaster HAF 932. The psu can be mounted in the bottom rear of the case or in the top rear of the case. In either location the psu can be mounted fan up or fan down. The bottom rear of the case is perforated mesh. I decided to mount the psu in the bottom rear with the fan down. My thinking was that the psu could pull in cool air and cool itself. Carpeting was not an issue. A few years ago I built a custom pc stand for a couple of dollars to keep pc's off of the carpet. For the CM HAF 932 I simply cut a hole in the stand directly below the psu fan. Everything seems to be okay.
a c 243 ) Power supply
February 9, 2009 10:41:14 AM

I've got an Aerocool reverse ATX case, bottom psu mount with a removable plate so that the psu can be mounted in either direction, also has a vented bottom for use with bottom fan psu's. I keep it on the desktop, so no worries about carpet dust.
February 10, 2009 1:00:00 PM

delluser1 said:
Huh?
The Stacker 832 mounts the psu at the top of the case, and has enough mounting holes to allow the psu to be mounted in either direction depending on wether you're using the case as ATX or have flipped the motherboard to use it as a reverse ATX case, this is in order to keep the wire bundle at what would be considered the back when looking in from the open side.
Obviously this is only possible with a flow thru psu ( 80mm on the back )
If you have a psu with a large "bottom" fan and have it mounted upside down, it's a personal problem, not a case that was designed to choke the airflow to the psu.


If you had the case you would understand. There is a large hole above where the PSU mounts which is designed to allow you to mount the PSU with the fan facing up so that your pulling in cool air rather than warm air from inside the case.
a c 243 ) Power supply
February 10, 2009 1:54:03 PM

ausch30 said:
If you had the case you would understand. There is a large hole above where the PSU mounts which is designed to allow you to mount the PSU with the fan facing up so that your pulling in cool air rather than warm air from inside the case.

I understand completely.
The blow hole was designed ( before Coolermaster incorporated it into thier cases ) as an exhaust to remove the hot air that rises to the top of the case, if it's not being used as one then it can obviously be used as an intake for the psu.
February 10, 2009 9:31:50 PM

delluser1 said:
I understand completely.
The blow hole was designed ( before Coolermaster incorporated it into thier cases ) as an exhaust to remove the hot air that rises to the top of the case, if it's not being used as one then it can obviously be used as an intake for the psu.

There is also a exhaust fan in the top of the case in front of where the PSU mounts. The hole above the PSU is designed as an intake for the PSU.
a c 83 ) Power supply
February 11, 2009 2:22:05 AM

Is the air for the psu also taken from the back of the case?

If so would it not just suck hot air back into the psu as it comes out the back and loops back into the case inlet?

a c 248 ) Power supply
February 11, 2009 3:35:42 AM

Nuke - Interesting idea. I don't think I've seen a case with a configuration similar to your diagram. The cases I used had the top of the psu almost touch the top panel. There's no inlet at the top rear as shown in your diagram.

My psu is bottom mounted with the fan down. Cool air is drawn through a blowhole in the bottom of the case and warm air is blown out the rear. The cheap stand I built a few yeasrs ago has the equivalent of a solid rear panel that prevents a continuous loop.
February 11, 2009 11:28:37 AM

nukemaster said:
Is the air for the psu also taken from the back of the case?

If so would it not just suck hot air back into the psu as it comes out the back and loops back into the case inlet?

http://img155.imageshack.us/img155/8066/psuairflowhp0.gif

It is actually similar to your diagram but the top back overhangs slightly between the PSU and the vent and the vent is recessed more to help prevent the flow of hot air back into the PSU. There is also an airspace between the top of the interior of the case and the actual top of the case.
a c 83 ) Power supply
February 11, 2009 12:28:50 PM

JohnnyLucky said:
Nuke - Interesting idea. I don't think I've seen a case with a configuration similar to your diagram. The cases I used had the top of the psu almost touch the top panel. There's no inlet at the top rear as shown in your diagram.

My psu is bottom mounted with the fan down. Cool air is drawn through a blowhole in the bottom of the case and warm air is blown out the rear. The cheap stand I built a few yeasrs ago has the equivalent of a solid rear panel that prevents a continuous loop.

My showings are a bad possibility of heat getting pulled right back into the psu to be further heated over and over

Your intake from the bottom works great as heat rises so your system always gets cooler air.

ausch30 said:
It is actually similar to your diagram but the top back overhangs slightly between the PSU and the vent and the vent is recessed more to help prevent the flow of hot air back into the PSU. There is also an airspace between the top of the interior of the case and the actual top of the case.

I was wondering if there was an extra inlet, at least it can get more air from the forward vent as well.
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