More than one CPU fan?

[stuckinasquare3]

Some coolers have two cpu fans. My mobo only has one cpu fan plug. Where would you plug in the second cpu fan?

 

[TidusZ]

Either into a system fan header on the motherboard or use a 3/4 pin to molex adapter and plug into the power supply.

 

[stuckinasquare3]

The CPU fan slot has 4 pins but some other slots have 3 pins. My motherboard says it has 3 SYS_FANs and 2 of them are 3 pin and 1 is 4 pin. I imagine I can't plug a 4 pin cpu fan into a 3 pin slot but I'm not positive if I can reach the 4 pin slot. I guess I'd need an adapter of some kind.

 

[Gillbot]

Or use one of these: http://www.pcsilent.de/ppic-MAXI--525molex3.jpg and one of these: http://www.pcsilent.de/ppic-MA...3-pin-fans-ystrom3.jpg

 

[BonzaiDuck]

Originally posted by: Gillbot
Or use one of these: http://www.pcsilent.de/ppic-MAXI--525molex3.jpg and one of these: http://www.pcsilent.de/ppic-MA...3-pin-fans-ystrom3.jpg

Basically, the product in the second link is the one I'd use, but instead of buying one, I'd just make one from scrap 3-pin plugs and wire-stubs. The only drawback (which I don't worry about, myself) is that you can't hook up two sensor wires to plug into a single motherboard pin. So you run the sensor wire from only one of the two fans to the motherboard, with the red and black wires connected in parallel. To clarify, I don't worry if I can't read the tach-sensor on the second fan.

This only leaves two issues of concern:

1) the fans are best chosen to be same make, model and therefore -- amperage.
2) The sum of the amperages for the fans should be less than the motherboard's limit.

As far as I know, the cumulative limit for all fans connected to the motherboard is the main consideration, assuming the way the motherboard fan circuits are wired. In that scenario, although the motherboard user-manual may report a "per-plug" amperage limit, you could exceed that limit on one or more plugs if the total amperage of all fans connected is within the cumulative limit for the motherboard.

Some motherboard CPU headers allow for thermal control with either PWM (4-pin) fans or voltage-varied thermal control with 3-pin fans, and the 4-pin CPU-fan header is built to accommodate a 3-pin as well as a 4-pin plug.

 

[stuckinasquare3]

Thanks all

 

[Gillbot]

Originally posted by: BonzaiDuck

Originally posted by: Gillbot
Or use one of these: http://www.pcsilent.de/ppic-MAXI--525molex3.jpg and one of these: http://www.pcsilent.de/ppic-MA...3-pin-fans-ystrom3.jpg

Basically, the product in the second link is the one I'd use, but instead of buying one, I'd just make one from scrap 3-pin plugs and wire-stubs. The only drawback (which I don't worry about, myself) is that you can't hook up two sensor wires to plug into a single motherboard pin. So you run the sensor wire from only one of the two fans to the motherboard, with the red and black wires connected in parallel. To clarify, I don't worry if I can't read the tach-sensor on the second fan.

This only leaves two issues of concern:

1) the fans are best chosen to be same make, model and therefore -- amperage.
2) The sum of the amperages for the fans should be less than the motherboard's limit.

As far as I know, the cumulative limit for all fans connected to the motherboard is the main consideration, assuming the way the motherboard fan circuits are wired. In that scenario, although the motherboard user-manual may report a "per-plug" amperage limit, you could exceed that limit on one or more plugs if the total amperage of all fans connected is within the cumulative limit for the motherboard.

Some motherboard CPU headers allow for thermal control with either PWM (4-pin) fans or voltage-varied thermal control with 3-pin fans, and the 4-pin CPU-fan header is built to accommodate a 3-pin as well as a 4-pin plug.

I would use them both, as I wouldn't want to run two fans from one mobo header without knowing what the fans draw and what the header can produce.

 

[BonzaiDuck]

Originally posted by: Gillbot

I would use them both, as I wouldn't want to run two fans from one mobo header without knowing what the fans draw and what the header can produce.

Assume same make and model 12V fan, then amperage is just A1+A2 for fans 1 and 2, which would then need to fit within motherboard spec -- most often stated in Amps. As I said, on some mobo's I've used, it is only the cumulative amperage that counts. It may be that on some other mobo's the amperage limit per plug is the relevant limit.

 

[Gillbot]

Originally posted by: BonzaiDuck

Originally posted by: Gillbot

I would use them both, as I wouldn't want to run two fans from one mobo header without knowing what the fans draw and what the header can produce.

Assume same make and model 12V fan, then amperage is just A1+A2 for fans 1 and 2, which would then need to fit within motherboard spec -- most often stated in Amps. As I said, on some mobo's I've used, it is only the cumulative amperage that counts. It may be that on some other mobo's the amperage limit per plug is the relevant limit.

BUT, those amperage ratings are running amps only. Fans can pull quite a bit more amperage at startup. This is normally not a problem with one fan, but with two you can drop the voltage due to the excessive loading on the channel. When you run two on one header you run the risk of dipping below a safe startup voltage thus even further increasing amp draw. Just because both fans together appear to be below the limit of the header amperage, it's not safe to assume it can supply the proper power all the time. (ask me how I know)

 

[daw123]

Originally posted by: Gillbot

Originally posted by: BonzaiDuck

Originally posted by: Gillbot

I would use them both, as I wouldn't want to run two fans from one mobo header without knowing what the fans draw and what the header can produce.

Assume same make and model 12V fan, then amperage is just A1+A2 for fans 1 and 2, which would then need to fit within motherboard spec -- most often stated in Amps. As I said, on some mobo's I've used, it is only the cumulative amperage that counts. It may be that on some other mobo's the amperage limit per plug is the relevant limit.

BUT, those amperage ratings are running amps only. Fans can pull quite a bit more amperage at startup. This is normally not a problem with one fan, but with two you can drop the voltage due to the excessive loading on the channel. When you run two on one header you run the risk of dipping below a safe startup voltage thus even further increasing amp draw. Just because both fans together appear to be below the limit of the header amperage, it's not safe to assume it can supply the proper power all the time. (ask me how I know)

Gillbot, how do you know?

 

[TidusZ]

I know one time my pc wouldnt boot up, it kept shutting itself off soon after I hit the power button, and I found out it was cus an mp3 player was plugged in trying to charge, even while the pc was turned off. I unplugged the mp3 player and all was good. That was a strange problem to fix.

 

[Gillbot]

Originally posted by: daw123
Gillbot, how do you know?

I wish I still had the pics, but I can't find them. Needless to say after loads of research to find the limit of a fan header on my BRAND NEW motherboard, I hooked up 2 fans (with a combined amperage below said limit) and when I powered the system up, it fried it. I mean, large plume of smoke fried it. That was a quick ~$200 down the drain right away.

Also, keep in mind that those amperage ratings on the fan sticker are not an exact reading but an estimate across the line of fans of the same model. I believe they are allowed to vary +/- 10% from the rating on the sticker.

 

[daw123]

Originally posted by: Gillbot

Originally posted by: daw123
Gillbot, how do you know?

I wish I still had the pics, but I can't find them. Needless to say after loads of research to find the limit of a fan header on my BRAND NEW motherboard, I hooked up 2 fans (with a combined amperage below said limit) and when I powered the system up, it fried it. I mean, large plume of smoke fried it. That was a quick ~$200 down the drain right away.

Also, keep in mind that those amperage ratings on the fan sticker are not an exact reading but an estimate across the line of fans of the same model. I believe they are allowed to vary +/- 10% from the rating on the sticker.

Ouch, I'll keep that in mind since I was running two (Lian li) case fans off a splitter on the Sys0 and Sys1 fan headers (four fans in total) on my X48 MB, which seemed to be OK (as in the MB didn't fry). I was planning to do the same with my X58 MB.

 

[BonzaiDuck]

Gilbot has a good point there -- the startup voltages are lower -- amperage draw initially would be higher. If you look at certain "beefy" fans, they are rated nominally at maybe 0.90A, but the label might report 1.10A (probably the peak amperage Gilbot mentions.)

Like daw123, I took my chances with three 140x25mm Sharkoons which were rated at a nominal 0.20+A each -- running them off one of the "sys" or "case_fan" plugs on the motherboard. Never had a problem with it. But you MIGHT LIKELY have a problem with beefier fans rated above 0.50A. And it all depends on the motherboard.

It may be a "best guess" on this and the underlying assumptions. For instance -- whether or not the 12V circuit for all the fan-plugs is a simple parallel arrangement (internal to the motherboard) so that only "cumulative" loading matters.

I can just say that the three Sharkoons -- hooked up to a single case-fan mobo plug -- haven't caused my 780i motherboard a problem.

This would also lead to considerations about whether to use a separate fan controller (or just run the suckers off the PSU with a Molex). For "other fans" I was lucky that the nVidia boards have an "ESA" feature and that Silverstone produced their "Commander" controller that provides for thermal fan control or just lower-than-100% fixed settings from within software.

 

[Gillbot]

Originally posted by: BonzaiDuck
Gilbot has a good point there -- the startup voltages are lower -- amperage draw initially would be higher. If you look at certain "beefy" fans, they are rated nominally at maybe 0.90A, but the label might report 1.10A (probably the peak amperage Gilbot mentions.)

Like daw123, I took my chances with three 140x25mm Sharkoons which were rated at a nominal 0.20+A each -- running them off one of the "sys" or "case_fan" plugs on the motherboard. Never had a problem with it. But you MIGHT LIKELY have a problem with beefier fans rated above 0.50A. And it all depends on the motherboard.

It may be a "best guess" on this and the underlying assumptions. For instance -- whether or not the 12V circuit for all the fan-plugs is a simple parallel arrangement (internal to the motherboard) so that only "cumulative" loading matters.

I can just say that the three Sharkoons -- hooked up to a single case-fan mobo plug -- haven't caused my 780i motherboard a problem.

This would also lead to considerations about whether to use a separate fan controller (or just run the suckers off the PSU with a Molex). For "other fans" I was lucky that the nVidia boards have an "ESA" feature and that Silverstone produced their "Commander" controller that provides for thermal fan control or just lower-than-100% fixed settings from within software.

Normally, the label will only report an average running amp draw, Not amp draw at startup.

 

[BonzaiDuck]

Originally posted by: Gillbot
Normally, the label will only report an average running amp draw, Not amp draw at startup.

That may be true for several fans, and I've never been absolutely sure for which ones. But I picked up some 92x25mm DELTA jobs that were rated by the reseller at something like 0.65A, when the label reports something at 0.70A or above (need to look at them again, but there is a discrepancy in that order of magnitude.) I think I spoke to the reseller, who confirmed that the 0.65 represents "average."

 

[Gillbot]

Rated by the reseller means nothing. Ratings should always come from the manufacturer.

EDIT: and of course they are ALWAYS an average for that model of fan. They would never test each individual fan and them label it. That's just silly to even consider from a manufacturing standpoint. As I said, they are rated but the ratings can vary as much as +/- 10%.

 

[wkwong11]

You can connect 2 3-pin fans to one 3-pin header with this:

http://www.svc.com/3pinyadapter.html

If your CPU fan has 4-pins, that means it has PWM function. (No worries though, you can plug the 3-pin fan right on in. You just can use the PWM function.)