9.6 watts too much for a motherboard fan header?

[MichaelD]

My motherboard (Abit IP35-Pro) has 6 controllable fan headers. The manual does not specify a max wattage rating for each header. I know that typically, this is 4-6 watts.

I want to control the fans' speed by using the BIOS (it supports this). Once down to 7-8 volts, they'll be pulling just a small amount of current. But it's the startup that is worrying me.

The fans I want to connect are 120mm, 2800RPM, rated at .80A/9.6W by the manufacturer.

I'm thinking this is too much and I'll blow the headers. Thoughts?

 

[Rubycon]

Going north of 10W is getting there. 12W starts making you think red. Really depends on the motherboard though.

 

[legoman666]

Originally posted by: MichaelD
My motherboard (Abit IP35-Pro) has 6 controllable fan headers. The manual does not specify a max wattage rating for each header. I know that typically, this is 4-6 watts.

I want to control the fans' speed by using the BIOS (it supports this). Once down to 7-8 volts, they'll be pulling just a small amount of current. But it's the startup that is worrying me.

The fans I want to connect are 120mm, 2800RPM, rated at .80A/9.6W by the manufacturer.

I'm thinking this is too much and I'll blow the headers. Thoughts?

they pull the same current regardless of voltage.

 

[Zepper]

Wrong, legoman! Go back and recheck your Ohm's law... Less voltage across the same resistance = less current. Fan headers used to be rated at 0.5A or 6W - I know some can source more current now, but I wouldn't chance it unless I knew for sure. I just always use a separate fan controller so I don't have to worry about it. Why such power hungry fans to begin with? I have fans here capable of 110CFM at only 0.4A... Check with the mobo mfr. if it's not in the manual. Often you'll find that info in the FAQ for the model.

.bh.

 

[legoman666]

Originally posted by: Zepper
Wrong, legoman! Go back and recheck your Ohm's law... Less voltage across the same resistance = less current. Fan headers used to be rated at 0.5A or 6W - I know some can source more current now, but I wouldn't chance it unless I knew for sure. I just always use a separate fan controller so I don't have to worry about it. Why such power hungry fans to begin with? I have fans here capable of 110CFM at only 0.4A... Check with the mobo mfr. if it's not in the manual. Often you'll find that info in the FAQ for the model.

.bh.

lol oops, somehow i had it in my head that the resistance would .... nvm lol, had it all backwards. Proceed.

 

[MichaelD]

I guess the fans that come with my 4U rackmount just suck, that's all. :laugh:

Being that I've already paid for them, I figured I might as well see if I can use them at all. The Antec 3-speeds should be here tomorrow...hopefully.

Though I don't know their exact specs, I seriously doubt the fan headers on this MB (Abit IP35-pro) could support three fans that pull .80A each starting up at the same time.

*poof*
/magic smoke comes out
me<--ahhhhh!
me<--
my wallet<--/slit wrist

 

[Zepper]

Oh, no wonder - OEM fans for a server box - never mind the ears. Well, maybe you could add a resistor to them in the red (or positive if not red) fan leads to drop the load a bit before the mobo has to deal with them. Easy enough to calculate the resistance required to get it down to a half amp or so - probably give enough CFM at that point anyway. The resistor you need for 0.5 A = 9.375 ohms - choose nearest higher value. Wattage value of the dropping resistors will need to be about 2.25W so maybe 10 ohm at 2W resistors will do you as the max you'll be getting off the mobo fan headers (in controlled mode) will be ~11V anyway.

.bh.