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Old 2009-04-17, 17:22   #1
akruppa
 
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Default DIY power up for small fan

I suppose this technically belongs in Hardware though it's rather different from the other threads here...

I have a small fan for my Northbridge chip. The NB fan connector on the motherboard does not take a speed signal, nor does the fan supply it. (Edit: changing fan speed with fancontrol did not work, either) It nominally takes 12V but is pretty noisy. It can run at 5V, too, and is very quiet then. The NB is meant to be passively cooled but gets a bit hot, a little extra air flow is all I need, so running the fan at 5V would suffice. However, the fan does not spin up reliably with only 5V - once it spins (e.g., by manually spinning it up), 5V are enough to keep it running.

I'm pondering a small circuit that applies 12V to the fan for a few seconds at power-up, then switches to 5V. This should be fairly trivial... an RC delay, a trigger and a switch. I don't have much of a clear idea how to design the circuit, though... if anyone has a suggestion, I could solder the stuff together without problem. Easy-to-get parts would be a plus.

Any suggestions?

Alex

Last fiddled with by akruppa on 2009-04-17 at 17:33
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Old 2009-04-17, 18:46   #2
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how about running it at 7V, connecting 12V and 5V?
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Old 2009-04-17, 18:51   #3
Uncwilly
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That is about as quick and dirty as one can get.
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Old 2009-04-17, 19:50   #4
xilman
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Quote:
Originally Posted by KriZp View Post
how about running it at 7V, connecting 12V and 5V?
I'm impressed!


Paul
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Old 2009-04-17, 20:05   #5
akruppa
 
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Quote:
Originally Posted by KriZp View Post
how about running it at 7V, connecting 12V and 5V?
Lol, I'll try that... though you kinda spoiled the problem with your horribly pragmatic, smack-in-your-face obvious solution! I was actually looking forward to soldering something...

Well, maybe it'll be too noisy at 7V or won't spin up, and I might get to solder something after all.

Alex
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Old 2009-04-17, 20:54   #6
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Seriously, you may screw up both your 12V and 5V at the same time, -- for the rest of the computer. EE is not "only" math, you don't wanna connect these rails, really.

You may want to feed from 12V through a some small R/C and that's all, no triggers, no switches. If you can determine the resistance of your fan and match it with a resistor, than you will have 6V on it, when the capacitor is charged. Then calculate the C... you know.


You can also do a variable resistor on the back bracket, ...even Tuniq Tower doesn't shy away from this solution! It comes with it.
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Last fiddled with by Batalov on 2009-04-17 at 21:13 Reason: You can also do a variable resistor
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Old 2009-04-17, 20:59   #7
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It's a pretty common solution I think, certainly not my idea You can always make a switch between 5-7-12 volts if you have a strong urge to solder something.
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Old 2009-04-17, 22:10   #8
akruppa
 
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I certainly wouldn't hook any significant load between the 12V and the 5V rail, but the fan has 0.6W power draw when running at 12V so I don't expect the PSU to mind it very much.

Alex
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Old 2009-04-18, 00:13   #9
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Quote:
Originally Posted by akruppa View Post
I certainly wouldn't hook any significant load between the 12V and the 5V rail, but the fan has 0.6W power draw when running at 12V so I don't expect the PSU to mind it very much.
Remember that the power draw is much higher until the fan gets up to speed. It's transient, but I'm not so sure the supply won't mind.
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Old 2009-04-18, 01:02   #10
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Quote:
Originally Posted by KriZp View Post
how about < snip > connecting 12V and 5V?
I still remember being at work one Saturday morning 30 years ago. I was in the "machine room", and suddenly noticed wisps of smoke rising from all of the tape drives. I yelled "Fire!", which alerted the electrical workers on the other side of the room that they had connected a 220V line to a 110V circuit.

Five years later, at a different employer, I arrived one morning to find that the DEC employees who had installed a new VAX backboard (DEC had warned us that our warranty would be invalided if any non-DEC employee did the installation) had connected 12V to 5V for a while. However, they assured us, there was no damage, the system still worked fine, and our subsequent occasional system failures were completely unrelated to that incident.
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Old 2009-04-18, 05:28   #11
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I think this is what you need.
Ue = 12V DC <- not AC!
Ua = 5V DC
If you take 7806 instead 7805 you have 6V Ua

Regards Andi_HB
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