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Decouple fans and hard disks supplies

Hello respected engineers! I've built myself a frame where I've put six hard disks for a RAID5 system, and six 8x8 cm fans to cool them down (they live in a quite hot environment!). The PSU is a 500W one however I was concerned that the fans may pollute the hard disks' supply too much.. since they're motors after all! I've accidentally put a finger into a fan (what a pain!), and this stopped the fan for a moment and also caused hard disk errors, and thus convinced me even more that I need to decouple the fans' motors supply from the hard disks supply.

But how? I'm just a hobbyst when it comes to electronics, so I may even intuite correctly, but I won't be really sure of my thoughts. Right now, they are that I need some big inductors (with as low DC resistance as possible), some (big?) capacitors and maybe also a big rectifier as well.

The best scheme I've thought so far is to link all 6 fans together in parallel, then apply power through a circuit like this:

6 fans--*-ind-*-ind-*-ind-*-ind-*-ind-*-ind---diode------*-----PSU +12V | | | | | | | cap cap cap cap cap cap 6 HDs---PSU +5V | | | | | | | Gnd ----*-----*-----*-----*-----*-----*------------------*-----Gnd

of course in the above scheme I don't even need them to be six, it's simply the more the better, I just happened to have six of them on my bench (more or less a coincidence).

But now the big question: what's the right size of the caps, also to not overload the PSU at start? Will the diode really help the hard disks to not get some bad bad bad back EMF? Notice also that the wire that feeds the fans is AFTER all the wires that feed the hard disks (otherwise the voltage drop caused by the fans would have affected the hard disks). Yes, at least I know this. ;)

Is my scheme ok? What can I do to improve it? I really care about the hard disks and the data they contain!

Thank you very much, Mike

Reply to
mike
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Why do you think that the fans are polluting the supply, anyway, are you exceeding the power supply 12V current rating?

It might be worthwhile using a seperate power supply for the fans. It really depends on how much power they need

martin

Reply to
martin griffith

Are the fans brushless type? For this many fans it is preferable if they are.

Also, I doubt the diode will do anything much since noise is of both polarities and will get through. I think a single choke and a large electrocap (eg. 10,000uF)on the fan supply (close to the fans) should be adequate. You could also do the same on the feed to the HDD's as well and I reckon she'll be sweet.

Reply to
Ross Herbert

Are they 12V fans ? You need'nt worry if so since 12V simply supplies the disk drive motors.

In any case your idea seems very strange. Modern fans like the ones I'm sure you're using don't have brush DC motors.

Graham

Reply to
Pooh Bear

...

I'd split the +12 supply (point X) as physically close to the power supply as possible - does it have two (or more) +12 wires coming through the grommet? I'd probably use one for the fans and the other for the drives, and add ferrite rings to each. (you _can_ get split ferrite rings that clamp over the wire, right?) So the drives' filter can go in Y, and then you include the PSU's output capacitance in the filter. :-)

I also doubt if you need six chokes and caps, but I'd use a moderate electrolytic, say 330-470uF, with a .1 ceramic in parallel with it.

Good Luck! Rich

Reply to
Rich Grise

Hi Rich, with respect for your sensible recommendation of wiring methodology (I took that as a given), the use of ferrite rings will only be useful in preventing high frequency EMI radiation progressing past the ferrite into wiring where it can then be radiated into the environment. The type of problem I gathered from the OP's description is more likely to be of lower frequency and higher amplitude voltage swings, for which a more hefty inductor using real copper wire is more appropriate. In fact, where he described the painful "stopping of a fan with his finger", the most likely reason for corrupting the the HDD supply was that the stalled motor acted as a momentary short circuit which pulled the supply rail down substantially. A small electro of 330 - 470uf is hardly going to help to maintain the supply near the nominal 12V for more than a microfart of a second, hence my recommendation for a more substantial value. Yes, hf decouplng/bypass with ceramic caps is also recommended.

Reply to
Ross Herbert

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