7V from molex, potential damage to psu?

Soldato
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A simple question but not one that I've seen convincingly answered yet. For those who are unsure, the idea is that the molex connector has a 12V, 5V and two earth lines. A fan wired between 12V and 0 experiences a potential of 12V and spins fast, one wired between 5V and 0 likewise but spins slowly and might not start. So 12-5=7V is a nice compromise, almost all fans will run at this.

However, the psu presumably isn't designed to cope with maintaining current between the 12V and 5V rails. Is there then a risk of damage?

The alternative is putting resistors in line. The resistors get hot, which suggests the 7V mod generates heat across part of the psu which may well not be intended to deal with this.

Can anyone tell me more?

Cheers
 
Don
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It won't do it any harm, all you're doing is feeding current into the 5V rail from another point other than the PSU - assuming you actually have something being powered from the 5V rail, which you will.
 
Soldato
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Ah, I think I see. The current fed into the 5V rail is more inclined to go into the other devices on the 5V line than into the psu. I imagine the 5V line will have marginally more ripple on it than it would otherwise, but it'll still be within spec so it's all good. I really must learn some basic electronics at some point.

Is it fair to say then that wiring one wire of a fan to the 12V line, and the other one to a three way switch connected to nothing/0V/5V will leave you with a fan switchable between off, 7V, 12V without causing any harm to the psu?

Cheers Jokester
 
Soldato
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I can't remember exactly now but I was discussing this with some other technicians at work and we concluded that it was a bad idea. I'll try to remember why.
 
Soldato
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I'd certainly be interested if you come up with anything. It doesn't *sound* a good idea, but Jokester hasn't steered me wrong yet.
 
Associate
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7v modded loads of fans. never seemed to bother any psu i have used. Still i would be interested in seeing Mr x plosions info, if u remember why you thought it was a bad idea please post.
 
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