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no -5V rail on my power supply, how to do 17V fan mod? - Page 3

post #21 of 51
Quote:
Originally Posted by DuckieHo View Post

If you have a +12v and +5v, the potential difference is 7v.... not 17v.
EE here as well.... but rusty.
BTW, redsunx is wrong.

Yeah sorry Got it backwards there. My fault smile.gif

I'm also rusty. wink.gif I edited it to reflect the correct information. Don't want to spread lies! smile.gif
    
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post #22 of 51
Quote:
Originally Posted by DuckieHo View Post

If you have a +12v and +5v, the potential difference is 7v.... not 17v.
EE here as well.... but rusty.
BTW, redsunx is wrong. That would be shorting the +12v and +5v circuitry... the results are unpredictable and possibly damaging depending on PSU design. It however would definitely not be +17v.

Yet another post to stop me from going insane. This thread is full of win.
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post #23 of 51
Quote:
Originally Posted by killabytes View Post

Voltage is nothing but a difference, that's it. If you have 12V and 5V and you do have 17V. Same if you had 12V and -5V, you'd have 7V.

Source: My degree in EE. wink.gif
While what you say is true (vaguely if I might add), you have to consider the application. Didn't they teach you parallel verses serial connections in your degree? Since both sources have the same ground point, connecting the positives of both sources becomes a parallel connection. Serial is only possible when the ground reference point of both sources are not connected. In this application they are the same point, so it does not work.
 
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post #24 of 51
Quote:
Originally Posted by just_nuke_em View Post

While what you say is true (vaguely if I might add), you have to consider the application. Didn't they teach you parallel verses serial connections in your degree? Since both sources have the same ground point, connecting the positives of both sources becomes a parallel connection. Serial is only possible when the ground reference point of both sources are not connected. In this application they are the same point, so it does not work.

We'd connect the 12V to the (+) red on the fan and the -5V to the (-) black on the fan.

How about some visual aids?

358

358



See the difference?

PS: I suck at paint.
    
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post #25 of 51
I'm not sure what you are trying to argue, since you don't list voltages, but I believe you are trying to show this:

497

There is not way to get 17V on a psu since there is no -5V rail. If you hook the +12V line and the +5V line, you will just get the difference between the two since they share a common ground point. Don't believe me? do Kirchhoff's voltage law on the loop and tell me what you get.
 
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post #26 of 51
Some power supplies still offer -5V.

20-pin_ATX_connector.png

wink.gif
    
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post #27 of 51
Wait, if running +12 (to positive) and -12 (to negative) should that run a fan rated for 24v? Am I understanding that correctly?
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post #28 of 51
Quote:
Originally Posted by killabytes View Post

Some power supplies still offer -5V.

20-pin_ATX_connector.png

wink.gif


Now that's just arguing a moot point, the OP clearly stated that his PSU had no -5v rail.

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post #29 of 51
Quote:
Originally Posted by killabytes View Post

Some power supplies still offer -5V.

20-pin_ATX_connector.png

wink.gif
In the sense of keeping with the OP:
Quote:
no -5V rail on my power supply

thumb.gif, so in that sense it is impossible to get 17V on the OP's resources. But if it did, 17V would be possible.

Do modern psu's even have -5V anymore? thinking.gif
Nope, was done away with in 2002. Wikipedia: ATX standards
Quote:
ATX12V 1.2
A relatively minor revision from January 2002. The only significant change was that the −5 V rail was no longer required (it became optional). This voltage was used only on some old systems with certain ISA add-on cards.

Edited by just_nuke_em - 1/3/12 at 1:03am
 
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post #30 of 51
Thread Starter 
Quote:
Originally Posted by KarathKasun View Post

Wait, if running +12 (to positive) and -12 (to negative) should that run a fan rated for 24v? Am I understanding that correctly?

yes.
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