12v + -12v ---> 2x172mm's @ 24v

[dobbz]

i have two Papst 6224/4 24v 172mm case fans running on a separate PSU's mobo connector. the positive lead is going to the 12v line, and the ground is going to the -12v line for 24v (actually 22v). the fans are rated for 12-28v 18W.

everything seems to be running fine. is there anything i need to look out for before the PSU blows up or something?

[Jabo]

-12v line is very low amp line and that;s why you are getting 22volts imo.
Psu is lowering voltage to give needed amps.
I never heard about such a way of getting 24 volt line out of ATX PSU....
YOu need more imput here

[dobbz]

well these are the fans: http://www.papst.de/pdf_dat_e/S32gb.pdf

ill probably end up running these at 12v actually. theyre just too loud at full speed.

[Holst]

I wouldnt recomend doing this.

The -12v rail will have a low amperage (2amps) you may well blow up your PSU doing this... and if your PSU blows it may kill other stuff when it does..

[NST6563]

If the positive is connected to +12v, then wouldn't the fan draw the current from the positive lead????

correct my thinking if it's wrong please.

[enzoR]

no.....it may draw from it... "but it travels" though the -12v back. . . the -12 v is made inside the psu theirfore their is a limit of how much current can pass though it.

[NST6563]

Originally posted by enzoR
no.....it may draw from it... "but it travels" though the -12v back. . . the -12 v is made inside the psu theirfore their is a limit of how much current can pass though it.

ahh..ok. I understand now.

guess he could try this link...I had 2 24v fans running off one of these from a dedicated power supply (ripped from a busted external drive enclosure).

http://murray.newcastle.edu.au/users...92/doubler.htm

[dobbz]

whoa. that stuff is way over my head.

[Ragnarok]

umm if u wanna use a voltage doubler for the sort of power output u need, then u gonna need some large ing caps..

as for the +12 and -12V, learn KVL and it will all become clear

[dobbz]

Originally posted by Holst
I wouldnt recomend doing this.

The -12v rail will have a low amperage (2amps) you may well blow up your PSU doing this... and if your PSU blows it may kill other stuff when it does..

OK. its been running 24/7 for three days, still full speed at 22v and nothing is hot. i was hoping someone could gimme a headsup about when i should expect it to blow. cant say for sure?

like i said, the label reads 12v-28v, so is there no harm in running em with 12v?

[Holst]

You can run them on the 12v line perfectly safely, its just putting them on -12 that may cause problems.

If you do want to run them on -12 then put a ammeter in the line and see what amps they are drawing.. if this is more than whats printed on your PSU then I wouldnt run them on -12.

Its impossible for me to say, if or when your PSU will be damaged, but I wouldnt run my own system like that.