Question about combining PSU rails

gruntledweasel · 04-29-2004, 11:23 PM

I'm familiar with using the +12V and +5V rails on my PSU to get +7V. +7V or a bit lower would be just about what I need. But could this be bad for the PSU at higher amperages, particularly with not much draw on the 5V rail? If the 5V rail floats up a bit high (and the +7V "rail" correspondingly lower), it's no problem (nothing sensitive being powered), but I don't want to fry the PSU.

edit: number mixup

bigben2k · 04-30-2004, 11:37 AM

Whatever amperage you draw on the 7 volt trick, must also have a return path to ground: the PSUs +5 volt line does not accept reverse current: the PSU will automatically shut down.

In other words, you must have something, anything connected to the +5 line that draws at least the same current.

Other than than, you may be introducing some transients from the +12 line, onto the +5 live. A +12v line is spec'ed with max 100 mV voltage ripple. The +5V line is spec'ed with max 50 mv voltage ripple. Probably nothing to worry about, but something that you might want to keep in mind, if you're a serious overclocker.

kronchev · 05-09-2004, 07:50 PM

bigben2K: can you clarify that first part? it sounds like youre trying to say it WONT work but ive tried it with countless applications and have always had success...

bigben2k · 05-10-2004, 09:50 AM

It does work.

You have to remember that PSUs are designed to have power come OUT of the positive voltage supply lines, then back to ground.

When you do the 7 volt trick, the power comes out of the +12 volt line, but then you're expecting it to come *into* the +5 volt line, which the PSU just wasn't designed to do. However... If you have something connected to the +5 volt line, then the power can keep going through whatever you have connected there (at +5), and return to ground.

Without any load connected between +5 and ground, the PSU's internal protection circuitry will shut down the PSU (in most cases).

04-29-2004, 11:23 PM	#1
gruntledweasel Cooling Neophyte Join Date: Dec 2002 Location: Irvine Posts: 63	Question about combining PSU rails I'm familiar with using the +12V and +5V rails on my PSU to get +7V. +7V or a bit lower would be just about what I need. But could this be bad for the PSU at higher amperages, particularly with not much draw on the 5V rail? If the 5V rail floats up a bit high (and the +7V "rail" correspondingly lower), it's no problem (nothing sensitive being powered), but I don't want to fry the PSU. edit: number mixup

04-30-2004, 11:37 AM	#2
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	Whatever amperage you draw on the 7 volt trick, must also have a return path to ground: the PSUs +5 volt line does not accept reverse current: the PSU will automatically shut down. In other words, you must have something, anything connected to the +5 line that draws at least the same current. Other than than, you may be introducing some transients from the +12 line, onto the +5 live. A +12v line is spec'ed with max 100 mV voltage ripple. The +5V line is spec'ed with max 50 mv voltage ripple. Probably nothing to worry about, but something that you might want to keep in mind, if you're a serious overclocker. __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101

05-09-2004, 07:50 PM	#3
kronchev Cooling Savant Join Date: Mar 2004 Location: Lawrenceville, NJ Posts: 254	bigben2K: can you clarify that first part? it sounds like youre trying to say it WONT work but ive tried it with countless applications and have always had success... __________________ Ghetto riggin'!

05-10-2004, 09:50 AM	#4
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	It does work. You have to remember that PSUs are designed to have power come OUT of the positive voltage supply lines, then back to ground. When you do the 7 volt trick, the power comes out of the +12 volt line, but then you're expecting it to come into the +5 volt line, which the PSU just wasn't designed to do. However... If you have something connected to the +5 volt line, then the power can keep going through whatever you have connected there (at +5), and return to ground. Without any load connected between +5 and ground, the PSU's internal protection circuitry will shut down the PSU (in most cases). __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101

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