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Since87 · 02-25-2003, 09:16 AM

Ok, I'm an EE and I've designed switching power supplies.

I would strongly recommend just getting a beefier supply, but...

There are ways to do this. A 7805 is one way, although it is extremely inefficient. One Amp of current in at 12V provides one Amp of current out at 5V. You get an extra 5W available on your 5V rail, while 7W are dissipated in the 7805.

A better way to do it would be with a switching DC-DC converter, but such may be difficult to find cheaply. Assuming reasonably easy to achieve efficiency; One amp of current in at 12V provides around 2 amps of current out at 5V. You get an extra 10W available on the 5V rail, while 2W are dissipated in the DC-DC converter.

The problem is that small regulators (switching or otherwise) are not usually designed to share an output with some other regulator. (Your PSU) The two regulators, which are both trying to control the output voltage, may get into a pissing contest that blows your system up.

If you are going to do something like seperate the 5V rail for the drives, and use the additional rail only for powering the drives, you can avoid the pissing contest scenario, but how much trouble is this worth?

There really is no simple method of doing this, that is going to be very effective and/or reliable.

Edit: Just noticed you have a 550W supply. I guess the best thing I can recommend is to add an additional AT supply just for powering all those drives.

02-25-2003, 09:16 AM	#10
Since87 Pro/Guru - Uber Mod Join Date: Sep 2002 Location: Indiana Posts: 834	Ok, I'm an EE and I've designed switching power supplies. I would strongly recommend just getting a beefier supply, but... There are ways to do this. A 7805 is one way, although it is extremely inefficient. One Amp of current in at 12V provides one Amp of current out at 5V. You get an extra 5W available on your 5V rail, while 7W are dissipated in the 7805. A better way to do it would be with a switching DC-DC converter, but such may be difficult to find cheaply. Assuming reasonably easy to achieve efficiency; One amp of current in at 12V provides around 2 amps of current out at 5V. You get an extra 10W available on the 5V rail, while 2W are dissipated in the DC-DC converter. The problem is that small regulators (switching or otherwise) are not usually designed to share an output with some other regulator. (Your PSU) The two regulators, which are both trying to control the output voltage, may get into a pissing contest that blows your system up. If you are going to do something like seperate the 5V rail for the drives, and use the additional rail only for powering the drives, you can avoid the pissing contest scenario, but how much trouble is this worth? There really is no simple method of doing this, that is going to be very effective and/or reliable. Edit: Just noticed you have a 550W supply. I guess the best thing I can recommend is to add an additional AT supply just for powering all those drives.