Getting -5V using a PICO power supply

[Pasky]

  So I want to build a little project box and plan to use a laptop power supply to power a 200W PICO power supply.  The PICO supply outputs 5V, 12V, and -12V.  However,  I am also going to be needing a -5V output as well being able to push 1A maximum on it.

  This is the PICO supply I was planning to use:

http://www.ebay.com/itm/250W-DC-12V-mini-Pico-PSU-PC-ATX-Power-Supplies-ATOM-HTPC-MINI-ITX-Power-Supply-/161624642521?pt=LH_DefaultDomain_0&hash=item25a19453d9

Is it as simple as wiring up a 7905 to the 5V rail to output -5V (EDIT: I know this won't work)?    If so how do I calculate the amount of current I will lose off the +5V rail that will be used to convert it to -5V 1A?  Is this the wrong way to go about this?  Thanks for any help and advice you can provide!

EDIT:

I think I misread the 7905 datasheet, it wants a negative voltage as the input, so that won't work...hmmmm

[Psi]

7905 needs a negative voltage. You could feed your -12V to a 7905 and get -5V.
But you cannot use it to convert positive voltage to negative voltage.

Sadly your -12V is only 100mA, so you cant get -5V at 1A

Are you sure you need 1A at -5V.  Most computers dont use anything like 1A on the -5V rail if they even use it at all.
The -5V rail was made optional in ATX spec in 2002 so to fully comply with the spec any PC manufactured after 2002 should not require -5V

[Pasky]

Yes, it won't be supplying computers, but an arcade PCB.  These usually used the -5V rail to power the sound and audio amplifiers and the 1A is necessary.

[wraper]

EDIT:

I think I misread the 7905 datasheet, it wants a negative voltage as the input, so that won't work...hmmmm

Did you expect it to be some kind of magic device reversing polarity? 

[Pasky]

EDIT:

I think I misread the 7905 datasheet, it wants a negative voltage as the input, so that won't work...hmmmm

Did you expect it to be some kind of magic device reversing polarity? 

Yes, .  For some reason I truly thought it could haha.  Reading up on charge pumps, supposedly this is what they do.  Trying to find something I can work with to get my -5V.

[wraper]

Yes, .  For some reason I truly thought it could haha.  Reading up on charge pumps, supposedly this is what they do.  Trying to find something I can work with to get my -5V.

Charge pump at 1A current is insane. You need Inverting buck-boost converter.

[Pasky]

I see.  Most are outputting 100mA only as well (buck boost).  Gonna keep looking, thanks.