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RE: PCI Power Rating
This information is in the PCI Specification. You should have a copy
available if you are designing anything PCI ... or at least one of the
books about PCI that covers much of the same information.
The maximum power for one card is 25W. There are two lower max. power
options, depending on the PRSNT* pins.
However, an N-slot system might not have N*25W available for all four
slots, i.e., all four at maximum. Such a system could check the PRSNT*
pins while powering up and decide not to power up if the installed cards
go over its total power capability.
The maximum +12V current is 500mA.
The maximum current that you could draw from +3.3V is 7.6A, because that
works out to 25W, assuming zero current on all the other supply
voltages. But this limit is "system dependent," so a given system might
not have the full 7.6A available for a slot even if it is the only PCI
card in the system. A system might balance the power capabilities
differently between the +5V and +3.3V (and +/-12V) supplies, rather than
making 25W available from +5V and 25W available from +3.3V.
The maximum current from +5V is 5A, with the same conditions as above
for the +3.3V supply.
> I understand that I should not be relying on the +5V supply as not all
> motherboards will provide this voltage. Is this correct ?
I don't know about that ... but there was a time when the +3.3V supply
was optional and many systems did not provide it.
Presently, both +5V and +3.3V power are required on all systems that
support PCI revision 2.2 or 2.3.
If there are motherboards that don't supply +5V, they would not comply
with the current PCI specs.