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FW: Maximum Power for add in card more than 25W

Brad, you find that the power supply pin current capacity of the PCI
connector exceeds 25W. Good motherboard and backplane design dictates that
the pin capacity be supported with trace capacity. If this is done, the area
you will get into trouble is the thermal environment of the chassis or the
capacity of the power supply in case there is an overall heavy load.

In honesty, you can get away with exceeding the 25W limit in otherwise
lightly loaded systems. There are several downsides, especially if your bad
citizen card is of a nature where more than one would be installed in a
single chassis. Your company image can also suffer as you design out of spec
products. There might also be liability issues if an engineer designs a
critical system that becomes non-functional in difficult thermal
environments because he utilized your out-of-spec card in good faith.   

In other words, it would be imprudent to design a PCI card that dissipates
more than 25W, even if you can get away with it part of the time.

Good luck, Jim

-----Original Message-----
From: Klaus Bahner [mailto:kgb@tcelectronic.dk] 
Sent: Tuesday, December 19, 2000 10:08 AM
To: brad yu (by way of Daniel Weaver <dan.weaver@znyx.com>)
Cc: pci-sig@znyx.com
Subject: Re: Maximum Power for add in card more than 25W



PCI doesn't allow any card to consume more than 25W. If your card
exceeds this limit than it isn't PCI compatible. Period.

Of course your card can eat more power. I'm not aware of any motherboard
monitoring the actuall current drawn by a PCI card, so the board will
still be powered. However, since you are beyond the spec, don't be
surprised if your motherboard gets damaged due to overload on the power
supply lines, especially the power supply traces on the motherboard
print are the ones melting down first ;-)
In this case code PRST1/2 as 25W.

Bottom line, if you are designing a board exceeding the 25W limit for
your own test purposes (i.e. a controlled environment), this may be OK.
But if you plan to release your board to the public, then this is a no

And keep in mind that there might be other cards, which consume some
power too.


> My question is, how if an add in card consume more
> power than the maximum power that is shown by the
> PRST1# and PRST2# pin (i.e 25W)? What will happen and
> what are the workaround so that my add in board which
> consume more than 25W can works. Thanks.
> Cheers,
> Brad
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