Hardware Reference
In-Depth Information
factorsforseveralyearstocome.TheATX12Vspecificationdefinesthephysicalormech-
anical form as well as the electrical connectors for the power supply.
From 1995 through early 2000, the ATX power supply form factor was defined as part
of the ATX motherboard specification. However, in February 2000, Intel took the power
supply specification out of the then-current ATX 2.03 motherboard/chassis specifica-
tion and created the ATX/ATX12V power supply specification 1.0, adding an optional
4-pin +12V connector at the same time. (Those with the +12V connector were called
ATX12V supplies.) The +12V connector was made a requirement in version 1.3 (April
2002),whereuponthespecificationbecameonlyATX12V.TheATX12V2.0specification
(February 2003) dropped the 6-pin auxiliary connector, changed the main power connect-
or to 24 pins, and made Serial ATA power connectors a requirement. The current version
isATX12V2.2,whichwasreleasedinMarch2005andcontainsonlyminorchangesfrom
the previous releases, such as the use of Molex High Current System (HCS) terminals in
the connectors.
As the ATX power supply specification has evolved, there have been some changes in the
cooling fan orientation and design. The ATX specification originally called for an 80mm
fan to be mounted along the inner side of the supply, where it could draw air in from the
rear of the chassis and blow it inside across the motherboard. This kind of airflow runs in
the opposite direction than most standard supplies, which exhaust air out the back of the
supply through a hole in the case where the fan protrudes. The idea was that the reverse-
flow design could cool the system more efficiently with only a single fan, eliminating the
need for a fan (active) heatsink on the CPU.
Another benefit of the reverse-flow cooling is that the system would run cleaner, freer
fromdustanddirt.Thecasewouldbepressurized,soairwouldbecontinuouslyforcedout
of the cracks in the case—the opposite of what happens with a negative pressure design.
Forthisreason,thereverse-flow cooling designisoftenreferred toasa
positive-pressure-
ventilation design
. On an ATX system with reverse-flow cooling, the air is blown out
away from the drive because the only air intake is the single fan vent on the power supply
at the rear. For systems that operate in extremely harsh environments, you can add a filter
to the fan intake vent to further ensure that all the air entering the system is clean and free
of dust.
Although this sounds like a good way to ventilate a system, the positive-pressure design
needs to use a more powerful fan to pull the required amount of air through a filter and
pressurize the case. Also, if a filter is used, it must be serviced periodically; depending on
operating conditions, it could need changing or cleaning as often as every week. In ad-
dition, the heat load from the power supply on a fully loaded system heats the air being
ingested, blowing warm air over the CPU and reducing the overall cooling capability.
