Hardware Reference
In-Depth Information
Note
The ATX supply features several voltages and signals not seen on earlier AT/LPX designs,
such as the +3.3V, PS_On, and +5V_Standby. Therefore, adapting a standard LPX form factor
supply to make it work properly in an ATX system is impossible, even though the shapes of the
power supplies are virtually identical.
However, because ATX is a superset of the older LPX power supply standard, you can use a
connector adapter to allow an ATX power supply to connect to an older motherboard using
AT/LPX connectors.
One of the most important issues with respect to power supply connectors is the capability to deliver
sufficient power to the motherboard without overheating. When talking about specific connectors, the
current rating is stated in amperes per circuit, which is a measure of the amount of current that can be
passed through a mated terminal that will allow no more than a 30°C (86°F) temperature rise over
ambient 22°C (72°F). In other words, at a normal ambient temperature of 22°C (72°F), when
operating under the maximum rated current load, the temperature of the mated terminals will not
exceed 52°C (126°F). Because the ambient temperature inside a PC can run 40°C (104°F) or higher,
running power connectors at maximum ratings can result in extremely high temperatures in the
connectors.
The maximum current level is further de-rated or adjusted for the number of circuits in a given
connector housing due to the heat of any adjacent terminals. For example, a power connector might be
able to carry 8 amps per circuit in a 4-position connector, but the same connector and terminal design
might be able to handle only 6 amps per circuit in a 20-position connector.
All the modern form factor power supplies since ATX have standardized on the use of Molex Mini-
Fit Jr. connectors for the main and +12V connectors. A number of connector housings are used with
anywhere from 4 to 24 positions or terminals. Molex makes three types of terminals for these
connectors: a standard version, an HCS version, and a Plus HCS version. The current ratings for
these terminals are shown in
Table 18.6
.
Table 18.6. Current Ratings for Mini-Fit Jr. Connectors
