Hardware Reference
In-Depth Information
The pinout of the +12V power connector is shown in
Table 18.14
.
Table 18.14. +12V 4-Pin CPU Power Connector Pinout (Wire End View)
Using standard terminals, each pin in the +12V connector is rated to handle up to 8 amps of current,
11 amps with HCS terminals, or up to 12 amps with Plus HCS terminals. Even though it uses the same
design and the same terminals as the main power connector, the current rating per terminal is higher
on this 4-pin connector than on the 20-pin main because there are fewer terminals overall. By
counting the number of terminals for each voltage level, you can calculate the power-handling
capability of the connector, as shown in
Table 18.15
.
Table 18.15. Maximum Power-Handling Capabilities of the 4-Pin +12V Power Connector
This means the total power-handling capacity of this connector is 192 watts using standard terminals,
which is available to and used only by the processor. Drawing more power than this maximum rating
through the connector causes it to overheat, unless the HCS or Plus HCS terminals are used.
Combining the 20-pin main plus the 4-pin +12V power connector results in a maximum power-
delivery capability to the motherboard of 443 watts (using standard terminals). The important thing to
note is that adding the +12V connector provides the capability to support power supplies of up to 500
watts or more without overloading and melting the connectors.
Peripheral to 4-Pin +12V CPU Power Adapters
If you are installing a motherboard in a system that currently doesn't have the +12V connection for the
CPU voltage regulator, an easy solution
may
be available; however, there are some caveats.
Power adapters are available that convert one of the extra peripheral power connectors found in most
systems to a +12V 4-pin type. The drawback to this is that there are two +12V terminals in a +12V 4-
pin connector and only one +12V terminal in a peripheral connector. If the adapter uses only a single
peripheral connector to power both +12V pins of the +12V connector, a serious power mismatch can
result. Because the terminals in the peripheral connector are only rated for 11A, and the two terminals
in the +12V connector are also rated for up to 11A each, drawing more than 11A total can result in
melted connectors at the peripheral connector end. This is below the peak power requirements as
recommended by the
Power Supply Design Guide for Desktop Platform Form Factors
(
www.formfactors.org
), meaning these adapters do not conform to the latest standards.
I did some calculations: Assuming a motherboard VRM (voltage regulator module) efficiency of
80%, a CPU power draw of 105W would just about equal 11A, the absolute limit of the peripheral
connector terminal. Because most CPUs can intermittently draw more than their nominal rating, I
