I’ve been looking at some of the search terms from Google that have led people to this blog, and I’ve noticed a few terms that, while sketchily or partially answered in places herein, nevertheless don’t have a section or article dedicated to them.

With this in mind I’m going to talk about the subject of voltage, in terms of the voltages found inside a computer, and the matter of which components use what voltages.

As you may or may not know, the power supply unit or PSU inside a computer supplies a number of different voltages to certain components. In the older computers; and this is going back a number of years, from the early 1990s to around the turn of the century, computers relied a lot on the 5 volt supply from the PSU, particularly in the case of the motherboard; therefore the 5 volt rails in older PSUs provided a lot more in the way of wattage than it does on the newer PSUs.

Conversely, the newer computers; from just after the turn of the century until time of writing, rely a lot more on the 12 volt rails from the PSU; and therefore the bulk of the wattage is provided via these rails in newer PSU designs.

Let’s not forget the VLT or Very Low Tension supply rails which directly supply the processor itself with current, though.

On older motherboards; where the processor was a lot less powerful than its modern counterpart would be, a less direct method of powering the processor or CPU was used; that being that the 5 and 12-volt supply rails provided the supply to the motherboard, and the processor’s supply of power was extracted from the onboard continuation of those rails, regulated down to 2 volts or below, and applied to the processor itself.

With the greater power-demands of advancing processor technologies, however, this method was abandoned, on more modern motherboards, in favour of a more direct approach; that of powering the processor with its very own VLT supply provided by the PSU itself, which is then further regulated and stabilised by onboard power-regulation components, and applied to the CPU as a perfect, smooth, and flawless DC voltage.

In this VLT supply, a substantial amount of power is required to be supplied to the processor: A substantial amount of wattage could be drawn at any given time by any given processor, and that amount of power has to always be available should the CPU require it. At such low voltages, a wattage of 50 watts or more would mean that a substantial amount of current, 25 watts or more, would be required to flow in circuit: A PSU has to be able to provide these variable amounts of high-current at any time; particularly on its dedicated VLT supply rails. This is one of the reasons that a separate dedicated supply to the processor has been introduced.

Moving on, and as we go up in voltage we come to the… No, not the 5-volt supply yet: There is, in fact, another, perhaps little-known, voltage range in between the 5-volt supply and the former VLT supply to the processor; and that has to do with the dedicated SATA power connector(s) from the PSU.

At this point I’m going to quote from Wikipedia ( link ), with the particularly relevant parts enhanced to bold type: -

“The SATA standard specifies a different power connector than the decades-old four-pin Molex connector found on pre-SATA devices. Like the data cable, it is wafer-based, but its wider 15-pin shape prevents accidental mis-identification and forced insertion of the wrong connector type. Native SATA devices favor the SATA power-connector, although some early SATA drives retained older 4-pin Molex in addition to the SATA power connector.

SATA features more pins than the traditional connector for several reasons:

• A third voltage is supplied, 3.3 V, in addition to the traditional 5 V and 12 V.
• Each voltage transmits through three pins ganged together, because the small contacts by themselves cannot supply sufficient current for some devices. (Each pin should be able to provide 1.5 A.)
• Five pins ganged together provide ground.
• For each of the three voltages, one of the three pins serves for hotplugging. The ground pins and power pins 3, 7, and 13 are longer on the plug (located on the SATA device) so they will connect first. A special hot-plug receptacle (on the cable or a backplane) can connect ground pins 4 and 12 first.
• Pin 11 can function for staggered spinup, activity indication, or nothing. Staggered spinup is used to prevent many drives from spinning up simultaneously, as this may draw too much power. Activity is an indication of whether the drive is busy, and is intended to give feedback to the user through a LED.

Adapters exist which can convert a 4-pin Molex connector to a SATA power connector. However, because the 4-pin Molex connectors do not provide 3.3 V power, these adapters provide only 5 V and 12 V power and leave the 3.3 V lines unconnected. This precludes the use of such adapters with drives that require 3.3 V power. Understanding this, drive manufacturers have largely left the 3.3 V power lines unused.”

Now we move up to the 5-volt lines: The P1 connector, which is the large power-connector which is attached to the PSU’s output leads and connects directly onto the motherboard, carries 4 x +5V pins and a single –5V pin. This has to do with a dual-balanced 5-volt supply, using the ground pins as a 0V source. (See here.) (The –5V line isn’t actually used very much, if at all, by a lot of modern motherboards.) I won’t go into exactly how the motherboard utilises the 5-volt supply, as that’s beyond the scope of this article.

Finally; the 12-volt rails: On modern power-supplies there are a number of 12V rails, and with good reason: These rails power optical drives, hard-drives, and sometimes a graphics-card will require a dedicated 12-volt supply too. – With all that load, especially if you’re running a RAID array with any given number of hard-drives attached, the overall 12-volt power-requirement can be quite large.

If you want more detail on a power-supply-unit itself, then I suggest that you follow this link. Comments are invited.