Drive Power Connectors

The power supply provides power to internal hard disk, floppy disk, CD/DVD and other drives directly, through four-wire connectors that are designed to attach to the rear of each drive. The four wires provide +5 V and +12 V power, along with two grounds, to the various drives that use them.

Schematic diagram of a large style hard disk drive cable
connector (the small style is wired the same, the shape and
size are just different, as shown below. The wires used for
these connectors vary widely from system to system. For
the larger style connector, AWG 16 to 18 are common. For
the smaller size, AWG 18 down to AWG 22 are
sometimes specified (since floppies draw little current).

The connectors themselves come in two basic styles. The larger size, often called a Molex connector (after the name of one of the big connector companies) is keyed by virtue of the connector itself being "D-shaped", and is used on most internal drives, including hard disk, CD/DVD, Zip and other removable media drives, and the older 5.25" floppy disk drives. The smaller size, typically called a "mini-plug", is used for the newer style of 3.5" floppies. It is also keyed, but in a different way than the larger connector, and actually secures to its mating connector with a retention clip of sorts.

Large and small disk drive connectors. The tab in the middle bottom of
the small connector (not shown) fits into a retention clip on the floppy disk
drive and holds it in place. The large connectors secure simply by friction
(which can be a lot stronger than it sounds like, believe me!)

The number of connectors that come with each power supply varies considerably. In general, the bigger the supply, the more devices the manufacturer expects you to run, so the more connectors are included. Totals can range from 3 or 4 connectors to as many as a dozen. Another factor is just general quality; some makers skimp on the connectors to save money, and make you buy adapters to let you run additional drives. These adapters, usually called "Y-splitters" or "Y cables" after their general shape (sort of :^) ) contain one male and two female large-style hard disk connectors, cost around $2-5 in the U.S,. and are available in most electronics or computer stores. They are increasingly needed in modern systems, because not only do newer systems have more drives, they also have more fans and cooling devices, which also often attach using a disk drive power connector. Do remember, however, that adding a Y-splitter doesn't magically increase your power supply's output capacity! It just gives you more connectors.

Warning: It is best to avoid Y-splitters if possible, for a few reasons. First, there have been reports of incorrectly wired Y-splitters. Watch out for them, as they have the potential to damage your equipment. (It's pretty easy to see if the adapter has been wired correctly by inspecting it carefully. Using an ohmmeter to test for correct connectivity is even better.) Second, they are an additional potential source of failure in the system, and are often cheaply made and hard to align and plug in properly. Third, they can further clutter the inside of a busy case. Fourth, every time you share two devices on a single connector, all the power drawn by the two devices has to travel down the same set of wires from the power supply. If you chain three or four drives off the same connector using multiple splitters you may exceed the current rating for the wires and/or connector. See the discussion of wire size and resistance in the section on the motherboard connectors for more details.

A Y-splitter cable for running two drives off a
single large hard disk cable connector.

Image � PC Power & Cooling, Inc.
Image used with permission.

Most systems only come with a single mini-plug connector--because most systems come with only a single floppy drive that uses it--but some may come with two. If you need an additional mini-plug connector in a system with only one, or if the first one breaks, you can employ a simple adapter to change a large style into a small style. (They may make them the other way as well, but I am not sure.) You may also on occasion see mini-plugs, or mini-plug adapters, that only use two wires instead of the standard four. They omit pins 1 and 2 because newer floppies may use only the +5 voltage. This won't cause problems in most cases, but beware.

An adapter to change a large-style drive
power connector into a small style connector.

Image � PC Power & Cooling, Inc.
Image used with permission.

A couple of final thoughts. The power supply fan in your PC runs directly off a connection within the supply itself, but additional fans within the case are growing in popularity, and typically each of these requires a drive connector. They don't draw very much power (but fancier electrothermal coolers can draw quite a bit). Finally, the hot-swappable drives used with RAID in newer server boxes typically do not use standard drive connectors for their drives. They make use of a technology called single connector attachment (SCA). In this scheme, special bays are installed in the system, which take either standard power connectors from the power supply, or a special wiring harness. The drives themselves plug into the bays and draw all their power and signals from a single mated connector, one half on the back of the drive and one half inside the bay. This allows them to be easily removed while the system is still operating.