Power Supply Types
There are two basic types of power supplies. There are AT power supplies, which are older and in older computers, and ATX power supplies, which you will find in virtually every new computer you can buy.
There are two fundamental differences between AT and ATX power supplies. First, the switch mechanism is different. AT power supplies use a normal on-off switch, which directly turns the power supply on or off.
ATX power supplies use a momentary switch which does not directly control the power. Instead, the switch signals the motherboard, which performs one of three actions:
- If the computer is off, the power supply is turned on (which turns the computer on)
- If the computer is on, the computer goes into power-saving mode (standby)
- If the switch is held for more than 4 seconds, the power is cut and the computer turns off.
Because of this difference, ATX power supplies are better for projects that require the second power supply to turn on automatically when the computer is turned on.
The second difference is in the motherboard connector: AT power supplies provide two 6-pin connectors (figure 1), which are easy to insert backwards. The ATX connector is a single 20-pin connector that only plugs in one way (figure 2).
Figures 1 and 2: The difference between AT (left) and ATX (right) motherboard connectors.
Both power supplies provide two types connectors for plugging devices into. These connectors are called Molex connectors, and they come in two sizes (see figure 3 and 4). A power supply will generally have a few of each size.
Figures 3 and 4: Large (left) and small (right) Molex connectors.
There is no difference between the two sizes other than the size itself. Both sizes provide the same amount of power to whatever device is plugged into it (12V and 5V). The small Molex connectors are generally used only for floppy drives. Large Molex connectors power hard drives, CD/DVD drives, and many fans and lights as well.
Note: You can purchase large and small Y-adapters if you run out of Molex connectors. Be careful when using the Y-adapters however, because if your power supply does not have enough power for all the devices attached (especially true for older, lower-wattage supplies), you can damage it.
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