3.2 Power Supplies

6-pin PCI Express

Newer video cards require more power than can be supplied through the PCI Express bus and from the main motherboard connector. The 6-pin PCIe connector, also known as a PEG connector (PCI Express Graphics), plugs in directly to the video card to supply the additional power. The 6-pin connector provides up to 75 watts. Instead of a 6-pin connector, some PCI Express cards require an 8-pin connector which provides up to 150 watts. Some power supplies have a combined 6-pin and 2-pin connector (6+2).

what does the Power Supply do?

Power supplies must be matched to the motherboard and case form factor. If you have an ATX motherboard, purchase an ATX power supply. Likewise, if you have a MicroATX motherboard, purchase a MicroATX power supply. The power supply converts AC current to DC current. AC (alternating current) is the type of current distributed through wall sockets. With AC, the voltage alternates (at a quick rate) between a negative and a positive charge. This type of current is good for appliances requiring a high current. DC (direct current) is the type of current used inside a computer. Negatively charged particles being drawn toward a positive charge create a direct current flow. This type of predictable reliable current is ideal for an application where a lower current is required. Standard ATX Power supplies provide + 3.3 volts, +/- 5 volts, and +/- 12 volts (DC power). Most modern components require +12 volt output. MicroATX power supplies only provide + 3.3 volts and +/- 12 volts (+/- 5 volt components are not typically used in MicroATX systems.) Each separate voltage output circuit is referred to as a rail. To avoid overloading one circuit, many newer power supplies have multiple +12 volt rails. Much like a circuit breaker in a house, separate rails allow you to distribute the power load between multiple circuits to prevent any one circuit from becoming overloaded. Each rail can power multiple devices. Most power supplies have the capacity to receive both 110 and 220 volt power just by toggling a switch (typically red) on the power supply casing. You can use this switch when using the power supply in other countries. When troubleshooting, make sure this switch is set to the correct voltage. 110 volts is used in the United States. 220 volts is used in many parts of Europe. Many power supplies have a switch on the back that turns the power on or off. Power supplies are rated in watts. The watt describes how much work or how much power can be supplied to various devices. The more devices you have in your computer, the more wattage you will require. You can calculate the system's wattage requirements with the following method: Find the wattage requirements of each individual circuit by multiplying volts by amps (W = V x A). Add the circuit wattage requirements together to find the total system wattage requirement. Power supplies include a fan that helps to cool the system. On older ATX systems, the fan direction blows air into the case and across the CPU. Newer (all current) ATX systems reverse the fan direction to pull air from inside the case (blow air out). System case fans help improve airflow. Current ATX cases typically pull (cooler) air in from the front, where the power supply and additional fans at the rear blow the (warmer) air out. An ATX power supply provides soft power. This is a condition where the motherboard always has power, even when the computer is turned off. This feature enables the operating system to power off the system and enables other features such as power on for network or other events. The power supply includes connectors for powering various computer components. When choosing a power supply make sure it includes the necessary connectors for your motherboard. Specifically, some motherboards and processors require an extra 4-pin and/or 8-pin connector in addition to the main 20- or 24-pin power connector. Power supply connectors are standardized following the ATX specifications. However, some computer manufacturers, such as Dell, produced power supplies with proprietary connectors. In some cases, the connectors are the same as ATX connectors, but the wiring positions might be different. When replacing a power supply, identify whether a standard ATX or a proprietary power supply is required. You can use a power supply tester or a multimeter to test a power supply. If the +/-12 volt supply coming from the power supply drops below +/-10.8 volts or if the +/-5 volt supply drops below +/-4.5 volts, then the unit is likely failing and should be replaced. To turn the power supply on for testing, insert a shunt on pin 16 and a ground pin (such as pin 15 or 17) on the motherboard connector. Symptoms of bad power supply include: The system does not start The system shuts off The system reboots Fan does not run or is noisy Never ground yourself when working on a power supply. Use a voltmeter (multimeter) to measure the voltage on internal connectors. Power supplies store dangerous voltages. Never open a power supply. Instead, replace the entire power supply.

4-pin +12 volt power

Starting with the Pentium IV processor (P4), CPUs required more power than could be provided through the main motherboard connector. In addition, processors are powered using 12 volts instead of 5 volts. The 4-pin 12 volt connector supplies 2 additional wires of 12 volt power. Note: This 4-pin connector is not the same as the 4-pin connector used in conjunction with the 20-pin motherboard power connector.

20-pin

The 20-pin connector is the main motherboard connector and supplies 3.3, 5, and 12 volts to the motherboard. On older motherboards, the CPU is powered through a 5 volt wire in this connector.

24-pin

The 24-pin motherboard connector replaces the 20-pin connector in older motherboards. The additional 4 pins supply an extra wire for 3.3, 5, and 12 volts. Some power supplies use a 20-pin connector with an extra 4-pin connector. Some 24-pin connectors allow you to disconnect 4 of the pins. You can plug a 24-pin connector into a motherboard using a 20-pin connector (leaving the 4 extra pins unconnected).

4-pin accessory power

The 4-pin accessory power connector (often called a 4-pin Molex connector) is used by IDE hard drives, optical drives, and other accessory devices. The connector provides both 5 volts (red wire) and 12 volts (yellow wire). Each power supply cable typically has multiple 4-pin connectors on the same cable. When connecting devices, try to balance the devices connected to each cable.

4-pin mini-Molex

The 4-pin mini-Molex connector provides both 5 and 12 volts and is used by floppy drives.

8-pin EPS +12 volt

The 8-pin EPS connector provides 4 lines of 12 volt power. This connector is used with some older dual processor systems or some newer quad-core processors. Depending on the processor and the motherboard, you might be able to use a single 4-pin connector instead of the 8-pin connector (all 8 pins are typically required for quad-core processors). Some power supplies have two 4-pin connectors (4+4) that are meant to be used together in the 8-pin connector.

SATA power cable

The SATA power cable has 15 pins and provides 3.3, 5, and 12 volts. As its name implies, it is used for powering SATA devices. You can use a special adapter to convert a 4-pin Molex connector to a SATA connector. When using an adapter, or on some power supplies, the connector only supplies 5 and 12 volts.