Chapter 3.2.1: Power Supplies
Power supplies are responsible for powering every component in a computer system. Power supplies perform the following functions:
*1) Convert AC power to DC power* -AC (alternating current) is the type of current distributed through wall sockets. The voltage alternates between a negative and a positive charge, which is good for appliances requiring a high current. -DC (direct current) is the type of current used inside a computer. Negative particles are drawn toward a positive charge, creating a unidirectional current flow. This type of predictable reliable current is ideal for an application where a lower current is required. *2) Provide components with the correct levels of DC voltage* -Standard ATX power supplies provide + 3.3 volts, +/- 5 volts, and +/- 12 volts of DC power. Most modern components require +12 volt output. -Each separate voltage output circuit is referred to as a rail and can power multiple devices. To avoid overloading one circuit, many newer power supplies have two or more +12 volt rails. These are known as dual rail power supplies. Separate rails balance the power load between multiple circuits, preventing anyone circuit from becoming overloaded. *3) Aid in thermal management* -All ATX power supplies have a fan that cools the unit. -The fan direction pulls cooler air from the front of the case and blows hot air out the back. -Older ATX units use a reverse air flow that blows air directly over the CPU. This method is not as efficient.
When troubleshooting a power supply, keep the following in mind:
*1) Symptoms of bad power supply include:* -The computer does not turn on -The computer sporadically shuts off or reboots -A broken or noisy fan *2) Before opening up the computer, rule out the obvious. Make sure:* The power cord is plugged into the wall. The power switch is in the on position. The voltage switch is set to the correct voltage. *3) Test the power supply using a multimeter or power supply tester. Voltage levels should be within +/- 5% of normal. If they aren't, the power supply is bad or failing and should be replaced.* -12 V rail should be between 11.4 and 12.6 volts. -5 V rail should be between 4.7 and 5.25 volts. -3.3 V rail should be between 3.1 and 3.4 volts. -When using a multimeter, the power supply can be turned on by bridging (shunting) pin 16 and a ground pin (i.e., pin 15 or 17) with a bent paperclip. -Because power supplies carry dangerous levels of electrical current, always take proper safety precautions. 1) Never ground yourself when working on a power supply. 2) Never open or disassemble a power supply. Always replace the entire unit. Some computer manufacturers, such as Dell or HP, produce proprietary power supplies. These power supplies might have a unique shape or use different wiring schematics on connectors. When replacing a power supply, identify whether a standard ATX or a proprietary power supply is required.
You should be aware of the following facts about power supplies...
-Power supplies should be matched to the motherboard and case form factor (i.e., match an -ATX power supply with an ATX motherboard or a Micro-ATX power supply with a microATX motherboard). -Most power supplies have a voltage switch that toggles between 115 and 230 volt electricity. 1) 115 volts is used in North America. 2) 230 volts is used in Europe. -Some power supplies eliminate the voltage switch and instead automatically switch between voltages as necessary. These power supplies specify a voltage range they can function in. -Many power supplies have a switch on the back that turns the power on or off. -Power supplies are rated in watts. A power supply's watt rating determines its maximum power output. To determine a computer's power requirements, use the following method: 1) Find the watt requirement for each component by multiplying volts by amps (W = V × A). 2)Add each value together to find the total watt requirements. -Alternatively, there are several online tools you can use to estimate a computer's watt requirements. -ATX power supplies provide *soft power*—even when the computer is turned off, the motherboard has power. Soft power allows the computer to be turned on and off by the operating system or over the network. -If your power supply does not have some of the required connectors (such as for the CPU, video card, or SATA device), you can purchase adapters to convert from one connector to another.
