Power Supplies with Robbie Chenault

Of all the components used in power supplies, such as generators, PC’s, modems and switches – most technicians understand the power supply the least. That’s unfortunate because power supplies are not all that complicated, and they are often the cause of puzzling, tough-to-troubleshoot problems. In today’s newsletter, we’ll explain some of the basics of power supplies, including how they work, what types are available, and how to test one for proper operation.

How a power supply works
The power supply takes wall current (120-volt, 60-Hz AC) and converts it to an appropriate level of DC voltage for the various components in a PC. Depending on the component, this can be +3.3V, +5V, or +12V. Generally speaking, the motherboard and any circuit cards use +3.3V or +5V, (newer motherboards and processors tend toward +3.3V, while older ones are usually +5V) and fans and disk drives use +12V.

Many power supplies also generate -5V and -12V, but those negative voltages are rarely used in modern systems and some of the newer power supplies do not even provide -5V support. Support for -5V is part of the ISA standard, but new systems being produced today are typically PCI-only, so they do not require this support.

What the wires do
Have you ever wondered why the plug from the power supply to the motherboard has so many different colored pins and wires? It’s to provide different voltages of power signals to the motherboard, which then parses them out to connected devices. The motherboard itself uses only +5V. Other voltages are routed to the ISA bus: -5V on pin B5, -12V on B7, and +12V on B9. Integrated serial ports on older systems use +12V; while on newer systems, they use +3.3V or +5V. All the other plugs coming out of the power supply—called Molex connectors—are for drives, and they provide +12V (yellow) and +5V (red) power, as well as two ground wires (black).

Types of power supplies
Power supplies are sold using two main specifications: the form factor and the wattage. Wattage is volts multiplied by amps. For example, you might see an ATX-style 250-watt power supply or an LPX-style 200-watt power supply.

LPX style is a descendant of the Baby-AT, AT/Tower, and AT/Desk type of power supply and is used primarily with Baby-AT style motherboards. The ATX style is used with ATX, Micro-ATX, and NLX-style motherboards. When selecting a power supply, you must make sure that it not only matches up with the motherboard type (so the connectors will fit) but also that it fits inside the case you are using. LPX-style power supplies have two six-pin connectors to the motherboard, while ATX-style power supplies have a single 20 pin.

For an ATX-style power supply, there is a single 20-pin connector, two rows of ten wires. The colors listed here are part of the ATX standard but are not required, so some off-brand systems might be different.

The wattage of the power supply refers to the maximum wattage of which it is capable. An extremely high-wattage power supply in a lightly loaded system is a waste, because the system draws only what it needs in terms of amps. That is not to say, however, that a high-quality power supply is a waste. High-quality power supplies can provide cleaner and more reliable power to a system and can help reduce sags and spikes from the wall current.

There are many other measures of a power supply’s performance, but these are not typically shopping specs. If you become a real hard-core hardware enthusiast, you might also want to compare the ratings of various power supplies for features such as MTBF, input range, peak inrush current, holdup time, transient response, over-voltage protection, maximum and minimum load current, and so on.

The next time you have a baffling hardware problem to troubleshoot, remember to check the power supply! For questions, please contact Robbie Chenault at rchenault@cccoa.com.