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Points to Know about Your Computer Power Supply Unit

written by: omega•edited by: Lamar Stonecypher•updated: 12/7/2009

Computer power supplies are the main source of energy for any desktop computer. Their operating scheme, the meaning of colored wires inside, and the relationship between voltage and current to output power is outlined in this article. How to measure power supply voltages is also demonstrated.

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    Power Supply Unit Basics

    Ever wonder what is happening inside your computer supply? Computers cannot run without a source of energy. They require a proper power unit in order to be fed. Since the first computers, power supplies have been used for feeding every electronic device or circuit inside a computer. But how do these electric generators work and provide electricity to the computers? There's no magic inside. A power supply simply draws AC (alternating current) voltage from an electrical source and converts it to DC (direct current) voltage.

    A computer power supply unit consists of several components inside. There are coils, capacitors, an electronic circuit board to regulate current and of course a fan to cool down the whole unit inside a power supply. Fans are the number one failure reasons for power supplies. It’s true- a $5 fan may cost you more than ten to twenty times that amount when it fails.

    Besides these components, several cables should be attached to the circuit board with a specific number of wire sets in distinguishable colors. These sets of wires are used to transmit different voltages to the main board and other devices connected to it. A computer power supply draws 110 volt of electricity from wall outlet which is in the form of AC voltage and converts it to much smaller voltages in DC form.

    Today's modern power supplies are equipped with several safety circuits checking flowing current continuously. If any kind of extreme condition exceeding its power output capacity is detected, the power supply simply shuts itself down and prevents any possible further harm to itself or to the mainboard.

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    Computer Power Supply Voltages

    In a computer mainly three types of DC voltage are required to run. 12 Volts is necessary to feed the mainboard itself as well as any new age graphic cards, 5 Volts is required for the chassis and CPU fan or USB ports, and 3.3 Volts is used for the CPU itself. 12 Volts is also applicable for specific "smart" chassis fans. Thus in broader sense, a computer power supply can convert 110 Volt electrical current (or 220 Volt for the European scheme) to +12V, -12V, +5V, -5V, and +3.3Volt.

    The electronic circuit board in a power supply is responsible for forwarding the converted electricity through dedicated cable sets in order to feed the devices inside the computer. With the help of the components cited above, AC voltage is transformed into clean direct current. Nearly half of the work done by a power supply unit is carried out by capacitors inside. They help regulate the smooth, clean currents to precious circuits in your computer.

    But be warned. Even if your computer is unplugged, there is still chance of electricity being stored inside your PSU even days after you pulled its plug. That's what capacitors are used for. They store energy to be used for a continuous work flow.

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    Power versus Voltage Output

    What makes a computer power supply unique and capable of feeding the computer with sufficient energy is simply its capacity to produce power measured in Watts in almost every unit out there. Yes, every power supply has a decent capacity in terms of maximum power they can produce within a certain time period, and this specific capacity often leads to confusion as well.

    Let’s make things more clear. Since power means voltage multiplied by current from Electricity 101, computer power supplies can be interpreted as a power source to the extent limited by their capacity to produce maximum Watts per operating cycle. This capacity is regularly misjudged by consumers. (Manufacturers are responsible for this confusion.) Because real capacity can be calculated by adding up all the voltage outputs of every bit of wire set multiplied by maximum current they can produce. This specific capacity reflects the real output power of a power supply, but manufacturers prefer to use a broad numbering format which may lead to false interpretation of real output power.

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    Powe Supply Unit Color Codes

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    Inside power supplies you see bunch of colored cable sets coming out with different sockets or connectors and different numbers of wires each. Color codes of power supply cables are as follows:

    1. Black Wires: Those are used to provide a ground for the current. Every other color should be paired with a black wire.
    2. Yellow Wires: This color denotes +12Volt
    3. Red Wires: This color denotes +5Volt
    4. Blue Wires: -12Volt
    5. White:Wires -5Volt
    6. Orange Wires: 3.3Volt
    7. Green Wires: Control wire to check DC voltage
    8. Purple Wires: +5V on standby mode
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    Checking Power Supply Voltages - Power Supply Ratings, Laptop Power Supplies, Using a VoltmeterA computer power supply's output voltages can be measured with a proper multimeter. Using multimeter's nodes, specific voltages can be checked. But due to the higher risk of any short circuit, user must always connect black cables with black one on the multimeter and other colored wire with the other note making a pair. Black cable must always be used in the pair. We also look at laptop power supplies and how to determine power supply capacity by adding up the individual circuit ratings.
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    Interpretation of a Typical Power Cable Plug

    Let’s take a look at a hard disk power cable, no matter if it is IDE or SATA. It has four cables attached to the connector at the end, starting with a yellow one, then two black wires in a row, and the red one on the other end. A hard disk uses both 12V and 5V at the same time. The 12V feeds the moving mechanical parts, and the 5V feeds the electronic circuits. So, that's why all these cable sets are equipped with 12V and 5V cables at the same time. Even the electrical connectors on the motherboard for CPU or chassis fans have four legs, keeping the motherboard ready for either 12V or 5V fans.

    Apart from black, yellow and red ones, other colored wires can only be seen in the main connector which directly goes to mainboard's power socket. These purple, white or orange cables are not used by consumers to connect peripherals.

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    How to Check Power Supply Voltages?

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    Ok, so far we are all familiar with colored wires and their capability. But can these voltage feeds can be measured by consumers in order to be sure if they are delivering enough voltage? The simplest answer for this question is of course a multimeter (voltmeter). Turn on your multimeter, switch it to a DC voltage range, preferably with a limit of less than 20 Volts, connect your multimeter cables into the proper sockets (again, black is always to be used for grounding) and simply touch any sockets with your multimeter cable needles. You need to check your multimeter's manual prior to make a measurement, and of course you need to start your computer, too.

    Any possible misconfiguration may lead to some unwanted results. You must be very careful while operating such an experiment. The easiest way to make a measurement should be checking the bigger plugs of your power supply. Since they have bigger holes to plug your multimeter needles, you can attach the black cable to black and other cable to either yellow or red cable. Never connect your multimeter's own cables to red and yellow cables at the same time. This usage causes a short circuit and may harm your motherboard. Please DO REFER to our warning at the end of this page!

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    Power Scheme of a PSU Explained

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    Power supplies are capable of delivering different voltage levels, but they are all limited to a certain extent in terms of power capacity. For instance through a 12Volt channel, a power supply may only produce 30 amps (30A) of current. Thus if such a scenario exists that power supply can only generate a maximum of 240 Watt through 12Volt cables. That rule applies both to 5Volt and 3.3Volt channels too. So, together with adding up all the maximum current capacity multiplied by specific voltages, you can reach a final maximum power value of the power supply. But some power supplies limit their 5V and 3.3V output capacity as you can clearly see in the photo. The photo on the left illustrates a typical power supply's output capacity.

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    Why do We need to Check Power Supply Voltages?

    In fact, checking power supply voltages is not a piece of cake at all. And there may be no need to do that. But interested users may give it a try in order to see whether their power supply is functioning properly or not. In our simple test, we found about a slight change from 5V, for instance. Furthermore, since low voltage currents are under consideration, using too many long cables may also have an effect on voltage delivery. This is often the case when you use a 6ft (2 meters) long USB cable for your pen drive. Due to voltage loss throughout the long cable, your fancy pen drive or external storage unit that draws its energy from the USB port may not work properly.

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    Do Laptops also have a Power Supply?

    Every electronics device need a source of energy to operate. Running on battery power does not mean laptop don't have a power supply as in desktop computers. The cable plus adapter duo used to charge a laptop battery is in fact a power supply for your portable computer. The adapter acts like a power supply and converts AC voltage to DC and feeds either the battery or provides DC voltage and current to the power control card of your laptop. Laptop adapters do not have a fan and may cause some trouble if too much heat is generated inside. That's why we saw in the news about millions of laptop adapters being pulled back in the past for a free exchange with brand new ones resulting millions of dollar loss for big laptop manufacturers.

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    Disclaimer Concerning Power Supply Voltage Check

    In order to check or measure voltages of a power supply, you must read your multimeter's manual and get the operating instructions. Even though you are an experienced user, there may have been some sort of short circuits while trying to check voltages of a Power Supply. Power Supply voltages should not be manipulated by any means. Items such as your CPU, memory, or graphic cards may require voltage variations, but these variations have nothing to do with your power supply. Your motherboard's BIOS takes control of those variations.