Computer Power Supply Page

    Computer power supplies

    The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer.In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan. Typical computer power supply generates the voltages needed by the computer motherboard accessories. A typical modern PC power supply generates the following voltages:

    • +5V (+-5%) at up to tens of amperes for motherboard electronics, disk drives and cards
    • +12V (+-10%) at several ampreres for disk drives and some cards
    • +3.3V (+-5%) up to tens of amperes for the majority of modern logic electronics in motherboard
    • -12V (+-10%) usully less than one ampere for some accessory cards
    Most other computer power supplies usually give voltage on this line, because those are the most commonly used voltage used in computer systems. Depending on the PC model, power supplies are rated anywhere between 150 and 350 W. A PC power supply on average is rated for 250-400 watts. PSUs above300W are unusual and only tend to come in servers, or machines thathave been designed for 'hardcore' applications such as gaming, where ahundred watt graphics card is not that unusual. Typically if you have 300W available, and the computer is going to be usingperhaps 150-220 W of that, depending on what's in it. Pc power are designed to provide +12, +5, -5 and -12 (usualy nowadays also +3.3V), with the power spread unevenly across those ranges. Grab an averagecomputer PSU and take a look at it, and there will probably be a tableon it listing how many amps can be delivered per voltage category.PC power supplies are mainly primary switching power supplies with power switches arranged in a half-bridge configuration. The outputs can drive the usual 20 A (+5 V), 8 A (+12 V) and 0,5 A (-12 V, -5 V) at approx. 205 W output power. (modern ATX power supplies add considerable amount of 3.3V to this).A typical efficiency of a PC power supply is around 75 %. A typical power PC supply measures around 140 x 100 x 50 mm (W, D, H) and weights around 300-400 grams. The switching frequency of approx. 33 kHz is usual for PC power supplies. The PC power supplies can be generally found at AT and ATX varieties. The older PCs used to use AT power supplies. Those power supplies supplied +5V, +12V, -12V, and -5V power to motherboard. Practically all new PCs use ATX format power supplies which have added to the picture following extra functions: +3.3V output, program/pushbutton turn on, standaby power (low current +5V output to some parts inside PC when the main power supply is off) and option to turn power supply off with software control.

    Laptop computers use slightly different approach for power supply. Modern laptop computer typically comes with a switched mode poer supply that plugs to the wall and supplies the needed power to the computer at some suitable low voltage. A typical voltage that those mains adapters supply to laptop are in 16-24V range, the actual voltage used can vary between different computer brands and models (check the computer manual and/or power supply markings for more details). The power supplied by this kind of power supply is typically in aroun d 40-60W range maximum (check your computer manual and/or power supply for information on your system). The internal power supply electronics inside the laptop then generate the multiple voltages needed inside the laptop (typically at least 5V, 3.3V and processor internal core voltage). If you need to power your laptop from car voltage (12V), you have two options to do this: use a DC to AC converter or a DC-DC converter. When you use DC to AC inverter, you first take the car power (typically 12V from lighter plug) and turn it to a normal mains voltage (110-120V AC or 220-240V AC depending where you live) power. Then the normal PC wall power supply is used to convert this power to voltage used by the laptop. This approach could work, but has it's downsides. The downsides are poor efficiency (power lost, both converter and laptop PSU get hot), and potential incompatibility with the DC to AC converter and computer power supplies. The DC to AC converters genrally do not like computer power supply type load (very non-linear load that takes high current splikes, can lead to unreliable operation and potential converter failure) and the computer power supply might not always like the non-sinusoidal mains power that is put out by most cheap DC to AC converters (can cause more heating on power supply, even power supply damages). An expensive high power sinewave DC to AC converter should work well with any load, also with computer power supplies, but is expensive. Another usually better approach is to use a DC-DC converter that replaces the original computer mains power supply. It takes in car 12V power and output the same output voltage that the normal mains adapter gives out. This kind of adapters are available from several laptop manufacturers. An adapter from the same manufacturer as your laptop is usually the easiest and safest choise, nut not usually cheapest option. Nowadays there are also quite cheap general purpose laptop DC-DC converters that can be adapted to be used with many different laptops. Those adapters have typically an adjustable output voltage (should be adjusted to match you specific computer). Just select an adapter that can be adjusted to your laptop operating and has high enough power rating (same or higher power rating as the original mains adapter), and things should work well. Please note that in some cases using a DC-DC converter not approved by the computer manufacturer can void your laptop warranty.

    Using PC power supply as general laboratory power supply

    Many people seem to have nowadays many old PC power supplies from old comouter aroudn them and seem to want to use them for some other applications. The PC power supplies supplies many voltages (+-12V, +5V etc.), but using those power supplies successfully takes some knowledge. The PC power supplies are generally switched mode power supplies that do not like run without a load (power supplies usually automatically shut down themselves then they see no load). The AT standard power supplies do not have any special signalto stay on. They are designed to work on certain power loadsrange correctly. If they are loaded less or more than they are designed to work they will shut down themselves. Generally you need to load the +5V output enough (usually 1 amprere to many ampreres minumum load needed depending onpower supply unit, youy might need to test this).The motherboard or old hard disk connected to power supplytakes at leas this minimum load, so keep the power supplyhappy. If you do not want such devices near you, you need to have some other type of load you can use, for examplewell cooled power resistor to +5V output or a 12V car headline bulb connected to +5V output (12V bulb willglow red/yellow when powered with +5V and take enough powerto guarantee that minimum needed load). The minimum load on +5V is needed, because PC power suppliesare generally regulated at +5V output. The controllign loopon the switching power supply works well on normal load conditions, but in case of too little load causes generally the+5V output to rise to too migh voltage, which leads to theshutdown of the power supply when it detect the overvoltage. Whatever power supply you want to use the best approach is tocheck the PSU spec: some require a minimum current draw on some rails. It is quite easy to provide this by means of a power resistor or power resistors. What is said above is usually valid for most other computer power supplies as well.All modern PC power supplies are nowadays built to match the newer ATX standard. Those power supplies have more signals on their connector for example for turning them on and off. If you plan to use this kind of power supply, you need to take care of the minimum load details told above. In addition to this you need to find out the details how you are supposed to turn this kind of power supply on. To activate ATX power supply, only PS_ON is needed. PS_ON can be activated by connecting ATX power output pin 14 (PS_ON) to pint 15 (ground). Some ATX PSUs also require a load on the main +5V to start (could be even damaged without enough load). Most but not all require a load on +5V to regualte properly, a few require aload on +12 as well. PWR_OK signal has nothing to do on ther controlling of ATX power supply operation. PWR_OK is an output from the supply. "PWR_OK is a "power good" signal. It should be asserted high by the power supply toindicate that the +12 VDC, +5VDC, and +3.3VDC outputs are above the undervoltage thresholds.


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