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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:
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 above 300W are unusual and only tend to come in servers, or machines that have been designed for 'hardcore' applications such as gaming, where a hundred watt graphics card is not that unusual. Typically if you have 300W available, and the computer is going to be using perhaps 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 average computer PSU and take a look at it, and there will probably be a table on 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.
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 signal to stay on. They are designed to work on certain power loads range 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 on power supply unit, youy might need to test this). The motherboard or old hard disk connected to power supply takes at leas this minimum load, so keep the power supply happy. If you do not want such devices near you, you need to have some other type of load you can use, for example well cooled power resistor to +5V output or a 12V car headline bulb connected to +5V output (12V bulb will glow red/yellow when powered with +5V and take enough power to guarantee that minimum needed load), The minimum load on +5V is needed, because PC power supplies are generally regulated at +5V output. The controllign loop on 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 the shutdown of the power supply when it detect the overvoltage.
Whatever power supply you want to use the best approach is to check 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, Some ATX PSUs require a load on the main +5V to start. Most but not all require a load on +5V to regualte properly, a few require a load 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 to indicate that the +12 VDC, +5VDC, and +3.3VDC outputs are above the undervoltage thresholds.
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