Computer Power Supply

• +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 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.

Laptop computers use slightly different approach for power supply. Modern laptop computer typically comes with a switched mode power 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 generally 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.

Check this out for more computer power supplies info and Computer Power Supplies on Amazon

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