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What happens When a PC Powers-up?

When you turn on a computer, its power supply must bring all the different voltages required by the motherboard and its peripheral components to acceptable levels, with enough current to power the whole system. These voltages include both plus (+) and minus (-) 12.0 volts (V), + and - 5.00 V and most likely +3.30 V (which may be converted to even lower voltages on the motherboard for the latest power efficient processors). [ Note: All these voltages will vary slightly for each combination of power supply, motherboard, disk drive(s) and various peripherals (since the number of electronic components drawing current alters a power supply's load), but its output voltages should never fluctuate upon reaching normal operating levels. If any of its outputs has an unacceptable amount of noise (or AC ripple^[1]), then one or more of the motherboard's components, such as its Memory, may fail the Power-On Self Test, keeping the PC's operating system from ever booting up.   In locations where a computer's AC input from the power company may experience problems or unexpected total loss thereof, the use of an uninterruptible power supply (UPS); which stores up a charge to power a PC for some reasonable number of minutes without any AC power, will allow one to save data and properly shut-down a PC before it uses up the stored power.]

Due to the sensitive nature of some IC chips (Integrated Circuits inside small metal, ceramic or plastic packages), especially the CPU (Central Processing Unit; which produces a great deal of heat, thus the reason for a large metal heat sink and fan on it), the motherboard is not allowed to power up until a power good signal is received from the power supply. This may take over half a second; a tremendously long time when compared to the speed of a CPU that can step through millions of instructions per second (MIPS), yet it often seems almost instantaneous to its human operator.

By the time we notice the sound of a drive motor spinning up or fan blades rotating, the voltages have already stabilized, power has been applied to all the chips (the interface chips that facilitate communications between the CPU and Memory modules; along with any audio, video or LAN cards plugged into bus connectors, and the controllers for peripheral devices such as keyboard, mouse, hard disk(s) and CD/DVD drives) and the motherboard's Clock-pulses are already synchronizing all those interactions. In what feels like a fleeting moment to us, the PC is already executing instructions from the BIOS (Basic Input/Output System) chip, and a short time later a speaker or transducer (built into the computer's chassis; note some motherboards may have a transducer mounted directly on them) beeps, one time, signaling the POST (Power-On Self Test) has completed without any errors.

The BIOS in Detail - A History of the PC's BIOS

NOTE: This page is still under construction

¹[Return to Text] In electronics, the terms "AC noise" and "ripple" often refer to what is usually a tolerably small, but nevertheless unwanted, AC component 'riding' on top of the DC output voltages. 'Ripple' is either the result of some Alternating Current that has passed through the rectified line frequency, or due to internal switching transients of a switching power supply, during the process of converting the AC input into the Direct Current required to power the equipment. When certain components of a PSU aren't 'up to standards' or begin to fail, this can result in more 'noise' or 'ripple' than your PC can tolerate, causing it to either shut-down, or never fully boot-up.

The power supply for this author's first self-assembled computer appeared to be functioning normally, since a voltmeter showed acceptable DC voltage levels under a small load. But the motherboard would never boot-up. Upon checking the outputs with an oscilloscope, we found too large an AC signal on at least one of them; undoubtedly the cause of our trouble. To convince the tech at the store, we simply asked them to connect a power supply from a working computer to our own, proving when it booted-up normally, that the first power supply was defective.

Revised: 15 December 2007 (15.12.2007).
Revised: 27 September 2010 (27.09.2010).
Revised: 21 October 2010 (21.10.2010).
Updated: 3 September 2012 (03.09.2012).

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