Understanding AC POWER PROBLEMS

Semiconductor manufacturers of the microprocessors and memory devices, of which our computer systems are based, are approaching the smallest electronic cell element permitted by the laws of physics. The electrical over stress required to destroy state-of-the-art electronics, in most cases, is 100 times less than devices available 10 years ago. The driving force to shrink device geometries to increase speed and storage capacity will continue to make computers even more sensitive to ac power.

The major cause of system malfunction and failures is ac power noise. Semiconductor devices should last for around 1,700 years before they run out of electrons and fail. To provide proper protection to maximize system reliability, it is important to consider IC component tolerances, power supply operation and the ac power source as an interactive system. Condensing years of research and field test results on this issue, the following parameters are given for ac power affecting computer-based system performance. We all know about transients, brownouts and spikes. But let\'s review some of the facts behind the terms:

* Voltage transients or noise greater than 10V peak-to-peak riding on the ac sine wave (between line and neutral) can cause internal component degradation and eventual failure. This normal mode noise exists at high frequencies and, depending n the amplitude, can result in immediate system failure or a slow erosion over weeks or months. During this degradation period, system lock-ups, resets and data transfer errors increase.
* Voltage transients or high-frequency noise greater than 0.5V peak-to-peak measured between neutral and safety ground can result in system logic confusion. This common mode noise is especially important to eliminate because all logic components are referenced directly or indirectly to the ac power ground. Also, computer peripherals rely on the green wire ground as a zero-voltage reference to ensure proper data transfer through interface ports.
* Neutral-to-ground voltage (measured with a standard ac voltmeter) higher than 0.5 VAC can cause similar logic confusion and system disruption problems. This low-frequency voltage, commonly referred to as ground loops, also can cause interface board failures on networked systems.
* High-impedance ac power source: Switch mode power supplies predominately used in currend-day systems require a stiff power source to draw the peak ac currents for proper operation. In the presence of a high-impedance power feed, ac sine wave flattopping occurs with an increase in reflected ac nose and an increase in system sensitivity to this noise.
* Loss of ac voltage: Another attribute of switch mode power supplies is the ability to ride thru momentary power losses. The internal high-voltage capacitor typically has sufficient energy storage to maintain system operation for one to two ac line cycles. Ac power loss greater than 33 msec generally results in a halt of system operation. It is important to understand that because of the nature of power distribution systems, extremely high-amplitue and damaging noise impulses always accompany loss of ac voltage.

About Author:
\"AC Power Specialists & Consultants\"
Wesley E. Pettinger
HiTech Marketing Associates, Inc.
http://www.theupspeople.com