At 07:04 -0700 5/27/09, w_tom wrote: > A UPS in battery backup mode can output some of the 'dirtiest' >electricity a computer may see. For example, this 120 volt UPS >outputs two 200 volt square waves with a spike of up to 270 volts >between those square waves; called a modified sine wave. No problem >for any computer. But may be destructive to surge power strips.
Analysis: w_tom must be using the 200V figure to represent the peak to peak voltage of the square wave. That's the difference between minus 100 volts and plus 100 volts with respect to the common wire and is what one would measure with an oscilloscope. The 120 volt value is called an RMS, for root mean square, voltage of what it quite close to a sine wave that is provider by the power company. The peak to peak voltage of that wave is 2 * sqrt(2) * 120 or 339 volts. RMS is used because it gives the same answers for heating power of a DC voltage with the same value as the RMS AC. A 120 volt incandescent lamp doesn't care if it gets 120 V RMS or 120 volts DC. Calculus-enabled folks will recognize RMS as as the square root of the integral of the square of the wave over one cycle. The "modified sine wave" is pretty bad but those circuits do pretty well at making the RMS value of their "square" waves come out to 120 V. What they do is modify the time ratio between on and off in response to the demand of the load. The waveform should be described as rectangular rather than square. Computers are happy with rectangular waves. In fact they would be happy with DC at about 339 volts. Motors are an entirely different story because the harmonic content of modified sine waves just heats them up without creating mechanical energy. That's one reason for 12 volt DC fans these days. The spikes from a UPS are not supposed to be there. They typically come from sudden interruption of current in an inductive load. That's not a computer but might be a motor or a badly designed transformer in the UPS. 270 volts is not terribly high but if repeated regularly it might damage the metal-oxide varistors that are placed in surge protectors after the UPS. -- --> In Christianity, man can have only one wife. This is known as monotony. <-- --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
