[Somewhat off-topic, but I see this misinformation so often I'll reply for the archives.....]
On 07/19/2013 01:01 PM, John R Pierce wrote:
On 7/19/2013 5:51 AM, Darr247 wrote:
It should also be a 'true sine wave' output when running on battery. Many UPS units output a 'stepped approximation' (typically pulse width modulation), which some computer power supplies may not like.
virtually all PC and server power supplies now days are 'switchers', and could care less what the input wave form looks like. they full wave rectify the input voltage to DC, then chop it at 200Khz or so and run it through a toroidal transformer to generate the various DC voltages.
Uh, while it's true that switching supplies are the norm these days, it's also true that a square wave or even a modified sine input waveform, even if the same RMS voltage, will have a lower peak voltage that might be out of range of the input rectifier(s) in the power supply.
Do the math; for a full square wave, V(rms) = V(peak) so a 120Vrms square wave has a peak voltage of 120V. In contrast, a sine wave with a peak voltage of 120V has a Vrms of only 84.8 volts. The input rectifiers, which work on peak voltage, when powered with a 120Vrms sinewave sees a peak voltage of 169.73 volts. Even power supplies that are rated 100-240Vrms aren't rated for peak voltages less than 141.42 volts. So a 120Vrms square wave has a peak voltage well below the tolerance for a typical 100-240Vrms rated power supply. This is the reason for modifed square wave UPS's, which get closer to the 169.73 peak voltage, but are loaded with odd-order harmonics that can play havoc with power-factor correction circuits in less-expensive but newer supplies.
So the waveform does matter, and a sine-wave or multi-step modified sine wave UPS is more likely to work, even with less well-designed supplies (I would say cheaper, but I've seen expensive supplies balk at anything but a true sine wave).
My standard example of this is the 90A 5V supply used in pairs in 3Com's CoreBuilder/CellPlex 7000 ATM switches. These were rather expensive and beefy supplies, but trying to power them with a modified sine UPS simply did not work, but a true sine UPS worked fine (and the glacial ATM PNNI reconvergence times when one or more of the five core switches went down for a short glitch wreaked havoc on our network!).
I also have here in production an older high-end industrial PC with a redundant power supply made by Astec that would consistently drop out with an alarm under modified sine wave UPS power. It works fine with the APC SmartUPS PWM-derived sine wave. A Cisco 7609 I have here, with non-Astec supplies, also does not work well at all with anything but a true sinewave UPS.
True-sine UPS's are desireable and even necessary for maximum compatibility, even with modern power supplies.