[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.
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