[ale] sometimes you just need to reboot NOW!

[Alex Carver]

On 2014-07-31 13:28, Michael H. Warfield wrote:
> On Wed, 2014-07-30 at 22:23 -0700, Alex Carver wrote:
>> On 2014-07-30 11:02, Jim Kinney wrote:
>>
>>> The good part is I can justify getting a BeagleBone Black box as an upgrade
>>> to the RasPi :-)
>>>
>>> Now if I can do a remote power detection and determine when the room power
>>> is running on battery without actually wiring anything to the units...
>>>
> 
>> What kind of UPS are they using?  If it's modified sine wave you could
>> make a filter that can hunt down the harmonics.  Relatively clean AC
>> will have low harmonics.  The modified sine will have very dirty
>> harmonics, enough to pick up and distinguish between the two modes.
> 
> You might be surprised.  It's been over 20 years since I looked at it
> but a simple inverter generating a triangular(ish) waveform into a 3
> break point diode shapper gets close enough to a sine wave that the end
> result is less than 1% THD without a heavy resonant circuit.  Most
> modern UPS with high efficiency switching supplies are significantly
> better than that.
> 
>> A true sine inverter would be harder to spot.
> 
> You're going to be really hard pressed.  Modern units might even
> generate cleaner sine waves out of those switching supplies than what
> you'll get from the primary power.
> 
> Regards,
> Mike
> 

1% THD is plenty to spot.  The wall power is nominally clean enough to
be well below 0.1% THD so you've got a 10x factor to play with.  All you
need to do is spot something near 180 Hz or one of the fractionals
(3/2f, 5/2f) to give away the source.

It's probably moot anyway.  I just double checked APC's offerings of
their SmartUPS series and they're all using full sine output.  Only the
very basic BackUPS series uses mod-sine.