Asimov wrote:

"Jeff Wiseman" bravely wrote to "All" (28 Aug 04 17:52:25)
--- on the heady topic of "APC Back-UPS 280B faulty - looking for ideas"
JW The setting on the potentiometer for the output voltage of the
JW inverter section was physically set originally at about 2/3 full
JW scale. With a 40watt load and the unit unplugged (output voltage
JW at outlet around 85 VAC), I turned the pot up to full scale. This
JW resulted in the output climbing to around 119 VAC.

JW Although I can (just) get the output up to a "working" level, I
JW would not trust it, it's stability, or load handling since
JW something has obviously gone wrong with it. I do not own or have
JW ready access to a 'scope so I could not examine the output
JW waveform or frequency (I own a Fluke 27 Multimeter)

I recall that most multimeters are calibrated with pure sinewaves and
their internal precision rectifiers measure the average value of the
waveform. Thus depending on the shape of the BUPS's output waveform the
reading may be inaccurate. The meter may indicate a correct reading
from the normal AC line but then read low when the BUPS is running.
I think an AC RMS meter would be needed for a proper calibration.


I believe that the Fluke 27 does read true RMS (it was a $300
meter 14 years ago). The fact that the 40 watt bulb also goes
extremely dim when the plug is pulled also reinforces my believe
that the reading is likely reasonable.


I would try loading the BUPS to a significant percentage of its
ratings and see how the output voltage is affected. If there isn't
much change, say less than 10%, then I would suspect the reference
voltage or the feedback divider chain. Resistors do go open circuit,
capacitors get leaky, solder joints and connectors can go bad, etc.


When I get the chance I'll try a 150 watt bulb (that's over half
the UPS's max rating) but I still find it a little difficult to
believe that a 280 VA UPS wouldn't work properly to protect a 40
watt load but I'm not real familiar with the normal operating
conditions of these. I have a Tripplite 550VA UPS that will
continue to properly supply just a simple 14 watt flourescent
light when it is unplugged with no side effects (although its UPS
type may be different than the APC I have).

And although circumstantial, I'm still convinced that there is a
non-battery related failure in the APC. If I could track down a
schematic or something it would really help.

- Jeff