The explanation of what I think is going on is a bit hard to describe: The base unit draws power from the highest voltage supply attached to the device using diodes to select between them. Where two supplies are roughly the same voltage it is sort of random which it considers the highest voltage since there are some diode tolerances involved - and if they are really close there is a possibility that it will pull from both.
When your power source failed, the wall wart was still most likely providing intermittent bursts of 12V, which the base unit was trying to use 100%, which would force the wall wart to drop power since it didn't really have much power available. I'm guessing if you had connected something like an oscilloscope to the wall wart input you would have seen voltages alternating between some low voltage and 12V fairly rapidly. The sitemonitor averages this long enough that you probably were seeing 8V because that was the average. Or stated differently: The wall wart would put out 12V, the base unit would try to use that since it was higher than your battery voltage, which would cause the wall wart to fail because it didn't have adequate input voltage, which would cause the base unit to switch back to the battery which would cause the wall wart to produce 12V again, which the base unit would use again, and the cycle would repeat, with an average voltage of ~8V. I'm guessing what is going on now is that the same thing is going on except that for whatever reason the wall wart is not dropping down below 12V like it was. Another mode that a wall wart with insufficient power can get into is that it raises it's voltage to just at the point that it provides some of the power to the sitemonitor and the battery provides the rest. If it tries to provide more, the sitemonitor will pull more power from it, causing the voltage to drop. So at this point, it seems like the wall-wart has switched to this 'tracking' mode for some reason - maybe there's a bit more or less power. I guess the executive summary would be that what you're seeing isn't 100% unexpected in the case where the wall-wart isn't being powered correctly but hasn't completely lost power. On Wed, Jan 20, 2021 at 7:34 PM Rob Genovesi <r...@corp.coastside.net> wrote: > Apologies in advance for the lengthy explanation, but hoping the > hivemind can help explain what might be happening here: > > We use packetflux sitemonitors to monitor utility power status and > battery voltage at many sites. > > Pwr1 is wired into a 12V wall wart power supply plugged into utility power. > > Pwr2 is wired to batteries of UPS. > > Pwr1 typically reads ~12V if the power is on and 0 when utility power > is lost and we know we are running on batteries. The other day is > started reading 8V so we new something was not right with utility > power. > > The battery voltage started falling, confirming utility power issue, > so a tech went to the site and plugged in a generator. > > As soon as the generator was plugged in Pwr1 went back up to 12V (even > though utility power was still funked). A meter on the AC confirmed > only 60VAC. > > Ever since then Pwr1 (AC) has followed the same readings as Pwr2 - > nearly 12V when the generator is running but once the generator quits > the voltage reading of Pwr1 drops and starts descending as the > batteries discharge. > > Attached is a screen shot of our monitoring of these events. > > Had the tech unplug the wall wart and Pwr1 went to 0V. > > I thought Pwr1 and Pwr2 were isolated from each other and not sure why > Pwr1 would start acting this way? > > Thanks in advance for any thoughts. > > > -Rob > -- > AF mailing list > AF@af.afmug.com > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > -- - Forrest
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