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Need a single use 120vac power outlet Answered

Ttrouble shooting HVAC issue. Looking for a device that plugs into 120vac outlet. On until micro/millisecond power outage and if power goes off, will not accept power again until reset.

Sort of a GFCI that trips NOT on a short, but on a power loss.

Issue is that it seems HVAC gets quick power glitch and the condenser/compressor goes off (then on), and the evaporator/blower (with thermostat transformer) does not go off. We think this is what might be happening and need to prove it. Compressor goes into short cycle because thermostat is always "hot".

Ohhh, forgot to say why - to prove that the issue is that the evaborator/blower does not go off with the power glitch, but the compressor does.


Jack A Lopez

10 days ago

Kind of the funny thing about AC mains power, in particular the single phase variety, is that it actually does turn itself off periodically.

In fact, the power turns off twice every cycle, or once every half-cycle, or approximately every 8.33 mS (milliseconds), which is (1/120) seconds.

Technically the power is only completely off (i.e. zero) instantaneously, but if you are willing to widen your definition of "off" to include those times when the instantaneous power is less than 10% of its average value, then there is actually an interval of time, lasting a few milliseconds, every half cycle, when this condition is true.

Yet somehow all our mains powered loads, or most of them, seem to ride through these "little dips" in instantaneous power, which occur every half-cycle.

But the big "dips" are different from the little ones, and I think that is the animal you are hunting; i.e. the power quality event you hypothesize to be messing with your compressor.

Which reminds me, the word, "dip," or "sag," is real jargon used by people who contemplate power quality. From the Wikipedia article for, "Electric power quality",

"A 'dip' (in British English) or a 'sag' (in American English the two
terms are equivalent) is the opposite situation: the RMS voltage is
below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute."


So you are maybe wondering about ways to detect these voltage dips when they occur?

One way to do this, is to have a cheap UPS (uninterpretable power supply)


plugged into the same circuit, and then listen to the noises it makes.

This can take some patience, as voltage dips can be happen rarely or frequently, depending on the character of the mains power. Where I live, out in the sticks, a plugged-in UPS is pretty much guaranteed to beep and click, at least twice a day, for no apparent reason. (Sometimes there is an obvious reason; e.g. it is one of my loads, like a washing machine motor starting, and pulling the mains voltage down briefly.)

The "beep" is a warning cry, to strike terror into the hearts of any users listening nearby. The "click" is the noise it makes when its relay actually switches its loads over to battery power, usually just for a second or so, before switching its loads back to the mains power.

But of course, you have to be near the UPS when it beeps, for to notice it noticed something amiss with the mains voltage.

Perhaps that is why you want to build a breaker tripped by voltage dip?

It would kind of like a mousetrap, but triggered by voltage dip. So that you would not have to hang around waiting for a voltage dip to occur. Rather you just set the thing, and then come back and check it later. Presumably a sprung mousetrap suggests some thing triggered it, and it is obvious in those cases when the trap contains a dead mouse.

One easy way to build such a thing, would be to just wire a relay, with a coil made to be powered by 120 VAC mains, so its coil is in series with the contacts that close when it is turned on. That circuit also requires a momentary push button, for to turn it on.

I am not sure how sensitive such a relay would be to sensing voltage dip; i.e. how long of dip, how many cycles or seconds needed to cause it to trip off (and stay off), but I thought it would be worth mentioning, just because something like that would be super easy to build.

As an alternative to this "mousetrap" approach, you might also consider building some kind of circuit to produce an attenuated (like by a factor of 1000, e.g. from 120 volts to 120 millivolts), and galvanically isolated, version of the mains voltage, and then just feed that to any kind of audio recorder capable of recording several hours of audio.

I mean the majority of that audio signal is just hours and hours of 60 hertz, "mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm,"

However, there will be something different at those moments when the voltage dips occur, if and when they occur.

I don't know if the dips are audible, but they would be visible when examining the signal with a sound editor, or perhaps detectable by examining the signal in some other way, like by writing a script to examine the signal data, e.g. a script in Octave, or MATLAB, or LabVIEW, whatever software you use for general purpose number crunching.

To make an analogy, recording several hours of the mains voltage signal, would be less like mousetrap, and more like video camera watching the barn or the kitchen, trying to capture images of things shaped like mouse, moving like mouse, for to detect the presence of mouse.

Although I admit the mousetrap approach is simple, and sometimes a simple approach is all that is needed.