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What would've caused my power to do this? #171021 11/17/07 11:49 AM
Joined: Jan 2005
Posts: 59
Dawg Offline OP
This morning I was online when all the sudden the computer goes off. Then come to find out my home suffered a power outage.

Ok, no big deal, except my monitor (LCD flat screen) would flash on and then right back off....then on and right back off....un plug it and it would stop...but plug it back in and it would do this all over again.

But wait! How could it do this? The power is out!

Well then I go in to use the rest room, and forget the power is out, when I flip the switch. There is a faint glow.

I look up and low and behold the light was illuminated, but barely. It almost looked as if it were on a dimmer switch on the lowest setting.

I tried a few other lamps in my home. All of the incandescent lamps did the same thing (the fluorescent tubes wouldn't do it)....the filament would glow an orangish color, sorta looked like those old vacuum tubes they used to use in radios.

Went and grabbed my multimeter, flipped it on AC volts and stuck the probes in the 120 volt wall outlet.

I got a reading of 24 volts AC out of my 120 volt AC wall outlet.

I called the power company and reported the outage. I got no explanation on why it happened but in about an hour I had power again.

I'm just curious as to what would've (or could've) caused this to happen? I've been in power outages before, but not ones where there some power still there.

Anyone know?


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Re: What would've caused my power to do this? [Re: Dawg] #171023 11/17/07 01:16 PM
Joined: Oct 2007
Posts: 56
SP4RX Offline
I can't actually explain what happened, but to me it sounds like there was some kind of back-feeding going on. As though 2 line-side phases of your supply transformer went down, but the third was still live. It would still induce voltage into the secondary windings, but the voltage would be much lower than normal.
This is just an educated guess though, I've never seen this happen before. But I'm sure someone here will have a definite answer as to what would've caused a situation like this.


Re: What would've caused my power to do this? [Re: SP4RX] #171037 11/17/07 07:05 PM
Joined: Jul 2007
Posts: 265
wire_twister Offline
I have had one hot leg go completly away at my house, due to a transformer failure. Never seen just 24 volts though interesting problem hope someone can explain.


Life is tough, Life is tougher when you are stupid
Re: What would've caused my power to do this? [Re: wire_twister] #171051 11/18/07 12:12 AM
Joined: Aug 2005
Posts: 265
Sixer Offline
We used to have brownouts a lot before the poco upgraded their system. The lowest voltage I got at a receptacle was 42 volts. Across the street would have full power, and other parts of the subdivision would have none. It's been a while but I do remember a lineman telling me it was something to do with a 3 phase distribution transformer. Can't remember what was the exact cause.


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Re: What would've caused my power to do this? [Re: Sixer] #171055 11/18/07 01:23 AM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
I suspect an open phase on the primary might get some voltage reflected on the secondaries of the transformers on that missing phase through the other loads if you had a mix of wye and delta connected loads.

Greg Fretwell
Re: What would've caused my power to do this? [Re: gfretwell] #171057 11/18/07 02:06 AM
Joined: Nov 2005
Posts: 141
Check Pilot Offline
I've seen that exact same thing - 24 volts - we had one of the phases crap out and someone in the local area went and started up a genset and backfed the current into the grid without making the proper offlodading from their genset back into the line. Another case of a know-it-all Home owner just getting things back to normal.

Re: What would've caused my power to do this? [Re: Check Pilot] #171058 11/18/07 02:37 AM
Joined: Oct 2007
Posts: 56
SP4RX Offline
The only thing I don't get about the scenario Check Pilot is describing is that the OP mentioned being at his (or her) computer desk when the power went off and seeing his (or her) monitor flashing off and on.

If the 24V were due to a backup power supply back-feeding the supply transformer, wouldn't there be a time delay between the power outage and the appearance of the low voltage responsible for the monitor's erratic behavior? The DIY'er who so ingeniously discovered how to re-supply his house with home-made electricity would surely need several minutes, at least, to acknowledge the power outage and fire up the generator.

Maybe I'm misunderstanding the wording, but didn't the monitor's weird actions begin at the moment of power loss?

Re: What would've caused my power to do this? [Re: SP4RX] #171079 11/19/07 10:21 AM
Joined: Dec 2001
Posts: 2,491
Texas_Ranger Offline
Computer stuff can do all sorts of weird stuff... my ex-gfs computer would act off as you turned off the switch on the back of the ATX power supply and pushed the power button on the front. The power LED lit up and the floppy drive started making weird "read error" type noises. Never managed to figure out what was happening there.

Re: What would've caused my power to do this? [Re: Texas_Ranger] #171084 11/19/07 11:47 AM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
I agree with Tex
These TSR supplies can build up enough DC voltage with low line voltage to try to power on but as soon as the full load hits it the supply "undervolts" and goes back down.

Greg Fretwell
Re: What would've caused my power to do this? [Re: gfretwell] #171085 11/19/07 12:40 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
You can get the same thing with the switched-mode (chopper) power supply found in many modern TV sets when fed severely under-voltage. There's enough juice to charge the reservoir capacitor, the start circuit will try to get the oscillator going, but as soon as it draws current the voltage drops too low on the capacitor, so it drops out again, then the whole cycle repeats.

The typical results is a regular flashing of a power/standby light, sometimes accompanied by a regular "ticking" sound, etc.

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