Re: Flourescent lamp -Wasting Power?
From: John Popelish (jpopelish_at_rica.net)
Date: 12/19/04
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Date: Sun, 19 Dec 2004 14:10:20 -0500
Jeff Wisnia wrote:
>
> Fred Bloggs wrote:
>
> >
> >
> > Ban wrote:
> >
> >> John Fields wrote:
> >>
> >>>> If the lamps would use 9W they would be *very* hot, I suspect the
> >>>> most current is not "real" in phase with the voltage, but imaginary
> >>>> as some capacitive or inductive load. I would guess 1-2W as you
> >>>> descibe it, much like the small low voltage xformers when idle. You
> >>>> cannot measure the power consumption with an amperemeter, even if it
> >>>> is true rms, but you need a dedicated power meter for it with
> >>>> 3connections at least.
> >>>
> >>>
> >>> ---
> >>> You can do it with an oscilloscope and a resistive shunt to measure
> >>> the current. Display voltage on one channel, current on the other,
> >>> determine the phase angle between them and then P = IE cos(phi).
> >>>
> >>> You could also do it with a current transformer once you've determined
> >>> its primary-to-secondary phase shift.
> >>
> >>
> >>
> >> John, what you suggest is potentially lethal, as you will have to
> >> connect a power line directly to the case and earth, hopefully
> >> tripping only the earth leakage circuit breaker.
> >> You will need two transformers, which are isolated on the secondary,
> >> or you could use an isolation transformer in the power line. But all
> >> this is *not* appropriate for an unskilled hobbyist.
> >
> >
> >
> > The OP is an MIT EE grad...note his signature file- he is definitely
> > "old school"- very hands on:)
> >
>
> You guys are correct, I didn't take the phase angle into into account, I
> just wired a 25 ohm wirewound resistor in series with the lamp and
> measured almost 2 volts ac across it (lamp switched off) with my olde
> Simpson 260 VOM. I can't properly relate the temperature my fingers felt
> to the real power being dissipated by that electronic ballast. The board
> was mounted in liberally slotted flimsy plastic cage in the center of
> the two nested circline flourescent bulbs in the lamp's bowl. There was
> good convection cooling through those slots, but who knows, maybe it was
> only 2 watts, that wasn't my point 'yknow.
>
> But, thanks for accepting that even in retirement I could probably drag
> my two channel scope off the shelf, hit the "hellbox" for parts and
> kludge up a setup to measure the real power - without frying myself.
>
> But, I'm reminded of the words of the MIT prof who tought a course in
> "rotating machinery". He was a Brit, and while we guys were hacking
> around with a bunch of 240v 3 phase apparatus with big exposed knife
> switches, motors without guards over their live parts, and stuff like
> that, he proclaimed, "You boys will never become real engineers until
> you learn to 'take' a shock". (Thanks for the mammaries.)
>
> I was obviously overly incensed by what I percieved as a wasteful and
> stupid design that I just multiplied current times voltage in my head
> and was probably wishing for a high number to further justify my angst.
>
> Nonetheless, the principle is the same, and I'm still wondering if there
> was any reason other than saving a few pennies to design those damn
> lamps that way.
>
> The two other "two tube" flourescent torchier lamps we have in the house
> have two separate skinny electronic ballasts hidden inside the pole,
> controlled by a single pole four position rotary switch which sequences
> like the one in a "3-way" incandescent lamp, except it controls the
> power to the inputs of the two ballasts rather than to the filaments of
> a 3-way bulb. (This I know because I've had to replace a switch in one
> of them when it got lazy had to be teased to get it to work.)
>
> Happy Holidays guys,
Designs like this, that expose electronics to line voltage,
continuously, and with no load, are subject to destruction by line
surges, a lot more than electronics that is only connected to the line
when the device is actually in use.
I just fixed my DVD player (blown rectifier and fuse) that got damaged
in a storm even though the unit was switched off. The switch was
between the rectifier and the rest of the power supply. Idiotic
design, if you ask me.
-- John Popelish
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