Re: Flourescent lamp -Wasting Power?

From: Jeff Wisnia (jwisnia_at_conversent.net)
Date: 12/19/04 

Date: Sun, 19 Dec 2004 13:29:23 -0500

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,

Jeff 

-- 
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"As long as there are final exams, there will be prayer in public
schools"

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