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#145488 - 05/26/06 02:10 PM This is rather strange
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Hi Guys,
The other day, I bought a whole heap of compact fluorescent lamps to fit my house out with.
These are Philips 20W BC type things.
Anyway, I was reading in bed last night and as it was getting late I turned out the room lights and proceeded to get some sleep.
Now I know that fluorescent lamps of any kind have a faint glow after you turn them off. However, I happened to notice out of the corner of my eye that the lamps were actually flashing at about a 0.3Hz rate.
Anyone ever struck this with compact fluorescents before?.
I was rather freaked out by this when I first saw it, you naturally assume that lights should not emit light with the switch turned off.
I woke up about an hour later and they had stopped, which was a relief, I was beginning to think the lamps might be haunted.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#145489 - 05/26/06 11:40 PM Re: This is rather strange
uknick Offline
Member

Registered: 02/10/06
Posts: 10
Loc: England
Could it be a residual discharge stored in the electronic ballast components.

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#145490 - 05/27/06 01:55 AM Re: This is rather strange
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Some sort of low-frequency resonance effect? 0.3Hz seems very slow for the components involved though.

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#145491 - 05/27/06 02:12 AM Re: This is rather strange
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
Interesting Mike,

I have seen a similar effect with green indicator lamps. These are not the Neon ones but got a different gas in them, probably Argon.

If they were turned of a glow remained for a while,
then,
when they were dark, and once in a while they flickered back on very briefly but not at a regular rate.

These indicator lamps did not have any electronics in them , only a 68 k Ohms resistor.
_________________________
The product of rotation, excitation and flux produces electricty.

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#145492 - 05/31/06 03:14 AM Re: This is rather strange
kiwi Offline
Member

Registered: 12/04/04
Posts: 347
Loc: christchurch new zealand
Trumpy I reckon those compact fluorescents are better for outdoor lighting and garages etc. They take a while to warm up, so they probably take a while to cool down, partially re-igniting several times after switch-off.

I guess you'll be changing the bedroom lights back to incandescants then ?

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#145493 - 05/31/06 04:49 PM Re: This is rather strange
gideonr Offline
Member

Registered: 12/18/04
Posts: 152
Loc: Edinburgh, Scotland, UK
These have electrolytic capacitors in them, that tend to charge up all by themselves due to their chemistry. A high voltage one that's been discharged can reach 30 volts or so. (Kilovolt caps need safety shorting straps if you are working inside equipment with them, eg high power radio transmitters.)

When the volts get high enough the circuit will burst into life momentarily, discharging the capacitor, and start over again...

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#145494 - 05/31/06 05:36 PM Re: This is rather strange
aussie240 Offline
Member

Registered: 07/06/05
Posts: 222
Loc: Blue Mountains, NSW, Australia
I have seen this effect with neon and fluorescent lamps...the reason is that such lamps will show a visible glow with microamps of current. How can such a current flow with the switch off? The wiring (especially if it's the usual twin cable) capacitance will be enough if there's a reasonable distance from light fitting to switch. Two ways to get over the problem are don't use a looped in switch wiring; ie. have the supply come to the switch before it gets to the light fitting, or simply put a high value resistor across the fitting; something like 470K 1W.
Why the pulsed effect with a compact fluoro?
Again, the slight current flowing will slwoly charge up the electrolytic filter capacitor in the CFL. At a certain voltage the lamp will attempt to start (hence the short glow). But in doing so it draws lots of current discharging the filter cap again...voltage drops and lamp can't fire. Now that the lamp isn't drawing current again the procedure repeats itself and functions as a crude relaxation oscillator.

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#145495 - 05/31/06 10:34 PM Re: This is rather strange
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Yeah,
It's interesting stuff, I've always associated an after-glow to a degree with the Cathode Ray Tube (or TV screen or oscilloscope screen).
Is it to do with the phosphor coating on the inside of the tube face that causes this or is it the fact that it takes a while for the EHT voltage in a TV to "die down" after intial turn-off?.
Anyone with knowledge of the old B&W sets would be familiar with the centralised dot in the middle of the screen.
Anyway,
It's actually stopped now (the flashing), I'd more or less put it down to "new-ness"
Kiwi,
I'm just doing my bit for the country, sure I won't save a wind farn being built, but every bit helps.
To be honest I like this 6500Kelvin colour and I'm saving power too!.
I'm not going back to incandescents, ever.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#145496 - 06/01/06 03:37 AM Re: This is rather strange
kiwi Offline
Member

Registered: 12/04/04
Posts: 347
Loc: christchurch new zealand
Trumpy its cosing me a small fortune replacing the "Spotone" incandescants here at my place. My whole house has downlights, 20 all up, and those Ro-80 lamps are always dying. Light Bulbs are a permanent fixture on our grocery shopping list ! !

My wallet wants me to put in long life compact fluorescents but I just don't like Fluorescent lights inside my house.

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#145497 - 06/01/06 04:05 AM Re: This is rather strange
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
Is it to do with the phosphor coating on the inside of the tube face that causes this or is it the fact that it takes a while for the EHT voltage in a TV to "die down" after intial turn-off?.
Anyone with knowledge of the old B&W sets would be familiar with the centralised dot in the middle of the screen.


The dot in the center of the screen was there for a short time while (a) the EHT voltage leaked away and (b) the cathode of the electron gun cooled to the point at which it would stop emitting electrons. This took much longer than the time needed for the vertical and horizontal scanning fields to disappear, hence just the dot and not a full raster.

Many manufacturers actually added a section on the on/off switch (or used some other similar method) to bias the tube into cut-off to prevent the dot.

The overall faint glow right across the tube face you can see in a darkened room after it's switched off is the persistence of the phosphor. I remember doing that with "Test Card F" as a kid! Turn off all the lights, then switch off the set and see how long I could still make out Carol & her companion!

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