Fluorescent 'flickering' when no supply!

[sparkybird]

Hi all

Hope someone can help me out. I have a bathroom mirror with a built in fluorescent tube light. It seems to work fine, but I have noticed that it 'flickers' even when switched off (it works via an integral pull cord). The flickering (like it is trying to start) is quite faint, so that it can't be seen in the daylight, only at night.

Is the problem something to do with the tube/starter/ballast/pull cord? Is there any way to establish the faulty part? I'm not very familiar with fluorescent lighting! It's a 40W tube wired as part of the upstairs lighting circuit.

Thanks

SB

 

[Steve]

its not faulty. Capacitance in the switch wires. Nothing to worry about. More common in smaller tubes which can fire at lower currents, CFLs do it more than straight tubes. The CFL on our landing light does it.

 

[sparkybird]

Crafty - really? It's been up for about 6 months and I've not noticed it before - I guess I was half asleep when I got up to go the the loo!!

I guess we don't normally look at our lights in the dark, so thanks for that. I'd be really interested to know (if there is an idiot's explaination) the physics behind the explaination, if you could be so kind...

Thanks for speedy reply

SB

 

[Steve]

When you have 2 wires running parallel, they produce a "capacitance", charge builds up on one of the wires.

With an incandescent filament lamp, this charge has somewhere to go, it simply continuously passes through the filament to neutral.

With a fluorescent lamp, however, they require quite a current to strike up, so it has to allow the charge to build up until it is able to "flicker" the current across the tube. The electrons cant pass through the un-ionised gas in the tube, it has to strike first, this is the flickering, but obviously there isn't enough charge to maintain the glow so it instantly dissapears.

The phenomenon is more common with 2-way switching, on landing lights, where the switch wires are longer, but I suppose it can happen to any fitting, to differing extents.

As far as i know, it has no detrimental effects on the life of the tube.

 

[Adam_151]

Unlikely to be capactance if worked by an internal pull cord (unless you have been adding suppresion caps across it), that effect generally appears on long switch drops, and its much worse if there is no cpc between live and switched live (unconnected cpcs, 2 way switching (where the cable is even longer) can do this is some configurations)


I notice that a florescent after being switched off continues to glow dimmly and pulse a bit after being switched off, thinking back to high school physics, the only explanation I can come up with is that when excited, the electrons in the gas and phospher coating don't all immediatly drop back down to lower levels realeaseing photons, but some stay excited for a while and then drop down, pproducing an afterglow effect, or maybe the fitting has residual charge that slowly trickles through the tube, (kinda grasping at straws here tbh)

EDIT: if you are intested in the whole capacative coupling thing, its here: //www.diynot.com/forums/viewtopic.php?t=20425 , (oh and it varies with frequency * (higher frequency = lower impedance) ... so you can have an arcing table lamp (imposing frequencies higher than 50hz on top of the mains) that trips the RCD with no earth in sight, as the earth leakage is across the cables in the socket circuit, so it can certainly cause some interesting faults

* capacative reactance = 1 / (2Pi x freq x capacator value) if you are interested

 

[sparkybird]

Thanks Adam.

Reading that post link, it seems to suggest that the 'problem' is
as crafty says, not really a problem, just normality.

SB

 

[Steve]

I was tired last night - that post probably isn't the clearest explanation of capacititive switch wires ever.

 

[ebee]

The only (noticeable) one I saw was in a pantry.
Turned out the Neutral was switched not Phase.
They`d got a cheap guy to rewire their house not me , but asked me to investigate it, so I advised them to get the cheap guy to rectify his own work

 

[RF Lighting]

I would say check polarity at the fitting and switch. If it does have phase and neutral reversed then it is possible that a bit of power is flowing through this fitting and back to a neutral somewhere else along the line.