Today I encountered a light fitting with a CFL in it which would not work. When switched on, the CFL would simply produce a dull pulse from one of the four U elements/tubes every 2 seconds or so. I swapped the CFL with an inc andescent lamp which worked but every 2 seconds or so, the incandescent lam p would dim for a small fraction of a second.
The CFL worked normally in another light fitting.
As far as I know the two light fittings are on the same circuit.
. When switched on, the CFL would simply produce a dull pulse from one of t he four U elements/tubes every 2 seconds or so. I swapped the CFL with an i ncandescent lamp which worked but every 2 seconds or so, the incandescent l amp would dim for a small fraction of a second.
Not another one wired up to the zebra crossing! :)
Filament lamps report what is, ie the supply to it must be varying regularl y... but why I've no idea.
A relative has a light fitting in which a CFL will flash dimly every couple of seconds, even when turned off. It isn't on a time switch or anything complicated like that. I must try an incandescent bulb in there to see what happens.
rk. When switched on, the CFL would simply produce a dull pulse from one of the four U elements/tubes every 2 seconds or so. I swapped the CFL with an incandescent lamp which worked but every 2 seconds or so, the incandescent lamp would dim for a small fraction of a second.
Cable capacitance. The cfl slowly charges up, pulses briefly, and repeat.
Another idea is that early dark sensors with solid state relays, were not terribly stable, and this sounds very similar to what you can get at low current drain on one of those. Are you sure there is no gadget like this or radio controlled switch in the circuit that might be using a dodgy solid state relay? Brian
The flashing CFL syndrome is common when switched off in some cases. There are two common causes;
One is where there is an electronic switch of some form (dimmer / PIR / remote control etc). These need their own power supply to work, however a common difficulty is that there is no neutral connection present at most light switch positions. Hence they have to cheat, and pass a very small current through the lamp in order to derive their own power. This works fine with a filament lamp, but causes odd results on LED and CFL lamps.
The second common cause is where there is a long length of "floating" wire in the circuit (common on two way switching circuits). Here the disconnected switch wire can "pick up" enough voltage from coupling with the adjacent live wires to be enough to allow a very small current to flow through the bulb again.
Either of these arrangements pose no problem with filament lamps since the tiny current will not cause it to light. With CFLs and LEDs etc they contain switched mode power supplies that rectify the mains and use it to charge an internal capacitor. This is what then runs the remainder of the power supply circuit in the lamp. So even a tiny current passing will eventually accumulate enough charge to trigger the lamp briefly.
It sounds like there is a CFL somewhere that is doing the brief flash routine, and that brief moment of conductance is in turn causing a small current surge and resulting voltage sag you are observing with the filament lamp.
Aye, Not many people have picked up on the incandesant dimming every two seconds or so, ie about the same rate as the CFL flashes.
When the CFL flashes it reduces it's current draw. The load decreases so the voltage drop across the more or less constant resistance incandescant drops, so it dims.
They could be in parallel but with a common faulty connection somewhere. The voltage being dropped across that connection.
Niether of the above quite fit the OPs description but none of the other explantions cover the regular diming of the incandescant.
When it is pitch dark and my eyes are dark adapted I notice that a CFL in the hall gives a faint flash every few seconds. It's on a normal circuit with no electronic switches or dimmer involved. As far as I can make out it is triggered by a (battery operated) smoke/CO detector about a metre away on the same ceiling. The smoke/CO detector emits a short bright flash every few seconds and the CFL seems to flash in sympathy. I keep meaning to put a piece of cardboard between the detector and the lamp to see if it is the flash from the detector causing the phosphors in the CFL to glow.
I don't think it can be any of the scenarios described so far. In the above scenario, current into the cfl is always tiny, as its limited by stray cab le C or the PIR etc. Its not going to have any effect on a filament lamp. T o do that you'd need very heavy current, like a cooker etc, to get a notice able Vdrop.
The only thing I can come up with is a faulty cfl, I've encountered one tha t did what the OP describes when powered up.
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