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CFL "pulses" when off!

Discussion in 'Hobby Electronics' started by [email protected], Mar 29, 2007.

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  1. Guest

    Hi All,

    All this recent talk of CFLs has reminded me of an odd situation that
    I've been meaning to ask about.

    About a month ago I bought a CFL for the fan/light combo in our
    bedroom (I hate those fittings, but it came with the house), and when
    I lie in bed at night and look up at the CFL, I can see it pulses once
    every three seconds or so. This is with the light switch turned off!

    This has me concerned somewhat, but I wondered if it was some
    interaction between the fan controller circuit and the CFL? The
    controller is one of those integrated into the wall-plate switch (4
    position rotary switch at top, light switch at bottom), and while I
    haven't popped it off the wall to look I assume it's some sort of
    electronic controller as opposed to the older tapped-transformer type.

    Can anyone think why this might be happening, and whether I should
    check out the wiring to the controller/light switch?


  2. Phil Allison

    Phil Allison Guest


    ** OK - here is a plausible hypothesis.

    Assume the lamp in the combo is controlled by a triac, which is in turn
    controlled by that wall plate.

    The triac has a snubber across it - ie a 47nF cap plus 100 ohm resistor.

    This snubber can supply a tiny trickle of 50 Hz current to a lamp in the off
    setting - but no way will this light an incandescent. However, it will
    slowly charge up the internal capacitors of a CFL.

    Typical CFLs draw almost no AC current until a threshold voltage of circa 80
    volts is reached - then the internal diac attempts to kick start the
    transistor inverter.

    So YOU see a brief flash of light, supplied by the stored charge as it is
    consumed - then cycle re- commences.

    Solution ?

    Fit a damn incandescent.

    ........ Phil
  3. Guest

    The lamp is controlled by a triac? I didn't expect that!
    Yeah, I agree, shame I hadn't done the reading that I've done in the
    last month about CFLs before I bought them. PF of 0.5 or worse? yuk!

    Thanks for the hypothesis, it makes sense. Seeing as the CFL is in an
    oyster style fitting, I doubt it will last much longer anyway :)


  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** I just tried a couple of CFLs I have here with small value caps in
    series ( ie 22nF and 47 nF class X) and found they turned into flashing
    lights at about 2 to 7 Hz.

    Then I tried a 820 kohms in series and got the same flashing but at a much
    lower rate.

    Changed to 270 kohms and now have a 13W CFL flashing at exactly 1 Hz.

    This is VERY BAD for the general usage of CFLs with any kind of electronic
    switch or dimmer - as the vast majority will have a snubber circuit with
    caps of around 22 nF.

    It is also VERY BAD if the house lighting wiring has even the slightest
    leakage current.

    A leakage current of 1mA is within acceptable standards - but turns most
    CFLs into flaming dicso lights !!!!!!

    Have alerted Rod Elliot already.

    His CFL page will be soon updated with this new disaster.

    ........ Phil
  5. jasen

    jasen Guest

    the 250VAC suppliy runs to the lamp holder and a branch goes from there to
    the lightswitch, this branch behaves like a capacitor and passes a small
    amount of electric current throug to te lamp where caharge builds up in a
    capacitor until it hits the threshold where the lamp tries to start.

    supply _----0----_ _/ _
    ================_ _========================____)
    ~-o o-~ \
    lamp terminals \
    this length of wire
    forms a capacitor

  6. kreed

    kreed Guest

    If there isnt a dimmer on the wall plate:

    Does the CFL flash only with the fan turned on - or all the time ?, if
    so does the CFL flash speed change with fan speed settings, or does
    it happen on one particular fan setting(s) only ?

    Most of those 4 position (off + 3 speeds) fan speed controls that I
    have seen are very basic and definitely not solid-state ;) they are
    simply 2 capacitors switched into series with the fan motor supply to
    reduce speed, ie:the rotary switch switches cap 1 (smallest value)
    into series when set to lowest speed, cap 2 (larger value) into series
    when switch is in position for medium speed, and when in position 3,
    the AC is supplied directly to the fan, giving full speed.

    You will find that there is some leakage current somehow getting to
    the CFL, and it is quite possible that someone has done the wiring
    incorrectly to the fan/light switch circuit, and by some set of
    circumstances, current is getting through those caps and via the CFL
    and back to ground or neutral. First thing I would check would be
    bodgy wiring and in reality this involves a qualified electrician,
    unless you have the skills yourself to deal with it and not get

    Other possibilities could (not likely but possible in the absence of
    anything else) be crappy quality / damp / insect / dirt filled light
    socket or connector that is conducting slightly, leaking current
    through the foreign matter back to the metal mains earth of the light
    fitting/fan assembly, or maybe even the active wire to the CFL running
    very closely, and/or for a very long distance, next to another active
    AC wire that induces a very minute current flow in the active line to
    the CFL..

    The CFL's "flashing" process is clearly explained in phil's post
    below (unlike some recent BLATANTLY crappy theories on here made by
    other posters in the last couple of days, he is quite correct.)
  7. aussiblu

    aussiblu Guest

    I had a similar problem and I wrote to the distributor as
    follows and got the reply below and some free Compact fluros.:

    The Reply:

    "My apologies for the delay. I was actually away on business
    travel last week and didn't get a chance to respond sooner.

    I have checked out your situation and can confirm that this
    rarely occurs we have found after much research that certain
    homes that are wired in a way where they are switching the
    neutral that this may occur with these lamps.

    I suggest just replacing them with our normal CFL's. If you can
    let me know what base these CFL's are (BC or ES)I will arrange
    to have 4 20W coolwhite CFL's to be dropped out to you, no

    My question:


    I just bought some of your Fairway cool white 20W Compact Fluoro
    bulbs from Bunnings (4 for $14.95) and unlike the old ones they
    replaced they emit a bright flash every minute or so after they
    are switched off. Is this normal? They're in the kids bedroom
    and they didn't notice this phenomenon with the previous compact
    fluoros. It happens whatever light socket in the house we use
    them in and these sockets are not 2 way switched and there are
    no timer switches or electronic switches in the house's light
    circuits. Some sockets have new switches so I do not think it is
    leakage across the switches. Is there any easy cure or is a
    matter of you get what you pay for with CFL and I need to buy
    another more expensive brand?"
  8. Guest

    There's no dimmer on the wall plate, and the flash rate is independant
    of whether the fan is on or off, or speed. It's a four-position
    switch (inc off)... but only caps do the speed regulation? Seems
    nasty. Hmmm curious now, I think I will take some measurements and pop
    the wall-plate off and just check that the wiring is ok and no-one has
    done anything silly like swapping conductors, etc.

    The flash rate never changes (one flash every 3 seconds) but the
    intensity does - I haven't worked out what affects the intensity yet,
    it's not affected by the fan settings either.

    It's not a big deal for myself and the wife but I could see it being
    an issue in a kids room as posted by Blue. Might set up some CFLs on
    the bench over the weekend and run some tests too...

    cheers all

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