Non dimmable CFLs on DIN rail dimmers

Discussion in 'General Discussion' started by Opto, Jul 20, 2011.

  1. Opto


    Jul 20, 2011
    Likes Received:

    If this has already been answered in the forum please accept my apologies but I have not yet been able to find the answer.

    I note that some non dimmable CFL?s work with the L5504D2A and L5508D1A dimmers fine when programmed for on/ off settings, but others flicker while the dimmers channel is set to ?off?.

    The installation instructions state that: ?Fluorescent lamps may be switched but not dimmed. For each channel connected to the fluorescent load, set the minimum turn on level to 100% . . . . .. A Power Factor Correction capacitor must be fitted". CFL's were not mentioned.

    An older version of the instructions (L5508D1A) dated 2000 found on the Internet state that "Non power factor corrected compact fluorescent loads can be switched but not dimmed with these products.

    On trying to find out about this on the internet I came across an article explaining that standard CFL?s should not be used with dimmers (dimmers in general ? not necessarily CBus dimmers). Here are a couple of links:

    - See the section ?Dimmer Principles?
    For a more general discussion on lighting types see:

    The links are posted with the kind permission of the author, who also replied:
    ?All normal dimmers will misbehave with CFLs unless the lamps are designed to be dimmable. The reasons are complex, but are pretty much guaranteed - especially with 2-wire dimmers (having no neutral connection). Even 3-wire/ 4-wire types are unlikely to work properly because the dimmer TRIAC cannot conduct while no load current is drawn (the first 1/3 cycle with almost all standard CFLs). Very few people are aware of the huge current spike, because electricians and C-Bus techs are unlikely to have an oscilloscope and current monitor as part of their kit.?

    So, could anyone on the forum advise if low cost standard non-dimmable CFL?s available in the UK should be used with the CBus dimmers L5504D2A or L5508D1A at all.

    Also, could anyone on the forum confirm that any dimmable CFL should work OK, even if it doesn?t dim much, and that the CFL electronics will prevent flicker while the dimmer channel is ?off?.

    NB: the non-dimmable CFL I thought was working fine did run quite hot after a while and it has been removed from the fitting now.

    Thank you in advance for any advice you are able to offer. I appreciate CFL?s may not be the ideal light source for some applications but we have a few circuits where the bulbs could just be accidentally replaced with CFL?s one day.
    Opto, Jul 20, 2011
  2. Opto


    Aug 3, 2004
    Likes Received:
    Adelaide, South Australia
    Putting CFL's on C-Bus leading edge dimmers has been covered quite a few times on the forum in the past. Here's a few links:

    The biggest issue when trying to run non-dimmable CFL's from a dimmer is the small amount of bleed current that exists on a dimmer channel when the channel is turned off. You'll probably find that the CFL will work fine when the channel turns on but you'll have issues when you turn the channel off. The CFL may flicker/flash or get hot when the dimmer channel is off. Even if it appears to behave nicely the lamp life may be reduced.

    To solve this problem, you can connect a C-Bus relay channel in series with the dimmer channel and then assign them the same group address. This will guarantee that off is really off. You will still need to program the dimmer channel so that it effectively doesn't dim, i.e. set the minimum level to 100%.

    Trying to dim non-dimmable CFLs is definitely a bad idea. Some lamps fail within seconds of operation under this condition, others go crazy and still others just turn off.

    Alternatively, you can fit CFL's that are designed to be dimmed. These types are expecting to have to manage the off-state leakage current (to a point) that practically all dimmers have. Even if you don't actually dim them, the lamp is designed to handle being connected to a dimmer.

    I've seen quite a few Philips Tornado CFL's and Megaman dimmable CFL's and these seem to perform well enough. Osram and GE also have some CFL's with a certain range of dimming.
    Newman, Jul 21, 2011
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