Whenever I flip on my dimmer switch, all four dimmable LED lamps flash on in unison, blink off, and then come on at the selected dim level. This happens even with the dimmer slider maxed out, so it's not a matter of trying to drive the lamps with too little power, which I know can cause flashing and flickering.

The flashing happens only on cold start, by which I mean, if I've just turned off the lamps and then flip them back on, they come on directly with no flash. Wait just a few seconds after flipping them off - a bit past the time they take to completely stop emitting light - and they will flash when you flip the switch back on again.

I don't know what could be causing this particular failure mode, so I don't know where to start debugging this problem.

For what it's worth, I first noticed this behavior when testing after wiring in the third lamp. When only two were wired in, no flashing was apparent.

Gory details:

I have a Lutron Diva CL 3-way dimmer. The "CL" designates that it handles CFL and LED dimming.

It's driving 4 Cooper Lighting HALO 4" LED retrofit kits, model RL406830WH.

The dimmer's package insert installation instructions state it can handle up to 150 W of CFL/LED load. Each of my LED lamps draws only 13.5 W, so all 4 together draw less than 55 W. This is well within the operating range of the dimmer.

The exact LED lamps I'm using are not listed among the "Approved Bulb List for Wall-Mount CL Dimmers", but several similar models by Cooper Lighting are.

ETA: I still would like to know why I saw this behavior, but the problem appears to have been the dimmer.

Disconnecting 3 of the 4 lamps showed flash-on-start still.

I swapped the dimmer for a standard S_3 and had no problem.

I then swapped in another instance of the same model. (Though the sticker on the side gave a different hardware rev, and the assembly location was St. Kitt's rather than China.) The new dimmer caused no flash on power-on, but it couldn't dim the LEDs nearly as low as the first instance. Frustrating.

  • 1
    Sounds like the dimmer circuit has a capacitor (works sort of like a very short term battery) that is not "charged" yet on a cold start, but on a warm start is already up to full potential. Not unusual to have "same" model from a different assembly plant using different components/circuits.
    – bib
    Sep 21, 2012 at 11:58
  • try the lutron toggle switch. I had the same problem and it seems the toggle is the best for led.
    – user14844
    Aug 31, 2013 at 16:21
  • 1
    This is the inherent problem in trying to dim LEDs by throttling their power source, effectively the LEDs have to use bucking to utterly defeat the dimming effort, then analyze the waveform to try to figure out what the dimmer is trying to ask for. Native LEDs are capable of infinite dimming, it's just lost in translation. Pro lighting uses a signal wire for dimming, it is adjusted 0-10 volts. Jul 27, 2017 at 16:27

6 Answers 6


As I don't know the interns of this dimmer it might switch when the alternating voltage (and therefore the current with resistive loads) crosses zero. To detect the phase (crossing 0V will occur at 0° 180° and 360° which is again 0° - you might want to look it up here: http://en.wikipedia.org/wiki/Phase_(waves) ) and the type of load (e.g. inductive loads delay the flow of current depending on their inductance and the frequency in relation to the voltage. I could not find a text on wikipedia describing that so here's a plain image: http://people.sinclair.edu/nickreeder/eet155/PageArt/phaseInductor.gif

To detect the type of load it has to observe the mains line for a short period of time without regulating it.

The issue does not occur on light loads because most dimmers can handle inductive, capacitive and resistive loads quite well on low power levels.

By letting the dimmer "cool down" you may erase its short term memory so it has to resynchronize every time you switch it on.

Again: As the internals of this dimmer (or better: the two) are not known I can't provide a better answer. A newer hardware revision usually improves the product. Also chinese manufacturers tend to produce lower quality products as seen here: http://hackaday.com/2012/08/15/buying-cheaper-electronics-and-not-saving-money/

  • Interesting. The LED lamps feed the 120 VAC mains power into a DC rectifier to drive the actual LED. I can see that without tearing anything apart. Without a teardown, I can't say much about the dimmer internals. The Chinese version is a newer hardware/firmware rev. It managed to dim the lamps lower and more evenly, but it had that blinding camera flash effect. Sep 21, 2012 at 15:07
  • LED's are driven with 2-4 Volts depending on the LED type. Those might be arranged in series to reduce the amount of components needed to control them. I'm glad that it works now :) Sep 21, 2012 at 16:39

I know this is an old thread, but I recently had this problem myself. Ended up being because of cheap bulbs. I have two of the same dimmers, one works fine, the other would flash once every time before turning on. I had just bought some "dimmable" LED bulbs on sale from Home Depot, and when I switched the bulbs for the LED bulbs I had in my other fixture the problem was solved. Guess not all dimmable bulbs are made equal. Hope this helps someone.


My wife bought a good quality ceiling light with 12 individual lights. I bought a trailing edge dimmer, but I found that the lights started to flicker after about 20 minutes. I carried out various fault finding with no success. I eventually contacted THORN who made the dimmer switch and they told me that it was probably a design issue and that they would send me, free of charge, new dimmer switches to try. I tried two before I got one which actually worked.

So what I'm saying is that sometimes the fault can be outside your control. It's worth phoning a few companies to see what they advise.


I have the same prob with the flash on start up. I replaced the first bulb with an incandescent and the flash went away. Not sure why.


You have to make sure that the dimmer is compatible with the type of bulbs you are using. They are not all interchangeable.


The inrush current of an LED on startup is similar to an incandescent bulb of similar brightness. If you have a 300 watt dimmer with four 15W LED you might think you are well within limits, when actually you have a 400W load. Either follow manufacturers recommendations or assume a 100W equivalent LED actually draws 100W at startup.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.