LEDs and Dimming

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charlie b

charlie b

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My question is whether I have an unsafe situation and need to do something about it. I replaced a light bulb in an overhead fixture yesterday. The fixture is controlled by a rotary dimming switch. The only spare bulbs I had were LEDs. The light works, but it has the same brightness regardless of the position of the rotary dimmer. Do I need to go buy another type of bulb, halogen perhaps, or can I safely leave the LED in place?

p.s. I believe it does not violate the forum’s “no DIY” rule to talk about a situation that would never have involved a professional electrician. If you don’t agree, feel free to report this thread. ;)
 
I will have to look at the package tonight. But even if it said it was dimmable, does that mean that a standard wall dimmer switch (probably just a variable resistor) would successfully dim the bulb?
 
That's the behavior you want from a non-dimmable lamp. You want it to stay the same brightness even when the input voltage varies. The lamp may even be designed for a wide range of input voltages.

The driver is doing its job - when the dimmer drops the input voltage, or when it drops the time that voltage is present, the driver will increase its input current to compensate.

As the input voltage gets close to zero, the current has to go up a lot. But LED's use such a small amount of power compared to an incandescent, it may not a problem for the wiring or the socket.

But is it safe? I'm not sure anyone can say for sure without knowing the dimmer and driver specifics.
 
Make sure the LED bulb is rated to be installed in an enclosed fixture as well. Some of the cheap versions are not

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The dimmer should be LED/CFL rated. Although most dimmers will work with LED bulbs that are dimmable (because most utilize a forward phase driver) they will burn out due to the in rush current of the LED bulb. If you are going to leave the LED bulb in place and you verify that it is a dimmable type, it would be best to update your dimmer to a compatible type.


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charlie b said:
. . . a standard wall dimmer switch (probably just a variable resistor) . . .
Click to expand...

Hi Charlie,

Incandescent dimmers have been variable resistors since the transistor was combined into them back in the early Sixties. The load current passing through the dimmer is regulated, at minimum, by a TRIAC. A triac is a solid state device that is essentially a pair of phase-controlled thyristors connected in inverse-parallel on the same chip. Similar to an SCR, once the device is turned on, the device cannot be turned off. This device is considered bi-polar and reverse voltage blocking.

In the basic incandescent dimmer each half of the triac turns off at the zero crossing of the AC voltage. The knob on the dimmer controls how long of a delay the turn on voltage pulse is before it is put on the gate of the triac. The turn on is very fast, and the load voltage then follows a regular AC voltage sine wave for the remainder of the half cycle until the zero crossing.

I think it is quaint that you are still thinking about the basic incandescent dimmer as a passive variable resistor.

The voltage of each half cycle, as it turns on is basically a square wave, meaning it is very noisy - full of many frequencies. The switching power supply of a CFL and / or the constant current driver of an LED require additional circuitry to handle the irregular waveforms that come out of a simple old style incandescent dimmer.

Once the dimmer itself gets additional circuitry to handle dimmable CFLs and dimmable LEDs, things become even more abstract in that each of the manufacturers lists on their web sites the make and model of other manufacturers equipment that is compatible with their electronic device in their opinion.

Most of my customers just look at me like I'm from another planet when I try to help them through this maze.
 
charlie b said:
p.s. I believe it does not violate the forum’s “no DIY” rule to talk about a situation that would never have involved a professional electrician.
Click to expand...
Well then, the rule has yet to be broken because no self-respecting DIY'r would EVER involve a professional electrician! :D
 
charlie b said:
Do I need to go buy another type of bulb, halogen perhaps, or can I safely leave the LED in place?
Click to expand...
I would not give it a second thought if it were mine as long as I fully rotated the switch. If I noticed excessive heat, I would put in a toggle switch if I did not care about dimming.
 
It doesn't turn on right at zero setting even when you have it maxed out. You usually get annoying shimmer when you try to dim non-dimmable lamps. This can be damaging to both the lamp/ballast and the dimmer.

Also, if you're contemplating installing a dimmable LED luminaire, I strongly advise that you use a portable hand dimmer cord and try out a few randomly chosen ones to verify that

1.) No excessive bzzzzzzztttttt!!!! sound from the LED ballast that HO would find objectionable. This is especially important for residential use when you might be relaxing to music, reading peacefully and other stuff. Ballast noise is a very very very common complaint. Barely audible noise can get many times worse when dimmed.

Better to figure this out before you install them and even better before receiving the entire shipment. Unfortunately, there is not rigid enough parameters for ballast noise when used in conjunction with forward phase dimmer. Some are so loud that your table lamp sounds like you're in a transformer room.
 
Thanks for all the information, everyone. The box that the bulbs came in explicitly states "non-dimmable." So at my next opportunity, I will buy a different type bulb and change it out. I can't replace the fixture itself. It is a Tiffany style that hangs on a chain above the dining room table, and my wife loves it.
 
170102-1157 EST

charlie b:

I suggest that you try a dimmable Cree and mate it with a Lutron CL type dimmer.

The Lutron CL is a very good two wire dimmer. Dimming characteristics are very good. At lowest dimming level you can remove power and reapply and the previous dimming level will be restored.

The dimmable Cree bulbs I have tried do not produce a huge amount of RFI (radio frequency interference) in the AM band. This is in contrast to some Feit units. However, I have had more failures with Cree than with Feit.

Any phase shift dimmer produces some RFI.

.
 
gar said:
170102-1157 EST

charlie b:

I suggest that you try a dimmable Cree and mate it with a Lutron CL type dimmer.

The Lutron CL is a very good two wire dimmer. Dimming characteristics are very good. At lowest dimming level you can remove power and reapply and the previous dimming level will be restored.

The dimmable Cree bulbs I have tried do not produce a huge amount of RFI (radio frequency interference) in the AM band. This is in contrast to some Feit units. However, I have had more failures with Cree than with Feit.

Any phase shift dimmer produces some RFI.

.
Click to expand...

Which Cree? Does it produce ballast buzzing?
 
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