Dimmable LED bulb question

Feb 4, 2009
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Not urgent more like something I can’t figure out.
I have some recessed ceiling lights. The cans take something like up to 65 watt incandescent. They work fine with LED what I can’t figure out is when you have a dimmable light and a dimmer switch/slider like the picture I am imagining top of the slider is 100% power in this case let’s stay with 65 watts. The bottom of the slider is zero percent, middle is 50 percent let’s say 32-ish watts. I now have a 40 watt equivalent dimmable LED that uses less than 8-ish watts why does that bulb dim as I slide the dimmer as in even at 20% on the slider I would think that would mean 65 x .2 or 13 watts which is over my current 8-ish watt LED.
Do dimmers not work by reducing power to the bulb? How do they work? How does a lower watt bulb know it should be at 50%

picture and damn I need to clean this
81CF154A-0FAB-43B0-921F-4A4A7E1CF5BA.jpeg
 

deadlyapp

Diamond Member
Apr 25, 2004
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First off you need to not think about power as if it is something controllable. Power is simply current / load and you can't modify that.

Edited - see power engineer's post below
 
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deadlyapp

Diamond Member
Apr 25, 2004
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Not exactly right. Modern solid-state dimmers work by blocking a portion of the AC wave but the frequency of the chopped wave is still 60 Hz.

https://en.wikipedia.org/wiki/Dimmer
Ah - you're right. I had originally thought that they were voltage variable but second guessed myself when reading about more modern dimmers. I thought they behaved more like PWM.

Thank goodness there's a power engineer on these boards!
 

Paperdoc

Platinum Member
Aug 17, 2006
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PowerEngineer is right, of course.

Additional info. I find using LED's with an older dimmer that they DO dim as old incandescent lamps do, but differently. The reason is that incandescent lamps give off light because they have a white-hot filament in them that puts out about 5% of the power as visible light and all the rest as invisible infrared (heat). When you lower the power feed to them the filament runs cooler and generates much less visible light, but still huge amounts or IR. At much-reduced power feed, that filament gets so much cooler that it gives out NO visible light, and for most dimmers that happens before the dimmer gets to minimum power. A LED lamp works differently, converting MUCH more of the power supplied into visible light. (The heat such a lamp creates mostly comes from the small circuits in the lamp base that power the LED part from the 120 VDC supply.) As the power supplied from a dimmer is reduced, this does reduce light output, BUT even at minimum dimmer setting the lamp still gets enough power to keep on putting out some light, and it never goes dark until you turn off power completely.

There are different types of circuits used in dimmers. Some types can generate brief voltage spikes in their output waveforms, but for an incandescent bulb filament that means nothing - who cares? For an LED and its feed circuit those spikes can cause damage or odd behaviour. LED lamp makers advise you should only use their lamps with a modern dimmer design that minimizes this issue. How to tell? Basically, use a very recently-made dimmer, or look for a label that says it IS safe for use with LED's. Indeed, some LED's are MADE for use with dimmers to protect themselves somewhat, and some say NOT to use with any dimmer. I have both types in our home. As it happens, I have one type not labelled for dimmer use, but used in a dimmer-fed fixture with a very old dimmer, and they work just fine. In fact, I just replaced one of those lamps that lasted 5 years of lots of use. So I've been lucky in the combo of dimmer I have and LED's that work with it.
 

mindless1

Diamond Member
Aug 11, 2001
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Also keep in mind that a dimmer designed for incandescent bulbs is not linear because incandescent bulb brightness isn't. At the 50% point on the slider, it is attempting to make the bulb half as bright, but needs more than half the power output to do so because incandescent is so inefficient at less than full power. Half the power means what the particular bulb is rated for, not necessarily 65W if it's not a 65W bulb installed.

LEDs do not suffer from this and sometimes the opposite, that at less than full power some provide more lumens per watt, until getting fairly dim where the losses in the driver circuit become more significant in factoring efficiency.
 
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