Dimmer for LEDs

[fredhargis]

I repalced all the bulbs in the dining light fixture with LEDs, 12 candelabra based 40 watt equivalent bulbs. The bulbs are GE brand, and labeled "dimmable". We have a dimmer on the fixture, but the dimming with the LEDs isn't as it was....more of a 2-level light, high and low. That doesn't bother me at all, but does the switch need to be an "LED compatible" switch for the complete range dimming?

[EatenByLimestone]

I think rheostats work by altering voltage. I'm not sure how that would work for LEDs.

A quick google search shows LED specific dimmers.

[fredhargis]

Hmmmm, I thought dimmers worked by pulsing the current. Anyway, I did see dimmers labeled as specific for LEDs, but I'm curious whether they might be regular dimmers that have been re labeled (with a price increase).

[TDKPE]

Dimmers for incandescent bulbs are usually just simple triacs with the turn-on point in the voltage cycle set by the knob or slide. It turns off when the supply voltage crosses the zero point of every cycle. The sooner in the cycle it turns on, the longer it's on before it shuts off. Rinse and repeat.

But LEDs have a constant current source built into them, and the capacitor hidden somewhere inside them is charged whenever the supply voltage is higher than it's stored voltage, while the LED is drawing current more or less steadily. So the on-off-on-off of 120V from a simple triac may result in periods of no capacitor recharging, then a lot of charging, then no charging - that sort of thing. Reduced range of dimming can result, and even on-off cycling of the LED. And LED's present a much lower load to a dimmer, though I suppose enough of them on a single dimmer would help. Many dimmers have a minimum load on their label, in fact.

I don't know what's different in an LED compatible dimmer, but presumably there's more sophisticated software (or firmware) that's designed around keeping the LED power supply charged to respond in a linear manner with dimmer control that mimics the response of an actual linear fixed resistance like a light bulb does on a common triac dimmer.

So, while I don't know exactly what they're doing differently, I'm sure they're much more involved than conventional dimmers. How's that for a non-answer?

[fredhargis]

Very good, then I wonder: will the life of my LEDs be impacted by the wrong dimmer? I could always just put in a fixed switch.

[TDKPE]

I honestly don't know, but I doubt it. I think.

LED dimmers are still going to be switching the power to the bulb on and off rapidly, but in a more controlled, or differently controlled, fashion I would think. And the bulbs still aren't going to see more than 120V (or 170V at the peaks).

I put an LED 'bulb', the kind that looks like a flattened light bulb so it works with the clip that snaps over the bulb itself, into DW's night table lamp. She leaves that darn thing on whenever she leaves the room, so it's on a LOT. Anyway, it works perfectly with the slide dimmer on a cord we use for them, which has been around since long before LEDs started getting popular.

Other dimmers, not so good. And the same is true of many or most photodiodes for DtD outdoor lights. Pulsing and never actually shutting off. Or the electronic timers I had for the front lights - same thing.

Oh, and part of the problem is 2-wire connection - dimmers don't usually use the neutral, and LED-specific dimmers may use a neutral for constant voltage to the control circuit. That may also be tied to the recent changes in the NEC requiring a neutral in 'switch legs' for lighting control. Same with timers, which use a neutral.

I'm not very current on this stuff, though. As you can tell.

[Woodenfish]

Traditional dimmers use rheostats to control current flow by variable resistance. Resistance creates heat which is energy wasteful. LED's are semiconductors. To maintain the color of the light a pulse width modulator is used to control the frequency of the current by creating a dwell cycle so fast the eye can not detect the fade of the light. If using the wrong technology dimmer you risk circuit damage to the dimmer and LED bulb. While shopping for a LED specific dimmer I found a few of these new dimers that actually sold for less than the older style and were designed to work with either LED's, compact fluorescent's and incandescents.

[Robert Adams]

fredhargis said:

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Very good, then I wonder: will the life of my LEDs be impacted by the wrong dimmer? I could always just put in a fixed switch.

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Yes it's quite harmful to the leds. Swap it out with a regular switch or an led compatible dimmer.

Now when you put the new dimmer in there are adjustments to say to set the minimum light level. If set wrong they can flicker a bit at low light settings but I have only seen that when running 2 or so on a dimmer. The more on the switch the easier it is on the adjustments.

Also beware that you really need a dimmer that has a neutral connection. The ones that don't still put a tiny bit of power through the bulbs to power the dimmer even when off. And some dimmers that have a neutral terminal still do that. The regular switch style with the slide dimmer that lowes sells still puts power though the bulbs when off to power the switch led. I have the model number if you want to avoid that one.
I ended up still using that dimmer in the kitchen cause I can't stand the Decora or the older flat paddle switches, I hate fumbling around in the dark trying to turn hose stupid things on where a regular switch is easy to find and flip.

[TDKPE]

Woodenfish said:

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Traditional dimmers use rheostats to control current flow by variable resistance.

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Not to be argumentative, but I haven't seen an actual rheostat dimmer for home use in decades. Too much wasted heat, which is why triac dimmers were developed; only the transition between full-on and full-off is generating heat, and that event is short-lived in relation to the half-cycle. The only variable resistance I'm aware of is in the trigger circuit for the triac. Like digital hi-fi amplifiers (true digital, not marketing digital); very little heat output since current isn't being regulated directly.

[Woodenfish]

You are correct sir, my bad for improper term.