LED and dimmer

bus_driverFebruary 14, 2011

I bought some LED lamps with Edison base online from Hong Kong. Very low price. The output is a bit too blue for my taste. Installed on a dimmer, the light dims only slightly with the dimmer at the minimum. Is there any technical or safety reason why LED lamps should not be used with a dimmer?

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Ron Natalie

Certain LED lamps will not work with the TRIAC dimmers that most common household dimmers are.

    Bookmark   February 14, 2011 at 6:20PM
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Is that to say that the LED fails to produce light, will light but is unsafe in doing so, or that the LED does light but will fail soon without hazard?

    Bookmark   February 14, 2011 at 7:04PM
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LED lamps and CFLs basically use similar electronics inside the bulb, with the exception that CFLs also use a DC-DC converter to obtain the high voltage for the tube.

They will take the incoming AC, rectify and filter it to DC, and then use a driver IC to supply the necessary voltage and current. In the case case of LEDs (and also CFL) they are trying to control the exact voltage & current, and by design, the circuitry attempts to correct for changes in the incoming AC. Which by the way is what the dimmer is doing.

When the LED lamp is put on a triac a number of things can happen: 1) The current demands of the LED may not be enough to keep the triac turned on. 2) The dimmed mode (or shortened phase) AC waveform may still be able to provide more than enough rectified DC voltage to keep the LED near full brightness. 3) Depending on circuit design it may cause overheating of the driver IC, and excessive current draw, opening an internal fuse - usually a fusible resistor - on the input of the internal electronics.

So, in answer to bus_driver's second post, my reply would be, yes it's safe (since there is internal protection), it may not produce light when set at low, or it may make no difference (his result), and depending on the design, it may shorten the life of the electronics in the bulb.

Several semiconductor manufacturers have been releasing ICs (marketed to the lamp mfgs) that address this issue. They still rectify the AC but also detect if the phase of it is shortened by a triac dimmer. They then internally use PWM (pulse width modulation) to drive the LEDs proportionally to the incoming phase.

As far as the LED lamp being too blue, there are currently 2 ways White LEDs are made. One is that inside there are actually 3 LEDs (RGB) that are mixed to produce white (just like a TV). The second, and now more popular, is using Blue/UV LEDs and coating the inside of the LED with phosphors to produce white light - just like fluorescent lights. Obviously, there is going to be a fairly broad quality range here.

Basically, lamp mfgs are trying to adapt to the existing infrastructure and built products to work with what we have. Also, most of these same technologies can be applied to CFLs as well.

    Bookmark   February 14, 2011 at 11:30PM
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