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Old 4th May 2013, 08:55 PM   #1
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Default Load on an AC circuit

I want to change a four fixture light array in my theater room from halogen bulbs to dimmable LED. The light is control by a Lutron IR electronic dimmer. When I changed out the bulbs for the LED's the bulbs would not completely shut off. I was told by Lutron it is due to the dimmer not seeing the same load on the circuit that the halogen bulbs generate. I am wondering if adding a coil to the circuit before the dimmer would generate a load great enough for the dimmer to sense?
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Old 4th May 2013, 09:21 PM   #2
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If the cause is the LEDs being a lighter load than the halogen bulbs, I think you would need to make the load seen by the dimmer greater. Not less, which is what adding coil series impedance would do. No?
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Old 4th May 2013, 09:38 PM   #3
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Quote:
Originally Posted by sofaspud View Post
If the cause is the LEDs being a lighter load than the halogen bulbs, I think you would need to make the load seen by the dimmer greater. Not less, which is what adding coil series impedance would do. No?
Your probably right. A single halogen bulb shows 18 ohms across the terminals. But, I have no clue how to size the resistor for wattage. The four dimmable LED are 13.5 watts each, if that helps.

Also, I assume I would place the resistor at the junction of the feed to the first light.

Last edited by NatureGuy; 4th May 2013 at 09:57 PM.
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Old 4th May 2013, 10:21 PM   #4
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The solution might be inside the dimmer, not at the load. What is the dimmer model? I see some of the Lutron dimmers shown as being low voltage load compatible, and some just incandescent/halogen/ceiling fan.
If 18 ohms is the cold filament resistance, that isn't of much use. The hot on resistance will be significantly higher. Paralleling a resistance with the LED load would be counterproductive IMO, though that ought to trigger the dimmer into operating correctly. I would see if I could tweak the dimmer first, hoping it would involve just a resistor change or two.
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Old 5th May 2013, 04:46 AM   #5
wwenze is offline wwenze  Singapore
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Ok, AC dimmers and LED lighting... here I go again...

AC dimmers work by conducting less than the full cycle of the AC voltage.

LEDs only work on regulated DC, so they're powered by an AC-DC power supply. And such power supplies have smoothing caps. So regardless of how strange the AC waveform is, the DC voltage at the cap is always peak voltage of the AC. Both ripple and peak voltage will depend on conduction angle, but again it doesn't matter that much because that DC is used to feed a regulated SMPS-cum-LED driver (they're the same thing but I'm just being clear).

In other words, the LED lamp will function normally even with imperfect or altered AC voltage, until it starts to flicker or totally not work at all.

Dimmable LED lamps have a smarter power supply that detects the presence of a dimmer, and from that decides how much power the LEDs should get.

And... this is where everything goes wrong because there is pretty much no standard regarding dimmer and LED lamp compatibility.

What you need to do is, contact the company that sold you the LED lamp with the "dimmable" feature drawn on the box, ask them what dimmer they used.

Last edited by wwenze; 5th May 2013 at 04:51 AM.
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