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Dimmable mains LEDs

Discussion in 'LEDs and Optoelectronics' started by flippineck, Sep 17, 2014.

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  1. flippineck

    flippineck

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    Sep 8, 2013
    Referring here to the sort of LED lamp which is commonly available in supermarkets etc, usually with a Bayonet or Edison Screw type connection, for domestic 240V AC use, and using a multitude of small discrete LEDs (rather than 'LED filament' or other new fangled technology) I guess similar is available in other countries albeit with a different voltage rating to suit the mains in the locality..

    I was asked a while ago to fit a chandelier that used these LED bulbs but the customer wanted it to be dimmable.

    I found that the majority of commonly available mains LED bulbs, were specifically specified as being non-dimmable.

    So, I went to the trouble of sourcing suitable LED candle bulbs (these were 3W ones with maybe 15 or 20 individual LED chips inside) that said 'dimmable' on the packaging.

    Rigged everything up and it didn't work.

    Turned out, that most dimmer switches work by chopping off a variable chunk from the back end of the sine curve. Full brightness, you get a full sine wave, as you go dimmer, the switch leaves the sine curve to rise from initial zero crossing, but then sharply drops the voltage to zero mid-wave until the next zero crossing.

    For some reason this trailing-edge chopoff doesn't work with most LEDs but, they work okay if you chop off the FRONT of the sine wave!

    Apparrently the trailing edge chop is a characteristic of the commonly available traic-type dimmer. Turns out there's another kind of dimmer switch circuit that uses a transistor instead of a triac, and that type does a leading-edge chop on the sinewave.

    Managed to get one of these, and it worked beautifully.

    I might have got trailing and leading edge and transistor/triac mixed up in the above description but hopefully you get the gist nonetheless

    Now, I have another chandelier. This one is fitted with six 4W mains LED bulbs, these say 'non-dimmable' on the packet.

    I just wondered what the difference was electrically (differences in the little power supply in the base of the LED?) and, although maybe not commonly available off-the-shelf in an electrical retailers, would it be possible to rig up some kind of bespoke circuit for dimming this type? Despite the labelling? Ideally without resorting to plain rheostats

    Sorry I've only got 6 of these bulbs & they're all in use otherwise I'd smash one up to inspect the circuit inside & provide pictures etc. They're 4 Watt LED candles from ASDA. E27 base.
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    That's an interesting question, I am guessing that it's load dependant as I know most dimmers have a minimum load current requirement. this is to ensure the TRIAC is triggered correctly and the holding current is maintained. As the LED light is dimmed this draws even less current and would make the situation worse. I have never looked inside a dimmable one but I would have thought they would need some kind of shunt to allow the TRIAC to work properly. I am not sure if there is a simple solution to this other than maybe hiding a small light bulb somewhere to act as a load, but this defeat the object of LED bulbs. You can obviously design something if you were starting from scratch, I think TI do the ICs.
    Adam
     
  3. Gryd3

    Gryd3

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    Jun 25, 2014
    Would it be worth experimenting with that idea and connecting a single incandescent to the lamp to see what happens?
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes it might work.
    Adam
     
  5. flippineck

    flippineck

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    Sep 8, 2013
    Just doing some reading around on it; it seems the most commonly used power supply in these lamps consists of a bridge rectifier across the AC input, with the rectifier's DC side feeding a storage capacitor. The LED is connected across the capacitor and draws it's current from there.

    Page I was reading does make the point though that all these lamps suffer from a massive lack of standardisation.

    http://www.ledsmagazine.com/article...-issues-to-ensure-compatibility-magazine.html

    bit more illumination on the innards of these bulbs here

    http://www.instructables.com/id/Transformer-less-12V-power-supply-out-of-LED-bulb/

    Was hoping if possible to keep away from incandescents.

    If I'm understanding things right, the way this power supply works is essentially that the LED just takes a very small current off the capacitor, which is acting like a dam holding back the huge force of the mains so to speak? In other words, if I was going to try dimming by controlling the AC input peak voltage rather than by cutting the phase like a traditional dimmer, I'll need to be cutting that AC voltage right down to a very low range compared to the 240V mains?

    Could I make a device that would give me say a controllable 0-5VAC from a 240VAC mains input, into this sort of bridge-and-capacitor arrangement? Without significant power loss in the device itself?
     
    Last edited: Sep 17, 2014
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    Understood, but if the addition a single incandescent alters the behavior of the LEDs due to a change in load that the dimmer is responsible for, it may open up other opportunities for a solution.
    Currently, only thing I can think of is trial and error with some dimming compatible LED bulbs, or swap out the dimmer for Triac Type.
     
  7. flippineck

    flippineck

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    Sep 8, 2013
    It wouldn't be hard to try a triac dimmer & change one of the bulbs for a small incandescent.. just a trip to asda and 10 minutes with a screwdriver so I'll give it a go. Worth the extra wattage if it keeps the missus happy, she's doing her nut about "it's all too bright now"
     
  8. Gryd3

    Gryd3

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    Jun 25, 2014
    The incandescent may not be required. As mentioned above, if the sum of current for the collection of LED bulbs you are running is sufficient to provide the holding current for the triac to turn on then you can chuckle to yourself and stick with LED all the way.

    Good find on those LED links by the way.
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    What I meant was add a bulb in parallel to the dimmer so it's driving an incandescent bulb. Not to replace any of the LEDs. You may be able to use a high wattage resistor in place of the bulb also. Once we know if this works then we can looks at that.
    Adam
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Just tell her it's so you can see how beautiful she is :)
    Adam
     
  11. Gryd3

    Gryd3

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    Jun 25, 2014
    Replacing an LED for an incandescent will do that without requiring any wiring modifications as the lights should all be wired in parallel anyway ;)
    Hopefully he tests the incandescent with the current dimmer though before swapping it out.
    If the incandescent makes a difference to the LEDs with either dimmer, than an extra light can be stashed away somewhere or a high power resistor like you mentioned.

    Sounds like lots of practice making excuse/compliment combos to the wife ;)
    Must work well
     
  12. flippineck

    flippineck

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    Sep 8, 2013
    I had an idea. Could I drive a small transformer, say a 6:1 turns ratio, off the dimmer then drive the LEDs off the transformer secondary? Or, have the transformer in front of the dimmer?

    I thought that might bring the AC input voltage range down to a suitable level whilst preserving efficiency?
     
  13. flippineck

    flippineck

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    Sep 8, 2013
    sorry about double post.. couldn't edit.

    I've tried what I believe to be a trailing-edge dimmer with a push on-off and rotary dim action on the same knob.

    http://www.answers.com/Q/Trailing_edge_dimmer_how_does_it_work
    http://www.chegg.com/homework-help/...x-t-generates-periodic-signal-y-alph-q1468714

    ... temporarily 'borrowed' from my earlier installation.

    It works with my 6 'non-dimmable' LEDs, they light up when I press the knob and they do get smoothly brighter as I turn the knob clockwise from minimum. They go up to full brightness as measured by my calibrated eyes ;-)

    However, they're also pretty bright when the knob's at minimum. Still way too bright for the missus.

    Looking at the second link above, which shows both the idealised waveform produced by the trailing edge dimmer, and also the waveform you are likely to actually see in practice, am I being realistic to think I can preserve efficiency by trying to run such a waveform through a small step-down transformer? Should I try stepping the mains down to say 100VAC *before* the dimmer using a transformer?
     
  14. Gryd3

    Gryd3

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    Jun 25, 2014
    I think you're relying on how the circuit for the LED is built.
    At least with a dimmer, you have a sort of PWM by chopping the waveform at the front or back... If you use a transformer, you will be constantly supplying a lower voltage than expected and could strain or damage the power supply in the LED bulbs. This is just a guess though, but I would hate to have you go through the work for no result.
     
  15. flippineck

    flippineck

    269
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    Sep 8, 2013
    Trip to the supermarket, bought a bog standard inline dimmer (leading edge chop / triac type? a guess - no info in pack. Cost a fiver i.e. next to nothing)

    Fitted it expecting a bang when I threw the breaker back on.

    Works! Well, works good enough for my needs

    There's noticeable mains frequency flickering (at all levels of brightness, a wave of the hand in the air reveals a lovely stroboscopic pattern if you look close) and at certain levels, the flickering gets real bad, more like a flast blinking or fluttering.

    However, there's a very low level of illumination in there somewhere that's well liveable with, and when the knob's turned up to full it's fully bright with no discernible flicker other than the slight 50Hz strobe if you're looking for it (which might have been there before even with the plain on/off switch i guess)

    That's good enough for my needs. As long as the switch or the bulbs don't pop, and my missus agrees she can live with the attainable level of dimness on at least one part of the dial, guess I'll be stopping here rather than trying to perfect it with transformers and additional loads etc.

    Interesting to note though, the dimmer switch I bought specifies a minimum load of 40W. My LEDs total 24W. So I've still not got it wired up correctly really. I'll try changing one of the six LEDs (yes they're all in parralel) out for a 20 Watt incandescent when I can find an 27mm screw base one. Just to see what happens.

    Non dimmable LEDs with the wrong type of dimmer, useable out-of-the-box. Whod'a thunk.
     
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