How many LEDs to a dimmer

Discussion in 'C-Bus Wired Hardware' started by l60mcm, Mar 24, 2012.

  1. l60mcm

    l60mcm

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    I have searched the forums but am still not 100% sure how many of these I could put on each channel of a 1a 8 channel leading edge dimmer. Or what information I would need to get to work it out. I have had great success dimming 1,2&3 of them from 1%-100%. But don't want to put any more on before I know it's safe to do so. Thanks
     
    l60mcm, Mar 24, 2012
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  2. l60mcm

    tobex

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    on leading edge dimmers = 300nF capacitance is your limit.

    on trailing edge = up to the limit of your channel supply (1A, 2A, .....)
     
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    tobex, Mar 24, 2012
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  3. l60mcm

    Newman

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    The limit for the L5508D1A and L5504D2A C-Bus leading edge dimmers is 300nF per channel.

    If the manufacturer doesn't say what the capacitance is then the only way to know would be to measure one. Start by measuring the capacitance across the input terminals of the lamp with multimeter. If you get a nice number like 33nF, 39nF, 47nF, 56nF, 68nF, 82nF, 100nF, then you can be confident in the reading. If you get an odd sort of number then the circuit arrangement inside the lamp is interfering with the reading and you'll either have to chase the manufacturer for the info or open one up and take a look.
     
    Newman, Mar 25, 2012
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  4. l60mcm

    l60mcm

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    Thanks both for the quick reply. One other quick question about channel load. When working out the load for halogen bulbs is it just ohms law 4x50=200w 200w/240v = 0.8a so 4x 50w halogen gu10 bulbs are safe on a 1a Chanel or am I missing somthing
     
    l60mcm, Mar 25, 2012
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  5. l60mcm

    Newman

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    That is correct for GU10-based 50W 240V MR16 halogen lamps (and other mains voltage lamps).

    For 12V halogen bulbs the transformer losses and the power factor need to be taken into account, so you need to allow a little extra. Even so, 4 x 50W ELV halogen MR16's can typically still be connected to a single 1A dimmer channel.

    As of 14th April 2012 the importation of ELV MR16's greater than 37W will be banned, so that's going to mean pulling out the calculator once more.
     
    Newman, Mar 25, 2012
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  6. l60mcm

    tobex

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    I was just about to ask "how do I work out capacitance". Thanks.

    I do have another question. What is the type of capacitor which should be used on the original model CBUS 4 channel dimmer. I want to balance a load with 2 LED's on it and one LED is flickering. Is there a model / brand / spec / construction of capacitor which would suit this application.

    I like this for testing

    http://www.directindustry.com/prod/schaffner-emc/feed-through-capacitors-15134-473197.html

    I presume that I must isolate the lines from power to do this test.
     
    Last edited by a moderator: Mar 25, 2012
    tobex, Mar 25, 2012
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  7. l60mcm

    Newman

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    If you are thinking of fitting a capacitor in parallel with your existing loads then the combined total of the capacitance of the lamps and the additional capacitor must not exceed 300nF. If your 2 existing lamps are 100nF each then you can only fit an extra 100nF in parallel with this. The capacitor needs to be an X2 type.

    The only caveat with this is that the real loading is too low relative to the capacitance the load may become unstable. The only way to know for sure is to try it out.
     
    Newman, Mar 25, 2012
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  8. l60mcm

    tobex

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    While I do not have this problem myself, my friend has 2 or 6 LED loads on a channel and one of the lamps flickers ever so lightly.
     
    tobex, Mar 25, 2012
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  9. l60mcm

    l60mcm

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    I have just contacted the supplier of the led?s and they can not give me the capacitance of the led?s. I have a multimeter that can read capacitance but I am unsure how to do it. Do I read the bulb when it is unplugged or do I test it in series or parallel to the bulb with the current on?

    Thanks
     
    l60mcm, Mar 26, 2012
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  10. l60mcm

    tobex

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    Measuring the bulb is probably much faster.
     
    tobex, Mar 26, 2012
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  11. l60mcm

    l60mcm

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    do you know the best/easiest way to do this
     
    l60mcm, Mar 26, 2012
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  12. l60mcm

    pat911

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    You need to measure the capacitance that the C-Bus channel would see. You would therefore measure the capacitance accross the same two wires/connectors that would normally attach to the C-Bus output, i.e. the 230vac input to the driver. You would do this whilst the unit is DISCONNECTED from the C-Bus output, and any other source of 230v for that matter. I have successfuly used this method to measure the input capacitance of electronic halogen transformers.

    Cheers
    Pat.
     
    pat911, Mar 26, 2012
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  13. l60mcm

    Newman

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    This is the technically correct way to do it but obviously requires opening up the switch board to gain access to the wiring and disconnect the circuit from the dimmer.

    Do not do this with the current on. That would be quite a dangerous thing to do.

    If you have a multimeter that measures capacitance then you can often just place the probes of the meter across the two terminals of the lamp. You do this with the lamp removed from the circuit, i.e. on a work bench or table. This won't give you the total but it will give you the amount of capacitance for each lamp, which you can easily add up given how many lamps are on the circuit.

    Just bear in mind that this occasionally doesn't work as some circuits inside the lamps can interfere with the multimeter reading. Post the reading you get back here and we'll comment on whether it sounds reasonable.
     
    Newman, Mar 27, 2012
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  14. l60mcm

    tobex

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    I just purchased an UNI-T UT136B. It looks strikingly similar to a FLUKE multimeter. But since I have several UNI-T units I am satisfied to have one more.

    http://img10.360buyimg.com/n1/1006/6eb8a407-adc2-47f8-8c84-10f8cc5e722c.jpg

    The grand price was $26. But I had to import it as there was nothing like that locally.

    The main reason for buying it was that it handled auto-ranging and start from 2nf.
     
    tobex, Mar 27, 2012
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  15. l60mcm

    Roosta

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    Hey Gents,

    I have a LED Downlight which the manufacturer is saying is 16w 80pF.. Does this sound right?

    Cheers,
    Nik
     
    Roosta, Mar 27, 2012
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  16. l60mcm

    Newman

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    If they said 80nF I would believe them, but not 80pF.
     
    Newman, Mar 27, 2012
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  17. l60mcm

    l60mcm

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    I did try the bulb on its own but the meter did not read any thing
     
    l60mcm, Mar 27, 2012
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  18. l60mcm

    Newman

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    Surprising. There should have been some sort of reading at least. The meter leads were plugged into the correct sockets on the meter and the meter was in the right mode?
     
    Newman, Mar 27, 2012
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  19. l60mcm

    l60mcm

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    I think I have done it correct but i could be wrong [
     

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    Last edited by a moderator: Mar 27, 2012
    l60mcm, Mar 27, 2012
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  20. l60mcm

    DarylMc

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    That's interesting.
    The only time I ever put a multimeter to check resistance across a GU10 halogen I got an open circuit.
    Tested a few by plugging into their live sockets and they all worked fine.
    Maybe it's hard to get a conducting area on the pins even if it doesn't look like it would be?
    If you have a bare socket handy on a fitting which is not installed can you give it a go by measuring your capacitance at the wiring terminals?
     
    DarylMc, Mar 27, 2012
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