Wirewound Transformers on Dimmer?

Discussion in 'C-Bus Wired Hardware' started by Ingo, Aug 11, 2007.

  1. Ingo

    Ingo

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    Hi,
    I am faced with a problem where I want to run my Pool light (1 x 12V/50W) and some Kitchen Pendant lights (3 x 12V/50W) from CBUS. The problem is that I only have two dimmer channels available. The Pool light isn't wired yet but the pendant transformers are installed but with no lights attached. When I switch the dimmer ON it trips the 1A breaker. If I enable the breaker again then it stays on.

    Do you guys think these transformers will give me hassles? I suspect the initial current surge might just be too much for a 1A breaker used for the pendants. I can put a larger breaker in or perhaps a fuse that will take the initial surge for a longer time period. Any suggestions?

    Oh, and the Pool transformer looks very crude, just a big mother of a transformer out of Noah's ark - looks that anyway. I am going to mount the transformer in my supplementary DB and run 12V to the pool about 12m away. Does that sound fine or should I replace the Pool transformer for a Possum, I have one spare?

    Thanks,
    Ingo
     
    Ingo, Aug 11, 2007
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  2. Ingo

    znelbok

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    you can put a d-curve breaker in (as opposed to the C-curve that you probably have now) to handle the in-rush

    wire wound tx's supposedly don't have capacitance, so they should be fine on a dimmer (as long as the load is less than 1A as well).

    You can run your 12m at 12V, just put in a thicker cable to cover voltage drop. You don't want to lose more than about 1V.

    The other option is to put a 13.5V transformer in and let the cable drop the voltage to 12V for you. Drop the voltage too far and the current goes up and everything heats up as well.

    See if you can find the DC resistance of the cable and then calculate the voltage drop using ohms law.

    Mick
     
    znelbok, Aug 11, 2007
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  3. Ingo

    Ingo

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    Thanks Mick, that sounds like good ideas. The existing cable is fairly thick, I don't have the guage # though but it shouldn't drop too much.

    Ingo
     
    Ingo, Aug 12, 2007
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  4. Ingo

    Darpa

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    Another possible solution to your Pool Light issue is to use 24v instead. This is common practice when running SELV (Safe Extra Low Voltage, or below 50V DC or 32V AC). If you can get your hands on a 24v globe that will fit your pool fitting, try using a 24v AC transformer instead. It's still safe if anything goes wrong, and you're much less likely to have any problems with voltage drop.
    Also, it would pay to put a suitably rated fuse just after the transformer.
    Just a thought :)
    Darpa
     
    Last edited by a moderator: Aug 12, 2007
    Darpa, Aug 12, 2007
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  5. Ingo

    JohnC

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    Lets say that the wirewound transformers were small cheap ones - like the Atco black wirewound units. Those have about 16W loss per lamp, so lets say 66W per lamp in total.

    The pool light should draw 66W which is 0.275A
    The 3 x dining lights should draw 0.825A

    Neither of these should cause a problem to the dimmer. They also shouldn't case a problem to the breaker either, as a C-curve breaker is designed for normal loads like ELV transformers. Also, a large degree of the lamp's inrush current is "absorbed" by the transformer, since it is incapable of supplying the full (many amps) inrush drawn by the lamp.

    So, it is alarming that you are experiencing MCB tripping. As an aside, why are you using the 1A MCB anyway - is this a requirement of the wiring rules in SA, or are you doing it to try and protect the dimmers?

    The large pool transformer is probably drawing a lot less power than a cheapy transformer - because lots of iron means efficiency! So, whilst it may look old and crude, you will probably find that it draws considerably less power than a more "modern" solution.

    There's nothing stopping you using Electronic trannies like a Possum, and the suggestion to use 24V is a great idea to reduce the voltage drop. A lot of installations use 32V as well, which will reduce the seconsdary current and voltage drop even further. However, be aware that Safety transaformers in 24V and 32V are MUCH more expensive than 12V transformers.
     
    Last edited by a moderator: Aug 13, 2007
    JohnC, Aug 13, 2007
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  6. Ingo

    Ingo

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    For the moment I am going to try the existing pool transformer but all depends on the voltage drop. Next will be the 24V option.

    As for the pendants, the MCB is there to protect the dimmer channel from possible overload. Rather have the MCB trip then damage a channel with overcurrent situations.
     
    Ingo, Aug 13, 2007
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  7. Ingo

    JohnC

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    Fair enough, however you should be aware that the tripping characteristics of an MCB means that they only trip quickly if there is a huge percentage of overload. For example, a 1A C-curve or D-curve MCB would take about 10 seconds to trip at 3 times rated current, or 2 seconds at 5 times rated current. In that time, the triac may still overheat or fail due to the overcurrent situation.

    In other words, in many overload situations an MCB usually reacts far too slowly to protect the Triac in the dimmer. From what I have seen in the field, if there is a full short circuit on the output the triac will often blow irrespective of whether there is a MCB or not.

    However, the great thing about having individual MCBs on outputs is that you can isolate the sub-circuit easily, which is very handy for maintenence.
     
    JohnC, Aug 14, 2007
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  8. Ingo

    Lucky555

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    Seeing as you sound like you know what you are doing it seems very unlikely that your circuits will end up overloaded 2 or 3 times channel rating. A short circuit fault can be caused by several factors including a lamp blowing (especially lamps facing upwards) As JohnC said a good short will see you saying goodbye to your dimmer triac long before the MCB trips. :( The individual MCB's per channel are still a good idea for safety reasons (major fault) and for isolation purposes. ;)
     
    Lucky555, Aug 14, 2007
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  9. Ingo

    Ingo

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    I measured the voltage at the Pool side and it's 12.3V. I am going to use the existing Pool transformer to drive the sealed-beam light. It looks like an old VW Beatle light with a high and low beam. The electrician wired both (75W & 50W) in parallel to give a total of 125W. The problem again is with the 1A breaker not withstanding the inrush current. I am going to change that to a 10A just to have individual control and not to protect the dimmer - we now know that any MCB is useless unless there is a major fault.

    Thanks for all the input.

    Ingo.
     
    Ingo, Aug 17, 2007
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