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LED transformers

Discussion in 'Electrics UK' started by hard-work, 14 Sep 2017.

  1. hard-work

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    There are 12v two LED strips with a wall switch for each.
    The idea is to have one LED transformer feeding the two.
    Transformer -> 2 gang light switch -> an LED strip on each gang.
    The switch will switch 12v not 230v.
    Anyone got any comment on this. The men at the counters in shops all say a transformer needs to be on each strip, probably trying to sell more. I see no reason why one transformer that is big enough cannot be used.
     
  2. SFK

    SFK

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    Hard-work,
    No issue on having one suitably sized LED Driver (not a transformer) powering more than one LED strip.
    Many do this when they have say a strip of LEDs under each of several shelves, all driven by one LED Driver.

    An [The only<Deleted] issue to watch out for in your setup is that the 12V wire does not get so long that Voltage Drop becomes and issue and the furthest away LEDs become dimmer.
    If an issue, this can be minimised by using thicker wire wire and by having the wire carrying the 12V going into both ends of the LED strip, or one wire into the middle of the LED strip.

    SFK
     
    Last edited: 15 Sep 2017
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  3. hard-work

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    SFK, thanks. The longest wire is about 4 to 5 metres and 1.00mm.
     
  4. SFK

    SFK

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    Hard-Work,
    That should be good.
    For LED strips I always pre-test the LED Driver and Strips on table first, as a pain-in-the-neck problem solving when looking upside down at bottom of a kitchen cabinet. :>

    My warning always with LEDs is to be extra careful/vigilant as you are dealing with both 240V and 12V and it is easy to mistake/forget which cable is which and perhaps touch the 240V!
    SFK
     
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  5. ericmark

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    A 12 volt strip light one would expect to be DC however it does not need to be supplied with ripple free DC it can be fed with a rectified supply directly from the transformer used to drop the voltage so the wave form could look like this [​IMG] or it could look a lot worse with a full off period in the mark/space controlled output. The LED is a current device and it can handle a set amount of power before overheating, but as long as the RMS voltage is kept to 12 volt having 16 volt spikes does not really matter.

    This means a cheap LED power supply be it either AC, or DC voltage regulated or current regulated can still cause RF interference. I note some do state interference free, which would hardly need stating if others were not interference free.

    The problem is to find which are OK and which are not you need to read the data sheet, it is often not mentioned on the sales advert, so many once you read the data sheet limit the cable length to 2 or 3 meters which also includes the length of the strip. It really does not matter what label is on the device, transformer, driver, power supply, or electronic transformer until one reads the data sheet one can't really say, OK with some you can see it is a simple wire wound transformer so you know not really a RF problem, but even if you can see capacitors inside it does not mean it is a smooth output.

    So some thing looking like this [​IMG] is likely OK designed to work from zero output and with a smooth output, but with these bricks [​IMG] when it says for LED lighting you really don't have a clue without a lot of reading which can have long output leads and which can't. In the main the strip LED lighting is only for decoration, often with a lumen per watt output little better than a tungsten lamp, 20 lumen per watt is common.
     
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  6. hard-work

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    How about this:
    [​IMG]
     
  7. JohnW2

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    The main problem with that approach is that it means that the power supply ('LED transformer) would have to be powered continuously, even when neither of the lights were on. That could have (small) implications in terms of energy usage/cost and may affect the life expectancy of the power supply.

    One could achieve what you want by using relays, so that the power supply was only powered up if one and/or the other light was switched on, but the cost and complexity of that would probably make it less sensible than just using two power supplies.

    Kind Regards, John
     
  8. hard-work

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    John, if there is no load on an LED transformer, why would it use power?
     
  9. JohnW2

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    They do, albeit not a lot - very few things in life are 100% efficient. Even a true (wire-wound) transformer will, but the sort of 'electronic transformer' (actually a witch mode power supply) certainly will have a quiescent ('no load') power consumption. The running cost of that quiescent power consumption is probably fairly trivial, but I would personally be more concerned about the possible effect on the life expectancy of the 'transformer' (and, I suppose, if I were prone to 'paranoia', also the theoretical fire risk of having something permanently powered up).

    I suppose it's a bit like leaving a TV, or whatever, in 'standby mode' rather than switching it 'properly off'. Again, there is a (pretty trivial) cost in terms of power consumption and a theoretical (extremely small) fire risk.

    Kind Regards, John
     
  10. SFK

    SFK

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    Hard-Work,
    The device you are showing there has an output of "11.5V AC" which is not suitable as you need "Constant Voltage 12V DC LED Driver" for your strip of LEDs.
    I used a Search for "LED Driver" on say amazon and looked for the ones with good write ups, not too cheap and not shipped from China.
    https://www.amazon.co.uk/s/ref=nb_sb_noss_2?url=search-alias=aps&field-keywords=LED+Driver
    https://www.ledhut.co.uk/led-driver-transformers.html?fq[base_type]=Driver

    However, as highlighted by JohnW as "the power supply ('LED transformer) would have to be powered continuously" 24/7 365 in your setup, so in your senario I personally would be extra diligent to get a good LED Driver (with a defined lifetime) as it will be on all the time and (as you have it before the switch) perhaps harder to isolate from the 240V when it fails.

    SFK
     
    Last edited: 14 Sep 2017
  11. hard-work

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  12. Iggifer

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    Used a few of those for myself, absolutely no complaints. Got 230VAC in and out and 3 sets of 12VDC outputs, which can be handy
     
  13. SFK

    SFK

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    Hard-work,
    I have never used a LED driver in 100% continuous use, so cannot answer your question.
    I have bought the amazon ones for personal DIY use, but as they are after switch they are only on for at most a few hours a day.
    Perhaps as an example this site (never used) shows LED Drivers with lifetime specifications, which is what I would look for if wanting quality (note you want teh constant voltage ones)
    Please login (registration is free!) to view this link.
    https://www.helvar.com/en/products/c-1/


    I would agree that the screwfix ones are likely to be of higher quality, and you have a recommendation from Iggiffrr. But I have never bought one of these as they seem low Wattage for the price and so I assume will only drive a short length of LED strip (I presume most LED strip needs about 5W per meter length). At that price you might be better buying amazon ones and having them after the switch and next to the LEDs removing the noise issues highlighted ericmark.

    But the big question I have for you is: what Wattage of LED lights are you wanting to drive, as this determines the Driver needed?

    Sfk
     
  14. Iggifer

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    I don't think it's the same for CV drivers, but if you switch the load side of a CC driver, it will blow up whatever is on the end of it each and every time.

    I don't know the internal processes of a CC driver, but it seems from some minor investigation, with no load applied as they're running they will ramp up to full voltage, applying a load whilst it's ramped up (as you would switching it on) will obviously blow it up. It must ramp up the voltage on switch on and stop when it gets to it set current.

    I would imagine the same problem is not present in CV drivers as the voltage is what's constant but it's something to bare in mind, and definitely don't buy a Constant Current driver, even if you know your exact wattage.
     
  15. hard-work

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    Thanks to all. :)
    One point. It may be possible to switch the led transformers. But a redundant earth wire needs to be used. Is this allowed? The bare wire can have a red sheath over it.
     
    Last edited: 14 Sep 2017

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