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powering a LED help

Discussion in 'LEDs and Optoelectronics' started by jarrett, Nov 18, 2013.

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  1. jarrett

    jarrett

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    Nov 18, 2013
    Hi, first post here. I want to backlight some old vacuum tubes. I'm looking for some help powering about 3 leds http://www.superbrightleds.com/moreinfo/high-powered/led-product/1076/1876/ can I use this driver http://www.superbrightleds.com/moreinfo/lenses-mounts/350ma-constant-current-led-driver/338/1284/ to power them by just connecting them in series and wiring it directly to this device or will this be overpowered? I'm looking to keep this compact as possible and don't have room for any other power supply I need something to work off of 120v supply. Any other suggestions are certainly welcome
     
  2. BobK

    BobK

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    Jan 5, 2010
    Yes, that driver is appropriate. With 3 1W LEDs this is going to very bright!

    Bob
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,419
    2,790
    Jan 21, 2010
    You may need to mount the LEDs on something to help keep them cool. The star heatsink may be sufficient, but attaching it to another large piece of metal (one that doesn't get hot) would help. Ensure that you don't short anything out doing this.
     
  4. jarrett

    jarrett

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    Nov 18, 2013
    Thanks Bob, If these are too bright for my liking could I add a resistor to dimm them down a bit? And Steve thanks for your suggestion about the heatsink ill see what I can find, maybe ebay.
     
  5. BobK

    BobK

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    Jan 5, 2010
    No, you cannot add a resistor to dim them if you are using a constant current driver, it will just up it's voltage to maintain the constant current!

    Bob
     
  6. jarrett

    jarrett

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    Nov 18, 2013
    So I changed my mind after looking around some and think I will go with smaller low power LEDs with a forward voltage of 3 to 3.5v and current of 20ma. I read the page on here titled "got a question about driving LEDs?" Very helpful by the way but I'm still not quite shure if I can use the same 350ma constant current driver. Does the 350ma just means that's the max it will supply and the LEDs will just take what they need? Or is there a compact way to power these LEDs with a 120v ac supply
     
  7. BobK

    BobK

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    Jan 5, 2010
    No, a constant current driver is just that, it will raise the voltage high enough (within limits) to make 350mA flow. If you put a 20mA LED on it, bad things will happen.

    Bob
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    LEDs running at 30 mA or less can be powered "directly" from the mains supply using a series capacitor and bridge rectifier. This is normally frowned upon because the LEDs are live, but if it's part of an enclosed product, and you're careful that everything is properly insulated and firmly mounted, some of these concerns are moot.

    The circuit involves a fusible input resistor and an input capacitor (which must be rated and approved for this application), feeding a bridge rectifier, feeding a smoothing capacitor with a zener diode across it, feeding a string of LEDs in series with a current limiting resistor. The circuit can be fairly compact - it would fit comfortably into a cube one inch on each side. It avoids transformers.

    If you are backlighting some toobs, though, you should already have a low voltage supply available for the heaters of the toobs. This is usually 6.3V (or 12.6V) and can easily supply a bit of extra current for the LEDs.

    What do you think?
     
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