led help or guidance appreciated please

Discussion in 'General Electronics Chat' started by MartinL, Oct 2, 2012.

  1. MartinL

    Thread Starter New Member

    Oct 2, 2012
    Hi Folks,
    I'm a cabinetmaker and recently designed and installed a large bedroom complete with led lighting above the cornice for a customer. These led's being straight forward and powered by a transformer from the 240v mains.

    I had some led's left and decided to put them to use in my boat. They are 5050 smd led's purchased in a string of 5 metres and can be cut to any length in multiples of 50mm, each 50mm containing three led lamps.

    They work and better still, if used throughout the whole boat, would reduce power consumption dramatically from over 20amps to less than 2amps if all switched on simultaneously.

    I have tested a sample of six led's in one light and left them on for overnight for use as a nightlight. They have progressively become dimmer with time, the splashproof coating has become hard and brittle. There is a simple answer and a solution to stop this happening.

    The boat's 12v dc system is is usually fully charged at 12.8v and most of the time in the marina, shore power is connected and when charging, the voltage rises to as much as 14.4v. I suspect it is current that is killing them.

    Any advice or solutions would be very much appreciated.

  2. crutschow


    Mar 14, 2008
    LEDs are current operated and can not operated directly from a voltage. They need a means to limit the current through them (often a resistor) to below their maximum rating. It sounds like you are seriously overdriving them and they are overheating.

    The 5050 has a 3V drop per LED and is rated for 60mA maximum. Thus the resistor required would be (14.4 - (3 x #Leds)) / .06 where #Leds is the number of LEDs in series. For a 3V LED and a 12V supply you shouldn't put more than three LEDs in series.
    MartinL likes this.
  3. cork_ie

    Active Member

    Oct 8, 2011
    Yes your guess is on the right track, the life of an LED can be dramatically shortened if the current is too high.
    There is an optimal current for most types of LED. It varies between different types and colours and you will really need to look at the specs. of the ones you have.
    Was there any datasheet or instructions supplied with them? If so it would be the best place to start.
    MartinL likes this.
  4. MartinL

    Thread Starter New Member

    Oct 2, 2012
    Its quite a search to get the datasheet but the general info is
    per 5 metre strip there are 300 5050smd led's
    rating is 12v with a total power consumption of 72w for the entire strip length.
    These led's are cool white and it seems that cutting a length which contains either 9 or 12 led's is sufficient to replace the existing tungsten lamps which are rated at 10w and of course draw far more current than the led's. Of course what I have been doing is wiring them directly to the 12v supply, the problem only comes to light when you leave them running for quite long periods and they cook themselves.
    I need to find out what to do to stop the cooking process, ie regulate the current they draw.
  5. Wendy


    Mar 24, 2008