Question about these LEDs

Discussion in 'General Electronics Chat' started by vindicate, Nov 21, 2011.

  1. vindicate

    Thread Starter Active Member

    Jul 9, 2009
  2. Wendy


    Mar 24, 2008
    Could be either. When you buy a cluster like this you are buying a finished circuit.
  3. SgtWookie


    Jul 17, 2007
    It's probably three strings of three LEDs, and only one current limiting resistor.
    If the typical Vf is 2v, then that's 6v for the LEDs, and 6v to 8v across the resistor; about 308 Ohms for the resistor to get 26mA current, and at 12v they'd have ~19.5mA current.

    Made as cheaply as possible, to extract as much profit as possible.
  4. Audioguru

    New Member

    Dec 20, 2007
    12V/9= 1.33V. But red LEDs are at least 1.8V so 9 of them could not be in series.

    I think they are connected in three groups of 3 in series and each group has its own series current-limiting resistor.
  5. vindicate

    Thread Starter Active Member

    Jul 9, 2009
    Isn't that dangerous in an auto application with the possible high voltage spikes?
  6. Audioguru

    New Member

    Dec 20, 2007
    The strings of LEDs have series current-limiting rersistors so voltage spikes will not be a problem.
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    I have seen some lamps of this general type incorporating a bridge rectifier input. When first I saw this I wondered why, as of course this wastes a couple of diode drops. It turns out that this has a few advantages:

    1. You can't connect it the wrong way round. Particularly useful when used to replace tungsten lamps which are not polarity sensitive.
    2. You could use it on AC (but not necessarily on the same voltage, and it will flicker more than a tungsten bulb).
    3. Reverse spikes will be turned into forward (maybe not perfectly, due to diode switching delays).
    Provided the diodes are up to it, that last may be a big help. Reverse breakdown is very bad for LEDs, but most can stand large short-term overload in the forward direction.
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    They are already current limited in the strip.

    I wouldn't expect them to last long, and I'd be sure to add a 1A fuse in series with it.