Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Led forward current cal

  1. May 30, 2012 #1
    We are using smd chip led 3528 which prescribe 20 miliamp standard forward current. I want to know basic calculation like those bulb. How do i find forward current?

    12 volt dc light, 48 pc smd chip, 16 parallel connection 3 series (as each chip volt is 3.5 to 4)
    Now we found that 3.14 watt is consuming this bulb means 0.26 amp consumption (12 volt). How we can found the each smd forward amp?
    Last edited: May 30, 2012
  2. jcsd
  3. May 30, 2012 #2


    User Avatar
    Science Advisor

    You have 16 parallel LED strings drawing a total of 260 mA, so each one is drawing (260 mA / 16) or 16.25 mA.

    Is that what you were asking?
  4. May 30, 2012 #3
    The bulb is 12 volt running dc but as each smd using 3.5 to 4 volt that is why first 16 in parallel and and those three (16 pc parallel each parallel) three in series. If 12 volt condition total 48 pc smd consumed 260 mili each how much forward current?
  5. May 30, 2012 #4


    User Avatar
    Science Advisor

    16.25 mA.

    The LEDs in series carry the same current.

    Note that there is probably a resistor in series with each string of 3 LEDs. It might be about 100 ohms in this case.

    Are you having a problem with this LED display?
  6. May 30, 2012 #5
    How do we calculate them always? Like we can get the reading amp how much consumed say 0.26 amp 12 volt. Now divide to three series each series 4 volt and each series 16 pc parallel chip. Got any easy calculation?
  7. May 31, 2012 #6


    User Avatar
    Science Advisor

    Do you mean "how would we design this, using these LED's ?"

    I like to allow 25% of the voltage for a series resistor, so if I had 12 volts, I would work out how many 3.5 volt LEDs could use 9 volts.

    It would be between 2 and 3 and this design uses 3.

    OK, so using 3 LEDs which each use 3.5 volts, that would be 10.5 volts.

    This leaves 1.5 volts for the resistor and the current is 20 mA. So the resistor has to be 1.5 volts / 0.02 amps or 75 ohms. To be safe, you would choose the next highest value which is 82 ohms.

    So, you have an 82 ohm resistor in series with 3 LEDs, each dropping 3.5 volts. The series string would draw 20 mA approximately. You could put any number of these across a 12 volt supply as long as the supply could deliver the current.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook