What resistor value

Discussion in 'The Projects Forum' started by Heinz57, Jan 11, 2010.

  1. Heinz57

    Thread Starter New Member

    Dec 16, 2009
    24
    0
    Hi all,

    I have 3 LEDs rated at 1.85 - 2.5v (max) and a current of 20mA. They are powerd from a 12v DC supply.

    I just want to check which resistor values to use. I worked it out at 600Ω. Is this correct?

    Cheers,

    Heinz
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You start out using their typical Vf @ current rating.

    Then calculate:
    Rlimit >= (Vsupply - (Vf_LED_total)) / Desired_Current (this is for three LEDs wired in series, not parallel)

    You are better off to use too low of a Vf in your calculation than too high.

    Is your DC supply regulated, or a "wall wart"? Wall warts are not typically regulated. If you are in doubt, measure the output voltage using a multimeter or DMM before you do anything.
     
  3. Heinz57

    Thread Starter New Member

    Dec 16, 2009
    24
    0
    I've just done:

    (12 - ( 1.85 )) / 20

    That's given me 0.5075. It doesn't seem right?

    The supply is a 12v DC model railway controller.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That would be for 20 amps. Divide by .02 (20 ma) and you get 510 ohms (actually 507, but 510 is a standard value).
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, it would be:
    Rlimit >= (12 - (3*1.85)) / 20mA
    Rlimit >= (12 - 5.55) / 0.02
    Rlimit >= 6.45 / 0.02
    Rlimit >= 322.5
    322.5 is not a standard value of resistance. A table of standard resistor values is here:
    http://www.logwell.com/tech/components/resistor_values.html
    Bookmark that page.
    330 Ohms is the closest standard value.
    Re-calculating to determine current:
    6.45v / 330 Ohms = 19.545...mA

    calculate wattage requirement:
    P = EI (Power in Watts = Voltage x Current)
    6.45v * 19.546 = 126mW; we double this for reliability; 252mW. This is less than 1% over 1/4 Watts, so you can use a 1/4W resistor.

    Now if you were running single LEDs with single resistors, you would calculate using just one LED's Vf as Beenthere did.
    Note that the wattage requirement will be higher.
     
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