+ Post New Thread
Results 1 to 3 of 3
  1. #1
    Full Member level 6
    Points: 3,158, Level: 13
    Achievements:
    7 years registered

    Join Date
    Nov 2010
    Posts
    341
    Helped
    2 / 2
    Points
    3,158
    Level
    13

    uln2003 base resistor

    in my circuit the input and output of uln2003 is 24v dc what value of base resistor can i use in order to drive a 24v 400ma relay.
    if my input is 5v then i can connect directely but for 24v what value of series resistor can i use. is 10k ok.

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 3
    Points: 7,294, Level: 20
    Achievements:
    7 years registered

    Join Date
    Jul 2010
    Location
    Sweden
    Posts
    967
    Helped
    374 / 374
    Points
    7,294
    Level
    20

    Re: uln2003 base resistor

    10k is a good value.

    How to calculate the approximate value that gives 5V at the ULN2803 input:
    Input current at 5V = (5V - 1.4V) / 2700 Ohms = 1.3 mA (ignoring the 100 uA in the internal base-emitter resistor)
    Wanted drop in the external resistor = 24V - 5V = 19V = R * 1.3 mA
    R = 19V / 1.3 mA = 14.6k

    10k will give a little more than 5V at the input, but they are specified to survive up to 30V.
    I would use 10k.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 80,983, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,275
    Helped
    4433 / 4433
    Points
    80,983
    Level
    69

    Re: uln2003 base resistor

    The input current is specified at 1.35mA with 3.85V applied.
    That means to drop the additional (24 - 3.85) = 20.15V at the same current, Ohms Law says R=V/I = 20.15/0.00135 = 14.925K.

    So 12K to 15K would be a better choice but 10K would still be safe.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



--[[ ]]--