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uln2003 voltage disparity, is it ok?

Discussion in 'The Projects Forum' started by melts, Oct 4, 2012.

  1. melts

    Thread Starter New Member

    Oct 4, 2012
    ok so I have a PIC running the some lovely little program I've put together and need to switch on an array of LEDs - about 40 LEDs per port over 5 ports with a mixture of normal and high brightness LEDs in each 'cluster'

    the PIC and other logic is at 5v, regulated down from 12v. I want to use the 12v to drive the LEDs and just want to make sure I've got the ULN2003 specs the right way in my head, that is, if the PIC drives a line high to 5v connected to the ULN2003 will that happily sink the LEDs on the 12v circuit, as they are sharing a common ground? Should I redesign the LED clusters to run on 5v and source a better 5v regulator instead?

    The LEDs are going to be about 30 meters from the PIC & ULN2003 so the use of the primary voltage seemed more fitting. After reading something I began to doubt my logic and just want to check it'll work. that and if i need any protection in the circuit to help the PIC out that'd be good to plan too, I am currently just planning on using the ULN2003 between the PIC and the LEDs.
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    How much current per port?

    40 LEDs x 8 x 20mA?

    The total maximum emitter terminal current of this chip is 500mA.

    I would use the 12V, not the 5V from the regulator.
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    the 500mA is for one channel. What will kill this chip more likely is power dissipation. How much heat that can be dissipated will depend on which package used among other things. This figure found in the Allegro datasheet is very descriptive. It is for the DIL package
  4. melts

    Thread Starter New Member

    Oct 4, 2012
    thanks for the info, I had not considered the multiple channel in use rating. I'm expecting each array, being of parallel and series, to draw around 150mA if I've got my calculations correct. I'll be measuring the assembled arrays now to confirm before putting them all in. If its a problem figure I'll look at driving them with PWM from the PIC.

    it sounds like there is no problem with the voltage difference, initially I thought there wouldn't be then read something about the darlington junction configuration requirements that got me concerned. Mainly been worried about the high voltage on the LED side causing high load demand passed onto the PIC, which doesn't really make sense.