Help needed to boost output current in a 7 LED chaser

Discussion in 'The Projects Forum' started by Tony Spence, Dec 7, 2014.

  1. Tony Spence

    Thread Starter Member

    Dec 20, 2013
    50
    0
    I have a 7 LED chaser circuit (BELOW) that i moded to get the results that i need.
    Firstly I increased the voltage in the whole circuit from 9V to 18V then i added 2 BC108C's in the darlington pair formation after each output leg of the 4017b chip that i used, before each LED output.

    I think i have blown the first transistors i used I have got new transistors which i believe turn on at 25V they are also BC108C's is there a way i can boost the voltage after the output pins of the 4017B chip to turn on my new transistors to give me the current i need, or would i be able to use another component to replace the Darlington pair with and get the same current gain.

    This is the circuit i started with: 7-Led chaser cct.jpg
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    How did you connect the transistors to the 4017?
    You will need a resistor between the 4017 and the base of the transistor.

    Bertus
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Something strange about the pin 3, because you have two pin 3.
     
    simo_x likes this.
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello ScottWang,

    You are right, the pin 3 to ground should be pin 13 (clock inhibit).

    Bertus
     
  5. eetech00

    Active Member

    Jun 8, 2013
    649
    112
    Hi

    I don't know how much current you want to drive the LED's but, for 18v, you could drive each LED at about 10ma like this. Adjust the collector resistor for higher current.

    eT
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    If the 4017 is also supplied with 18V you are lucky it has survived. 15V is the recommended maximum.
     
  7. eetech00

    Active Member

    Jun 8, 2013
    649
    112
    Hi

    Datasheet say 20v max, recommended 3-18v
    But...I wouldn't run it at that voltage....;)

    eT
     
  8. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    A ULN2003 chip makes a nice simple driver for this application
     
    absf likes this.
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    Interesting. The National Semiconductor datasheet says 18V max, recommended 3-15V.
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    395
    Tell us about your LEDs & connection arrangement.
     
  11. eetech00

    Active Member

    Jun 8, 2013
    649
    112
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    I saw that datasheet before, it shown that the "100% Tested for Quiescent Current at 20V", be careful the Quiescent Current different with Working Current.
     
    Last edited: Dec 8, 2014
  13. Tony Spence

    Thread Starter Member

    Dec 20, 2013
    50
    0
     
  14. Tony Spence

    Thread Starter Member

    Dec 20, 2013
    50
    0
    View attachment 76896
    Above is how I connected the two transistors to make the darlington pair (two BC108C's). I added them before each output LED and connected the LED to the emitter. I dont know how much current is needed to my output this is also why i need help, but what i do know is that the modifications i made to the below circuit work and that i need help perfecting it.
    View attachment 76895
     
  15. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    The attachment 76896 has something wrong, I can't see it.
     
  16. Tony Spence

    Thread Starter Member

    Dec 20, 2013
    50
    0
    hi current gain darlington pair transistors.png
     
  17. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    I'm not sure why you wan to using 18V, that is too high for a led driver, and the darlington pair need a Rb resistor to limiting the current.

    For many applications that you just need to using one bjt to be the led driver, it is unnecessary to using two bjts.
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    If your LEDs are the usual indicator type with a max current of ~30mA you don't need Darlington drivers.
    Here's how you could do it (I've shown only 3 LED sub-circuits, but you get the drift).
    LED-chaser.gif
     
  19. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    I just tried the downloaded file, but the CD4017 was missing?
     
  20. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    The CD4017 model is in CD4000.lib, included in the zipped Part1 of the LTspice library files freely downloadable from the Yahoo LTspice User Group. But any CMOS gate model should be usable just to demonstrate LED driving via an NPN transistor.
     
Loading...