1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Electronics Transistor swap / upgrade

Discussion in 'Modding' started by bigal, 4 Apr 2005.

  1. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    Hi people!
    I have thsi basic chaser circuit:

    [​IMG]

    but the outputs are going to be switching alot of LEDs, about 20 of them and at 20ma each that is 400ma. the 2n3904's there are not powerful enough for this (max 200ma) so i was wondering what i could swap them with that would give more juice. The whole thing is going to work off 5v.

    cheers!

    on a similar note, if i wire 20 LEDs in parallel with one reistor (all same) then will the resistor be the same as if i wired 5 in parralel (same as above)..

    Also, my calculator says i need 1watt resistors for this... is this true?

    thanks alot! :)
     
  2. biff

    biff What's a Dremel?

    Joined:
    8 Jan 2004
    Posts:
    1,253
    Likes Received:
    2
    An MJE3055T will do you just fine. Well perhaps a little overkill, but they are very common which means cheap.

    You can use a single resistor for all 20 LEDs no problem. Here's a sample of hoew to calculate it since I don't know the forward voltage drops of the particular LEDs you're using. I'll assume a 2.5V drop for the LED's and a 0.8V drop for the transistor. So 5V - 2.5V- 0.8V = 1.7V. Divide that by 20*20mA = 4.25Ohms (4.7Ohms is a common value). Then for the wattage W=I²*R = (.4)²*4.7 = 0.75W. A 1W would work but a 2W will give a little better safety margin.

    Hope this helps.
     
  3. cpemma

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    I'd go for a 2N2222A, it will take over 600mA and the base current (led current/gain) will be a lot less than with the 3055 so less drain on the 4017. Plus they're cheaper and just as common. ;)
     
  4. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    lol, just remembered i have about 20 2n2222's in stock for a project that got scraped... heh..

    i also forgot that transistors had voltage drops in my equations, good job you reminded me! :)

    BUT
    I forgot again... as there is two channels, red and blue, each has 10 channel's, channel 10 of red is joined (before the resistor) to channel 1 or blue and so on... This will mean alot of current, more than 400ma, i knew i forgot something.. so in total, 40leds at 20 ma = 1 AMP! so i need something that can handle at least 1 amp!

    i think this needs TIPs as 2n2222's have 800ma limit... they might just fall over at that..

    i will look into MJE3055T

    thank for the quick responses guys!



    EDIT:

    anything from here suitable:

    http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=80010&CTL_CAT_CODE=30397&STK_PROD_CODE=M34420&XPAGENO=1


    or here

    http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=80010&CTL_CAT_CODE=30397&STK_PROD_CODE=M34419&XPAGENO=1
     
    Last edited: 4 Apr 2005
  5. cpemma

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    I'd be tempted to redesign that circuit to just use 5 transistor switches with steering diodes to them (or you can have 6 switches for a bit longer back & forth like this ).

    For 1A you need the high gain of a darlington IMO, a gain of under 50 would want over 20mA from the 4017, pushing it a bit. With high loads you'll find the 4017 output voltage will sag a lot below the supply voltage, not good with the emitter-follower boost method you show.
     
  6. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    i am not using that circuit as it is shown... its 10 outputs are powering separate LED's... anyway i gather gain is hFE so anything with about 75 hFE would be suitable? what characteristics am i looking for?


    surely anything on this page:


    http://www.rapidelectronics.co.uk/r...CAT_CODE=30398&STK_PROD_CODE=M60869&XPAGENO=1


    would be suitable then. like the BC517 (at only 10p, since i need 10 of whatever, price is guiding me...) :eyebrow:
     
  7. biff

    biff What's a Dremel?

    Joined:
    8 Jan 2004
    Posts:
    1,253
    Likes Received:
    2
    If they are separate LEDs and not dual colour, dual anode LEDs then you can use the 4017 to drive a FET in common source mode. Use one that will turn on fully with only 5V to the gate like the IRLxxx FETS. Then you can power a 1000 LEDs if you like with minimal/no load on the 4017.
     
  8. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    well they are dual colour, but the are separate, so all the cathodes are joined to ground and then some blues are joined togetehr and the same led's reds and botehr run through resistors to the output of whatever this is... the idea is that the thing is like a line, the top of the line lights up BLUE and the bottom lights up RED. they move towards the center and pass in the middle (purple) and continue to each otehrs end.

    so FET's are the answer..

    i cant find any IRL fets in here:

    http://www.rapidelectronics.co.uk/rkmain.asp?PAGEID=20671&ctl_cat_Code=30400

    can anyone give me a lead on rapid.... there must be something suitable that they have... :sigh:
     
  9. biff

    biff What's a Dremel?

    Joined:
    8 Jan 2004
    Posts:
    1,253
    Likes Received:
    2
    If you were to use a fet in common source then the anodes would be connected to +5V and the cathodes would be connected to the drain of the fet. But if the leds aren't totally separate, i.e. if yours have separate anodes and a common cathode then this wont work.
     
  10. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    alright, slight change of plan... If i have 20 transistors and have 2 to an output will that work, so total LED current = 400ma, 2n2222A transistors (max 600ma) then i hook 2 transistors, LED strings per transistor, so each "Channel" of the 4017 will be activating 2 transistors each switching 400ma.

    will that work? :idea:

    cheers
    Alex
     
  11. cpemma

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    The limit here is the max power, 0.6W. In the above circuit, a darlington will drop about 1.2V (2 junctions), so current is limited to 500mA. Saturated as a switch it will only drop about 0.6V so you can (just) get the 1A rating.
     
  12. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    so a BC517 would work ok... what about my 2n2222 idea? :)
     
  13. cpemma

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    I got out of synch with the thread, watched a film in the middle of replying last time. :D

    Two transistors from an output to run 2 different loads would work fine. 2N2222 will stay cooler than BC517, & they're only on 10% of the time which helps. :thumb:
     
  14. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    GREAT

    i will order some more to be safe, i only have 20, 4 are ex user so i better make sure i have some spare!

    thanks bud! :)
     
  15. nick01

    nick01 What's a Dremel?

    Joined:
    6 Nov 2004
    Posts:
    598
    Likes Received:
    0
    The circuit as shown is not ideal if you want to use many diodes. I doubt that the 4017 can drive all that base current. You may need darlington transistors to drive the LED current, but then the BE voltage drop increases and nothing may work any more.

    If you are not stuck with a PCB that you already have you should consider a transistor array that is made for purposes like this, e.g. http://www.stmicroelectronics.com/stonline/books/pdf/docs/1370.pdf (currently $2.31 at www.digikey.com, also available in the UK). The array would be a low side switch and the LEDs would be connected in series to some suitable supply through a resistor. With a 48VDC supply you could even put all 20 LEDs in series with a resistor.
     
  16. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    sorry but the design doesnt allow serial use of the LEDs, Mainly because they are bicolour and share a common cathode..

    any other options or will the dual transistor idea work... i hear that a 4017 can give about 25ma per channel... :idea:
     
  17. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    just another thought, what about keeping the origional transistors and then on the emmiter's of them hooking the two 2n2222's ?

    i need to order stuff as the product has to be ready by sunday (its an art project and thats the hand in date) :eyebrow:

    THANK YOU PEOPLE!
     
  18. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    any idea what voltage drop i will get on the 2n2222's?
    they have 5v at their collectors and at their bases.. :)


    EDIT:

    here is the final version of the 4017 logic part with dual 2N2222A's per output.
    Anything missing? please tell me! :)


    [​IMG]

    thank you


    EDIT #2 : Heres the Beta version of the display PCB, the pads on the emmiters of the above transistors go onto the big 1Watt (actaully gonna be 3watt) resistors round the outside of the thing. When i know the emmiter voltage drop (i guess about 0.9V) then i will work out what resistors i need for them. It has 134 Blue / Red leds in the middle and 99 Green leds round the outside.

    [​IMG]

    Thanks again,
    Alex
     
    Last edited: 5 Apr 2005
  19. bigal

    bigal Fetch n Execute

    Joined:
    8 Oct 2004
    Posts:
    609
    Likes Received:
    0
    after discussions with Turbokeu, i have added base resistors to the layout or 10 or 4.7k (experimentation) and that the output will be 4.5V.

    Problems sorted! :)
     
  20. nick01

    nick01 What's a Dremel?

    Joined:
    6 Nov 2004
    Posts:
    598
    Likes Received:
    0
    In my datasheet 25mA is the absolute maximum rating. The High level drive capability is 4mA. http://focus.ti.com/lit/ds/symlink/cd74hc4017.pdf
     

Share This Page