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PNP Transistor Arrays.

Discussion in 'General Electronics Discussion' started by Gr3mlin, Jul 14, 2016.

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  1. Gr3mlin

    Gr3mlin

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    0
    Jul 29, 2011
    Hay guys, so my project im currently working on has 6 RGB LEDs, I am going to running them from a PIC. BUT, im using the PWM outputs to drive the RGB channels (Neg) and then i though i would just run the individual LED's from the PIC IO pin. but after thinking about programming and stuff, i decided i would be running all the LED's RGB channels for that LED at the one time. SO the current draw will probably exceed 20ma, which would kill the PIC's IO pin. Normally i would just wack a uln2004 in there to drive the LED's but cant because the LED Channel are Positive. and uln's are NPN.
    SO basically the only thing i can think of doing is running the 6 LED's via PNP transistors. but if anyone has a better idea could you let me know..
    Thanks.
     
  2. AnalogKid

    AnalogKid

    2,409
    685
    Jun 10, 2015
    The UDN2981 is an 8-channel source driver. The output stage is an NPN darlington, so the max output voltage is about 2 V below the rail. 5 V in, up to 50 V out. If you need the output to pull all the way up to the + rail, then go discrete with a 2N4403 or 2N3906 PNP and two resistors per line.

    ak
     
  3. Gr3mlin

    Gr3mlin

    20
    0
    Jul 29, 2011
    Thanks man, i had a quick look, no longer able to get them, i think i'll just go with discrete transistors this time around and in future common the anode, not the cathode! :S if only i designed the circuit first! :(

    Gr3mlin
     
  4. AnalogKid

    AnalogKid

    2,409
    685
    Jun 10, 2015
    He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast.
    - Leonardo da Vinci, 1452 - 1519
     
    Harald Kapp likes this.
  5. Gr3mlin

    Gr3mlin

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    0
    Jul 29, 2011
    Wow, thats so kind of you to say! I have a quote aswell!
    "False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutary pleasure in proving their falseness; and when this is done, one path towards error is closed and the road to truth is often at the same time opened." - Charles Darwin

    See I actually thought that doing it this way would assist me further in the circuit aswell as in the programming of the PIC, as my previous, all be it slightly different project, had way more components and since I was only going to run one LED and Colour at a time in the program, i would have been able to drive the LED directly from the IO pin as it would have kept the current draw per io pin below the limit of 20ma.
    But this changed as a result of working out a more efficient program.

    So.. Have I learnt that this design although will work, is not the perfect way of achieving my goal? Yes. Am I going to rectify this error in future designs? Yes. So does this make me a fool? No. But thank you for your quote, its an interesting one. Although I will likely never use it, more so when the person you are saying the quote to has already suggested they have seen the error and will rectify it in future designs, "i think i'll just go with discrete transistors this time around and in future common the anode, not the cathode! :S"
     
    Last edited: Jul 15, 2016
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