Design Critique: Using PIC16F690

Discussion in 'The Projects Forum' started by tigerapna, Mar 30, 2012.

  1. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    Hello,

    I was wondering if someone could critique my design if they can see any major flaws or a better solution.

    Thanks!!!
     
  2. Motardo

    New Member

    Sep 21, 2011
    20
    2
    With 2k2 base resistors, you've only got 2 mA or so base current into the 2222s, so you should keep the collector current less than 20 mA or so to keep the transistors in saturation. But you've got 17 mA through the coils, plus about 24 mA through the LED's? That seems like a lot for the LED's. I would drop it to about 5 mA (4k7 resistors in series) if you don't need them super bright. Or else keep the LED's how they are and use 1k base resistors to increase the drive.
     
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  3. chimera

    Member

    Oct 21, 2010
    122
    2
    okay.. the micro part looks okay. reduce the LED current limiting resistors,
    but I am more concerned with your power supply.

    1- You are using a 24Vrms transformer.
    2- You have no smoothing cap on it (other than the one in your LM7805 circuit)
    3- The input voltage to the LM7805 is going to around 33V (rectified and smoothed)
    4- That means that your power dissipation across the poor regulator is going to be huge. You are going to be needing an incredibly huge heat sink.
    5- Recommendation: Use a switching buck regulator IC to drop the voltage down to 10 or so volts (LM2576-adj) and then use a 7805. This way, no heat sink (or a small one). http://www.ebay.com/itm/LM2576S-Adj...t=LH_DefaultDomain_0&var=&hash=item231b959833

    Enjoy!
     
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  4. nerdegutta

    Moderator

    Dec 15, 2009
    2,517
    785
    Hi,

    All of the above and:

    • Add a capacitor near the IC's VDD pin, to GND.
    • Consider switch bouncing in you code.
    • Sink the LED instead of source it. (RC3).

    A good short book is "Microcontroller Interfacing Circuits". Nearly all kinds og interfacing is talked/explained/showed there.
     
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  5. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    Thank you all for your input. I was hoping for power supply ideas :).
     
  6. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    Will making the PCB layout for this power supply be difficult with the LM2576 and 7805? Will it be easier to just use a 6V step down transformer with the 7805?
     
  7. Motardo

    New Member

    Sep 21, 2011
    20
    2
    The worst case current draw through the 7805 is only 8 mA or so (three transistor bases drawing 2.13 mA each, plus two input switches drawing 500 uA each, plus say 1 mA for the uC clock (probably much less). So if the 7805 drops 28 V at 8.4 mA, that's less than a quarter Watt. It doesn't really need a heatsink, but a small one wouldn't hurt.
     
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