Schematic review, and possible code help

Discussion in 'The Projects Forum' started by Interitus, Sep 18, 2010.

  1. Interitus

    Thread Starter Member

    Nov 8, 2009
    34
    0
    I've made up a schematic to control an "Octobrite" led driver with an arduino, with midi Input. I was hoping somebody would be able to take a look over my schematic and see if there's anything that should be added or changed. I'll be powering the system with an old computer PSU, but programming of the arduino and some testing will be done with USB.
    My schematic:
    [​IMG] Note: U43 will be a MM74HC14N-ND, and the mosfets will be RFP30N06LE.

    I've not yet tested any Midi functions.

    Also, I've been having problems with the code for this project as well. There seems to be a problem with code for fading between colors. My old math algorithm(line 242,249) was crashing after a few loops of the code, and the new math(line 245) is causing a flickering during some of the transitions. I've exhausted my math expertise, and I cant think of a better way to do it. You can read the code here: http://bcas.tv/paste/results/NIjXph40.html
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    And not a return in the bunch?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Those MOSFETs have a pretty hefty gate charge. You really don't need that high of a Vdss rating.

    Consider using something like an IRLU8721:
    http://forum.allaboutcircuits.com/showthread.php?t=29644
    Vdss=30,Rds(on)=8.4m, Qg=8.5nC. By cutting the Vdss to something closer to what you need, the gate charge is considerably reduced, and even the Rds(on) is about half that of the MOSFETs you've specified. Besides, the IRLU8721's are in a more compact package; it'll save you board space.

    You have 47k resistors from the gates to +V. They should be pull-down resistors instead, so that if the uC is out of the circuit or faulty, the MOSFETs get turned off.

    Note that if you have unused INPUTS on that 74HC14, you should connect them to GND or +5v.
     
  4. Interitus

    Thread Starter Member

    Nov 8, 2009
    34
    0
    I'm guessing you mean the code? Each of the functions modify variables within the object, and shouldnt need to return anything.


    My only concern with the IRLU8721 is the on curve, Id / Vgs. Seems to turn on fairly sharply before tapering off. Since I'm controlling LEDs, I need them to turn on evenly at varying rates. A person in IRC is telling me it shouldnt affect my LEDs. And oh yeah, I just clued in, I'm not doing VARYING voltage levels to control the leds, I'm doing PWM, so switching full on/off lots per second. Ok. the 8721 is 10 cents cheaper too, so thats good.
    The 47k resistors are there as pullups at the recommendation of the manufacturer of the Octobrite board. "a pullup is necessary, since the octobrite is a current sink driver."
    Just the inputs of the 74HC14 need to go to GND?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill misinterpreted your schematic. I see your supply/return lines.

    Trust me on this. If you are not going to use gate drivers, you want your Qg (total gate charge) as low as possible, within reason. I don't know what your Id requirement is. If you want a really nifty MOSFET in a DIP package that can sink up to 2.4A, Vdss rating of 60v, look up the IRLD024. They won't jack up your breadboards like the TO220 packaged ICs will. But, they have a gate charge that's much less than the IRLU8721's.

    They don't know what you are sinking current with.

    I do.

    Yes, just the UNUSED INPUTS.

    Leave unused OUTPUTS disconnected.
     
  6. Interitus

    Thread Starter Member

    Nov 8, 2009
    34
    0
    The IRLU8721 looks good for my purposes. I was reading the curves/datasheets wrong.

    Actually, I talk with the manufacturer in IRC frequently, and he has seen my schematics, and knows exactly what I'm doing with his board. He makes a lot of projects with LEDs himself. I'm just going to get him to check over the schematics one more time before I order all my parts.
     
Loading...