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[beginner question] Filter Capacitor? Flyback Diode? Help!

Discussion in 'Robotics & Mechatronics' started by MathGeek, Dec 26, 2005.

  1. MathGeek

    MathGeek New Member

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    Ok. Let the truth be told. I am a beginner.

    I am building a simple two motored walker.

    In order to move, the motors rotate in one direction for a second and the other direction for a second, so forth. In order to move forward, each direction must last the same period of time, ie one second. You can imagine there are lots of fluctuation here.

    So, the brain is 16F628 programmed by ICD2. The motor control is done by Texas Instrument CD74AC139E dual 2-to-4 line decomplexer(very handy for small motors).

    OK, here is the problem. The motors are supposed to rotate in each direction for one second. If i put two LED's instead of a motor, each LED blink for one second, alternating each time.

    However, when I put motors, the timing is off and motors start to rotate in one direction for half a second or some times totally skips a cycle. I came to believe that my 16F628 is being interfered or confused and instructions are not carried out properly. I came to suspect that this is caused by fluctuation that my motor is creating. (If you have any other idea, please tell)

    I don't have any filter cap, and I don't even know what it is. I heard I have to put a .1 uF capacitor across a IC. So, woudl I put a capacitor across my CD74AC139E? Do I put a filter cap across my motor terminals also?

    To my understanding, flyback diodes are for one-direction motors, and since my motor rotates both direction, flyback diodes are useless?

    Thanks. I need any advice quickly.
     
  2. addyismad

    addyismad New Member

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    'flyback' or free wheeling diodes r nothing but a mechanism to give a path to the back emf of the motor.
    as far as keepin it 'on' for half a second is concerned, check out the response time of ur motor.
    i think it is not able to respond so quickly.

    as far as the capacitor goes, it is connectd so tat just in case the voltage across the motors drops, it can provide a current path to the motor and avoids any discontinuity. u shudalso connecta cap b/w vcc and gnd for the same reason.
     
  3. bloody-orc

    bloody-orc New Member

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    you need a lot of caps. put 100n cap to pic and all other IC's in circuit as close to the power pins as possible. then take 6 100n caps for your 2 motors. connect one across the motors leads, one starting from 1 pin and other end to motors metal shell and other 3 also that way, so you have a closed ring of caps. that will taake away any sparks that may ocure. you can put this diode thingy also, i dont have any xp with it so i dont know if it makes a difference (i'm not using it most of the time. onl with relays).

    forgot to say, if you are using a voltage regulaor, then this needs to have el lytic (polarized) caps also. 10-100uF on both input and output will do just fine.
     
  4. dave

    Dave New Member

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  5. MathGeek

    MathGeek New Member

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    I put a 0.47uF capacitor across each motor's terminal. Problem solved. Timing is perfect now.
     

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