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555 Timer - less then 50% duty cycle question.

Discussion in 'Electronic Basics' started by John Popelish, Jul 28, 2004.

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  1. Either should work. The silicon diode will cause slightly more change
    in duty cycle as the battery voltage falls, but whether this is good
    or bad depends on what you want.
    Either can work well. The mosfet is simpler to connect, because you
    don't need to limit the current and this lowers the total battery
    current, but the difference is small.
    Sounds like the switch is on during the 2/3 part of the cycle, instead
    of the 1/3 part. Try using the opposite polarity transistor and
    switching the opposite side of the supply to the motor. That should
    invert the part of the pulse that is turning the switch on.
    A schematic would be very helpful. Please post to A.B.S.E. or email
    me one.
     
  2. cpemma

    cpemma Guest

    I've built the simpler single 555 PWM circuit
    http://www.cpemma.co.uk/555pwm.html & found schottky diodes (BAT42) gave
    slightly nearer 0-100% duty than silicon (1N4148). But I wouldn't make a
    special purchase of them to get the advantage. ;)
     
  3. Rubicon

    Rubicon Guest

    Hello.

    Using a calculating program for a 555 timer operating at less then 50%
    duty cycle the diagram that the program displays has two diodes. One
    from pin#7 to pin#6 and the other from pin#2 with R2 to pin#7. I
    understand that the formulas work through these diodes but what
    exactly should they be? Signal IN4148, Schottky or another type?

    In addition when switching a motor via the 555 what's the best choice
    of switch? A transistor or a mosfet or some other device? I have tried
    both transistor and mosfet to switch a motor at approx 33% duty cycle
    but failed to do so correctly. At the moment it runs faster than it
    should when it shouldn't. That is a 555 monostable output through an
    NPN transistor switches another 555 astable (555 V+ to V+, V- to
    collector) that switches another NPN transistor or mosfet (Motor V+ to
    V+, V- to collector/drain) to control the motor. At the moment the
    motor runs as soon as power is applied. Not sure what's wrong as the
    wiring seems O.K.

    Regards,

    Andrew.
     
  4. John Fields

    John Fields Guest

    ---
    Check "Duty cycle controller" on abse and

    http://pdfserv.maxim-ic.com/en/ds/ICM7555-ICM7556.pdf

    for the timing equations so you can figure out the RC's
     
  5. Rubicon

    Rubicon Guest

    John,

    Thanks for the help. I'm in the process of making a basic schematic of
    what I've done so far and when completed I will make it available for
    you to have a look at.

    I certainly do appreciate your advice.

    Regards,

    Andrew.
     
  6. Rubicon

    Rubicon Guest

    Thankyou,

    Andrew.
     
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