PWM question

Discussion in 'General Electronics Chat' started by mazur50, Jan 22, 2008.

  1. mazur50

    Thread Starter Member

    Feb 5, 2007
    28
    0
    Is it posiable to do PWM with no IC.

    I want to make a varable motor control but not use a ic or 555 timer.

    I would like to just use a pot and a few caps and a mosfet. do you happen to know and circuit diagrams.

    thanks
    Mike
     
  2. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    why don't you wanna use a 555? it's the simplest oscillator ic there is. you don't wanna get down to transistors do you?
    do you have any suggestions?
     
  3. mazur50

    Thread Starter Member

    Feb 5, 2007
    28
    0
    I would like to make it as simple as possiable so i can learn different things. I have already done one with the 555 timer.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can use the pot to control voltage on the FET gate to pass more or less current through the motor. But that's not anything like PWM. Some means of generating pulses with variable widths defines PWM.
     
  5. mazur50

    Thread Starter Member

    Feb 5, 2007
    28
    0
    how would i hook that up. do you have a example dia.
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    In my opinion, using discrete components is a good way of learning, but is is the worse way of making things as simple as possible.

    "Reinventing the wheel" is a good choice if you want to do serious learning.
     
  7. mazur50

    Thread Starter Member

    Feb 5, 2007
    28
    0
    just trying to learn how things work. trying to get the basics out of the way before i dive into IC and Cmos and pics.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    The basis for most PWM circuits is a triangle wave generator followed by a voltage comparator. You would need to create discrete versions of these two components and ensure they will work together.

    Discrete Triangle wave generator:

    http://www.diyaudio.com/forums/showthread/t-57885.html

    Framework of a comparator:

    http://home.cogeco.ca/~rpaisley4/Comparators.html

    You should learn the circuits, how they are constructed and the logic behind them. Otherwise, learn how a FET and a BJT works, and expect a steep learning curve. Engineers stand upon a tall and historical epistemology.

    Steve
     
  9. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    A few years back I had a need to control the speed of a 2 volt motor (a mirror ball rotator), and as most I/C's wont work at that voltage, I used a modified 2 transistor multivibrator circuit. It worked and still works. I never checked the waveform, But it's probably fairly awful, and might be a bit hard on the output device if a high current motor were used. I think the circuit went something like this..... I havnt put any values, as I can't remember, and cant be bothered to re-work them out.

    [​IMG]

    (opps, ignore the polarity of the motor symbol)... ;)
     
  10. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    great, gadget, :)
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I tried simulating that circuit with various values and various transistors, but no joy.
     
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