Need HELP P.W.M

Discussion in 'The Projects Forum' started by ddude, Nov 18, 2013.

  1. ddude

    Thread Starter New Member

    Nov 18, 2013
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    Hello I'm new to this forum..

    I'm in need of desperate help!

    Posted is a schematic of a P.W.M circuit that is powered by a 12V DC PS.

    As the pot. is turned from 0-50 the motor is supposed to spin CW, and 50-100% CCW.

    This circuit is not working!

    For the transistors, instead of the ones in the schematic i have replaced them with the NPN CB547B and the PNP C557B

    Am I using the wrong transistors? Am I missing anything?

    I am not allowed to use another 555 timer. I only have to use the H bridge format.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Here is your schematic...

    [​IMG]

    You will need a inverter of some sort for one side of the H Bridge, and you have improperly biased Q7 and Q8. Voltage followers (common collector) do not invert.

    This reads like homework or I would just give you a solution. Is it?

    Bill's Blog

    Pulse Width Modulation
     
    Last edited: Nov 18, 2013
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    Here is one little hint. Current flows through a transistor in the direction of the arrows. Does it do that in all your transistors?
     
  4. ddude

    Thread Starter New Member

    Nov 18, 2013
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    Thank you for your replys, this circuit is for some personal use. Would the inverter go between the base's? and ill flip the transistors
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    If you feed both sides of the H-Bridge with the same signals then the load between them will see no change. Yes, you need the inverter in front of one of the H-Bridge drivers. It can be a simple transistor, or another 555 wired as an inverter.
     
    Last edited: Nov 19, 2013
  6. Wendy

    Moderator

    Mar 24, 2008
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    If this is a serious circuit I could offer some other critiques and improvement ideas, but lets see what you do first.

    What is the max current draw of the motor?
     
  7. ddude

    Thread Starter New Member

    Nov 18, 2013
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    Can you please show me how to wire it up? I have been trying, but no luck. the motor is a 6-15V small DC motor, im guessing it can take around 1A.

    This circuit is just a hack job i did quickly to demonstrate to a friend of mine
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    1 Amp is a pretty hefty load. A small motor will use a lot less. Lets start with the 555 circuit. It is close, but it could use tweaking.

    555 PWM Oscillator

    The H bridge will take a bit more. Lets see what changes you would do first. If you have to hand draw a picture then take a digital photo, then use the "Go Advanced" feature on the site to post it.

    Measure the current the motor draws, this is a needed parameter. Try loading it with your fingertips (if this is indeed a small motor) to get an idea of worst case current draw.

    Remember, both side of the H bridge should look identical. Then add an inverter (such as a common emitter transistor) in front of one side.
     
    Last edited: Nov 21, 2013
  9. Wendy

    Moderator

    Mar 24, 2008
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    Now you done it, I now have a new 555 article stuck in my head. Such projects take about a month to write and prototype, so don't wait for it.
     
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