pwm using 555

Discussion in 'The Projects Forum' started by fourty, Sep 8, 2014.

  1. fourty

    Thread Starter New Member

    Oct 13, 2013
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    hi i designed a pulse width modulator using 555 the problem
    is that the magintide is not stable,i simulated the circuit in multisim and with a breadboard using osciloscope plz can someone help me?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Are you saying that the simulator and the breadboard with real components BOTH produce that waveform?
     
  3. fourty

    Thread Starter New Member

    Oct 13, 2013
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    yes:((( i can't find the problem
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I honestly don't know how to get that output from a 555. Scope the voltage on the timing capacitor. Maybe it will yield a clue.
     
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  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Curious, where did you get that circuit?

    A common and different version is here:
    upload_2014-9-9_17-16-49.png

    John
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Or, closer to home, there's the 555 PWM section of Bill's blog. (Oddly, that link fails for me right now. Hopefully it works for you.)
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. tom_s

    Member

    Jun 27, 2014
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    same picture both locations :) playing around with that circuit now and it works like a charm
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Which "both." Obviously, the schematic I posted and the schematic in the link for its source are the same, as one would expect.

    However, the schematic posted by the TS is different. Look at the output/gate drive. In the TS's design, the "output" (Pin3) is used presumably for the gate drive, at least that is the source for the oscilloscope trace. In the DPRG design, "Discharge" (pin7) is used. Pin 7 is used in several designs as an output, but it is not the same as pin 3.

    John
     
  10. wayneh

    Expert

    Sep 9, 2010
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    I'm still at a loss to explain how you could get that trace on the output pin of a 555. I would say it's a measurement artifact (such as a shifting ground) but the OP says the simulator gives the same result.
     
  11. tom_s

    Member

    Jun 27, 2014
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    thanks John, deem my interpretation of 'A common and different version is here:' incorrect

    i read as A common (as image displayed) and assumed and different version is here: as another version.

    kind regards,

    Tom
     
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