Help with Pulse Generator Circuit

Discussion in 'The Projects Forum' started by joncad, Sep 16, 2013.

  1. joncad

    Thread Starter New Member

    Sep 16, 2013
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    Hello Everyone. I am hoping someone can help me with a circuit (see attached) I am working on. It generates a pulse of a set length when a button is pressed. I have it working ok, but I am having trouble adjusting the pulse length (see attached) to exactly what I want. Right now the fall time (A) is 400us, the length is 35ms, and the rise time to 75% full voltage is about 11ms. What I can't seem to figure out is how to reduce the fall time and rise time without reducing the length. I really would like to have a fall time of less than 5us and a rise time to 75% of about 4ms with the length staying around 30ms. I feel like I should be able to do this by adding another resistor or capacitor somewhere, but nothing I do seems to work. Any help you could provide would be very helpful. Thank you.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Use a 555 in a debounce circuit.
     
    #12 likes this.
  3. joncad

    Thread Starter New Member

    Sep 16, 2013
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    I should have added that although I know I can do it better with a 555 or other IC, I need to have the circuit pretty much as is or with different values or just an added resistor or capacitor. Thanks for the suggestion though.
     
  4. #12

    Expert

    Nov 30, 2010
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    You are suffering from the wide variation in gate voltage per current of the mosfet. A bjt would make a snappier transition, but use a lot more current.

    An op-amp would be perfect for this job, but you could build a differential pair with 2 transistors to get a sharper switching point.

    Bottom line, this is the wrong transistor, and you might need more than one of the "right" transistors.
     
  5. joncad

    Thread Starter New Member

    Sep 16, 2013
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    I am going to experiment with some different jfets. Thank you.
     
  6. joncad

    Thread Starter New Member

    Sep 16, 2013
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    1
    OK, so I have tried a few different mosfets with no success. Can anyone tell me what specific mosfet specification I should look at? I have tried contacting a few manufacturers, but have not really gotten anywhere.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I don't think any MOSFET will get you there on its own, although I suppose there must be some out there with a much narrower gate voltage range.

    You could use a BJT or BJT darlington to give a snappier transition of the gate voltage as #12 suggested. But this will draw some current and mess up your timing. Fixable, but a pain. Depending on the current through the MOSFET, you might be able to eliminate it and just let the darlington do the work.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Hysteresis. You can add a second transistor and some feedback to make a "schmidt trigger" type circuit, which will snap quickly between one output state and the next, even if the cap voltage is rising slowly. :)
     
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  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's how you could add hysteresis. Both FETs need to have a very low (~1V) turn-on voltage.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Nice one. :)

    I think there would be benefits going to NPN bipolar transistors instead of FETs. I've done similar using BC337 NPN which will saturate Vce below 0.1v with very little base current.
     
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