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do i understand this circuit?

Discussion in 'General Electronics Chat' started by allaccess, Sep 12, 2017.

  1. allaccess

    allaccess New Member

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    what path do the holes take when the capacitor discharges?
    [​IMG]


    Step 1:
    I believe the the negative side of the capacitor charges through the 22R. I think the positive side of the capacitor charges through the 300K R.
    At this point, the capacitor is now charged to 3v.
    Step 2: the capacitor discharges through the 300K, through the LED, then through the PNP ebc, finally reaching the negative side of the capacitor.
    Then the cycle repeats.



    Is there any way I can confirm my understanding (or yours) ?
    Right now, all I have is a milliamp meter.
    Thanks for your review/comments.
     
    Last edited: Sep 12, 2017
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    YAMV!

    Yet another MultiVibrator...

    58a.png 58b.png

    I dont think that avalanche has anything to do with it; just normal turn on, turn off when Vbe exceeds ~0.6V

    The two transistors create a non-inverting amplifier. The RC network provides positive feedback.
     
    Last edited: Sep 12, 2017
  3. allaccess

    allaccess New Member

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    MikeMI, thanks for your response. I finally got the path thru which the cap discharges, I think.
     
    Last edited: Sep 12, 2017
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    LTSpice is a Windows program that can be downloaded at http://www.linear.com/

    Most oscillators (including on-line ones) require that you artificially create a bit of noise to get the oscillations started. Noise in a real circuit serves the same purpose, but most simulators don't do that automatically. Note that I created the initial shock to get the circuit going by specifying "startup" in the .TRAN directive (read the LTSpice help file for an explanation).

    Note that the yellow trace in the original sim is what would be displayed on a milliampmeter (ignoring the ballistic averaging of the needle dynamics).
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    It charges through the 330K; it discharges into the base-emitter diode of the NPN. The net current into the capacitor's +plate has to exactly balance the current out of the +plate...
     
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    The left side of the cap is clamped to about 0.7v or so.
    It charges through the 330k AND the base emitter diode of the first transistor.
    Eventually the second transistor turns on, which could make it charge in reverse so that should be checked as this looks like a polarized cap.
     

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