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    Discrete transistor multivibrator.

    Just playing around with the 2 LED flasher circuit in Short Circuits 1 magazine in multisim.

    I notice if you reduce the capacitor value to say 100nF the signal starts to get fairly eratic and untidy.

    Any tips on cleaning up the signal?

    •   Alt8th August 2012, 03:40

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    Re: Discrete transistor multivibrator.

    It should work, can you post your circuit? Also how are you determining that the signal is erratic and untidy?
    J



    •   Alt8th August 2012, 05:49

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    Re: Discrete transistor multivibrator.

    Quote Originally Posted by jasonc2 View Post
    It should work, can you post your circuit? Also how are you determining that the signal is erratic and untidy?
    The signal from a 555 astable multivibrator is always very consistent in multisim. But with the discrete tranasitor multivibrators you get this sort of business......in multisim at least:



    Do you reckon a discrete transistor Schmitt Trigger based oscillator would do better. Like this one for example:
    http://en.wikipedia.org/wiki/File:Sc...ransistors.svg

    - - - Updated - - -

    Here is a 741 Scmidtt Trigger but it isn't doing what I understand it is suppsed to do:



    Based on: http://talkingelectronics.com/projects/OP-AMP/OP-AMP-2.html => THE OP-AMP AS A SCHMITT TRIGGER



    •   Alt8th August 2012, 08:15

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    Re: Discrete transistor multivibrator.

    That noise should not be there. Try reducing your maximum time step in multisim.
    J



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    Re: Discrete transistor multivibrator.

    As a first step, can't you try to generate waveform plots with readable axes in Multisim. This would allow to discuss the results. Up to now, I can't say but "looks wrong, somehow".

    If I understand right, you are showing the same waveform for transistor and 741 circuit.



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    Re: Discrete transistor multivibrator.

    Quote Originally Posted by FvM View Post
    As a first step, can't you try to generate waveform plots with readable axes in Multisim. This would allow to discuss the results. Up to now, I can't say but "looks wrong, somehow".

    If I understand right, you are showing the same waveform for transistor and 741 circuit.
    Sorry I ahave managed to over write one of those images below so now it doesn't match the text. The image showing the noise is of a pretty stock standard transistor multivibrator that you would use in an LED flasher. But in multisim at least they never seem to give a real tidy and clean signal.

    Just for fun I am trying to implement a voltage boost circuit to take 12V to 18V.

    Given the trouble I was having with the 'noisy' transistor multivibrators I hunted around a bit more and found this fully designed booster circuit that includes regulation:


    Here is is in multism:


    I have swapped the transistors used by the author to others I have on hand and it seems to still work very well.

    I also changed up the zenner diode to get the output voltage to 18V under load and this has worked well also as you can see.

    But I am a little puzzled by the circuit. There must be a multivibrator in order to drive the inductor and it has to be around the two BC337's. But it doesn't look anthing like the nice symetrical transistor multivibrators I have been playing around with previously in multsim. Can some one explain the multivibrator part of the circuit to me?

    The author has used a pretty heavy duty transistor on the inductor. Since I wouldn't be drawing large currents from this thing I assume I can get away with a far less heavy duty transistor.

    Interesting........ If I reverse the battery the circuit still functions but I only get -100mV out. But if I can figure out which components need to also be reversed it looks as though it will function nicely as a negative voltage booster as well.

    That noise should not be there. Try reducing your maximum time step in multisim.
    I assume you mean the time base scale Jason. I have fiddled with that a bit but it doesn't seem to make any difference - I still get noise.

    Is that noise and artifact of multisim or is it likely to occur in a real circuit?
    Last edited by boylesg; 8th August 2012 at 17:34.



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    Re: Discrete transistor multivibrator.

    I am referring to the maximum time step simulator option, which in multisim is the TMAX option described here: http://www.ni.com/white-paper/12774/en

    Try setting it to e.g. 1usec (1e-6).

    It is probably a simulator issue assuming your circuit is built correctly.

    Post your circuit so we can see resistor values and part numbers (that's why I asked).
    J



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    Re: Discrete transistor multivibrator.

    Quote Originally Posted by jasonc2 View Post
    Try setting it to e.g. 1usec (1e-6).
    That seems to work - much less noise.



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