# Discrete transistor multivibrator.

• 8th August 2012, 03:40
boylesg
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?
• 8th August 2012, 05:49
jasonc2
Re: Discrete transistor multivibrator.
It should work, can you post your circuit? Also how are you determining that the signal is erratic and untidy?
• 8th August 2012, 08:15
boylesg
Re: Discrete transistor multivibrator.
Quote:

Originally Posted by jasonc2
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:

http://www.gregsindigenouslandscapes...ics/Noname.jpg

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:

http://www.gregsindigenouslandscapes...ics/Noname.jpg

Based on: http://talkingelectronics.com/projects/OP-AMP/OP-AMP-2.html => THE OP-AMP AS A SCHMITT TRIGGER
• 8th August 2012, 14:50
jasonc2
Re: Discrete transistor multivibrator.
That noise should not be there. Try reducing your maximum time step in multisim.
• 8th August 2012, 16:12
FvM
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. :-(
• 8th August 2012, 17:21
boylesg
Re: Discrete transistor multivibrator.
Quote:

Originally Posted by FvM
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:
http://freecircuitdiagram.com/wp-con...wer-supply.gif

Here is is in multism:
http://www.gregsindigenouslandscapes...cs/Noname3.jpg

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.

Quote:

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?
• 8th August 2012, 17:51
jasonc2
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).
• 9th August 2012, 01:52
boylesg
Re: Discrete transistor multivibrator.
Quote:

Originally Posted by jasonc2
Try setting it to e.g. 1usec (1e-6).

That seems to work - much less noise.