Re: Curious oscillator circiut

On Fri, 17 Feb 2006 15:02:56 +0000 (UTC), kensmith@xxxxxxxxxxxxxxx (Ken Smith) wrote:

In article <8hhbv1ph0jv9qpfbapke6c63tb7a0edm9j@xxxxxxx>,
Mike Harrison <mike@xxxxxxxxxxxxxxx> wrote:
Probably one to file under 'interesting-but-probably-not-useful circuits
that behave a little
strangely'...

http://www.electricstuff.co.uk/oddosc.html

I've built basically this same circuit so I assume yours is doing the same
thing. In my case it was a high voltage NPN transistor that did it.

The collector load needs to have a high Q for it to happen. When the
collector junction goes reversed, it stores charge in the base. This is
what causes the flat spot you see just as it comes out of the reverse
situation. You are seeing the storage time of the transistor when
operated in this funny way. This storage time is what is adding energy to
the tuned circuit.

You can see if this is true by putting a diode in series with the
transistor to prevent the reverse current. This will completely kill the
oscillation if it is the same effect as I saw. If getting the coil to
ring down is what you want, a Schottky diode in series may be the final
answer.

Since you are running at a low voltage, you could add a small dual
Schottky as a "Baker clamp". This will clip the positive peaks.

Vcc
!
--- Vcc
^ !
! !/ e
---/\/\/\-++--!
! !\
--- !
^ !
! !
----+



In my case the fix was to tri-state the MCU pin that was driving the transistor instead of driving
it high.

.

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