Curious oscillator circiut

[Mike Harrison]

Probably one to file under 'interesting-but-probably-not-useful circuits that behave a little
strangely'...

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

 

[Tim Shoppa]

Probably one to file under 'interesting-but-probably-not-useful
circuits that behave a little strangely'...

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

3.3K at the base, beta of over 100, means maybe most of an amp through
the power supply, right? I'm thinking it's an interaction between the
PS's overcurrent shutdown (notice the nice "flat" spot in the curve)
and the load. He didn't put the PS voltage on the scope too, that's the
first thing I'd look at!

Tim.

 

[Ken Smith]

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
---/\/\/\-++--!
! !\
--- !
^ !
! !
----+

 

[Mike Harrison]

3.3K at the base, beta of over 100, means maybe most of an amp through
the power supply, right? I'm thinking it's an interaction between the
PS's overcurrent shutdown (notice the nice "flat" spot in the curve)
and the load. He didn't put the PS voltage on the scope too, that's the
first thing I'd look at!

Tim.

There was a 47uf tantalum cap across the PSU, so not that.
Remember the transistor is biased off DC-wise after the initial kick to start it.

 

[Mike Harrison]

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.

 

[Don Bowey]

Probably one to file under 'interesting-but-probably-not-useful circuits that
behave a little
strangely'...

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

It appears to be a blocking oscillator....... But I think there is more
going on at the base than the scope trace is showing.

Don

 

[Ken Smith]

[... funny oscillator ...]]
Vcc
!
!/ e
---/\/\/---!
!\
!
To LC

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

I'd worry about that in production. When the collector goes above Vcc, it
will pull the base above as well. A current will flow through the
protection diodes on the PIC.

 

[Mark]

Mike,

It might be just due to the collector to base capacitance.

Bypass the base to the emittter.

Mark

 

[Rich Grise, but drunk]

Mike,

It might be just due to the collector to base capacitance.

Bypass the base to the emittter.

Mark

But, but, but, nobody's got the point yet, that the whole circuit is
STOOPID, but works anyway!

So much for simulation! ;-P

Cheers!
Rich
--
-----BEGIN GEEK CODE BLOCK-----
Version: 3.1
GAT(E P) dpu s: a++ C++@ P+ L++>+ !E W+ N++ o? K? w-- !O !M !V PS+++
PE Y+ PGP- t 5+++)-; X- R- tv+ b+ DI++++>+ D-? G e+$ h+ r-- z+
------END GEEK CODE BLOCK------

 

[Pooh Bear]

Probably one to file under 'interesting-but-probably-not-useful circuits that behave a little
strangely'...

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

Once you get the tank circuit oscillating, and the transistor's turned off, there's no load to
'sap' power and stop the oscillation.

Try adding an R in series with the L.

Graham

 

[Tony Williams]

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.

I think that is exactly it. There's a very similar explanation (base
charge)
for the behaviour of an old (1957) push pull dc-dc converter.....
centre-
-tapped collector-collector saturating transformer, cross-coupled base
resistors, etc. At switchover, there is a spike at the collector of the
OFF-going transistor, which is transferred by the transformer into
a reverse-voltage spike onto the collector of the OFF (but ON-coming)
transistor. This is supposed to pre-charge it's base so that it comes
ON much faster.