AW: [sdiy] Zener-oscillators

[harrybissell]

Congratulations gentlemen

I think you are well on your way to inventing the four-layer diode...
maybe even the UJT ???    :^P

H^) harry

Magnus Danielson wrote:

> From: "Czech Martin" <Martin.Czech at Micronas.com>
> Subject: RE: AW: [sdiy] Zener-oscillators
> Date: Mon, 31 Mar 2003 17:09:33 +0200
>
> Martin,
>
> > I've included a drawing of the transistor DC curve,
> > as well as a curve tracer snap shot.
> >
> > It's only visible in heavily doped collector junctions,
> > thus reverse mode operation.
> > The thing is that the collector (it is the emitter!)
> > base junction breaks down
> > in avalanche. Once a certain amount of holes is in the base,
> > it will turn on the emitter base junction in normal
> > conduction and diffusion (emitter is really collector here).
> > This current will diffuse to the collector where
> > it will push the avalanche. Positive feedback!
> > The voltage across the transistor will collapse,
> > it will then look like a resistive branch.
> > Go under the minimum voltage to sustain the avalanche
> > and it will turn off.
> >
> > There is avalanche, "pushed avalanche", high injection
> > and normal current amplification involved at the same
> > time.
> >
> > Circuits that only want emitter to base zener will
> > therefore not connect the collector pin.
> >
> > Before reaching the break over point, the leakage due to
> > beginning avalanche will rise, if the current source
> > can not live with this the circuit will hang there.
>
> OK. This starts to makes sense. The transistor emitter-collector breakdown I-V
> explains what happends.
>
> I've seen two recent reports of this type of oscillator actually working.
>
> I've also tossed at BC550C under a 100 ohm resistor and a pretty good sine for
> straight and X-Y scope. Not a very elegant setup if you want a good I-V curve,
> but it does gives a hint. The breakdown is clearly visible and when you drive
> it as hard as -9.1 V it kicks in and bends in the other direction pulling more
> and more current falling back quickly to about -7.2 V which is a platau under
> which it pulls current (I drive mine as hard as -25 mA).
>
> A Zener-diode (6.2V out of a bag here on the table) has quite a different
> story to tell. So does the emitter-base setup. No, the emitter-collector stuff
> is what makes this fun.
>
> It's also interesting to see how the voltage of the base changes through the
> cycle. Near the abrupt slope it has about the same shape, including the abrupt
> slope. Looking at the base is like probing into the middle of all this, since
> it is the P between the N of the collector and the N of the emitter.
>
> Got to hit bed, oscillation is next lab-assignment for tomorrow!
>
> Cheers,
> Magnus