> -----Original Message-----
> From: Peter L. Peres [SMTP:plp@ACTCOM.CO.IL]
> Sent: Tuesday, November 05, 2002 9:09 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: Norp12
>
> On Tue, 5 Nov 2002, Michael Rigby-Jones wrote:
>
> *>Surely the transistor will only just conduct enough to keep 0.6 volts on
> the
> *>base, the transistor will not be able to be entirely switched off until
> the
> *>cap is charged.  This would effectively cause the relay to switch off
> very
> *>slowly.  I certainly don't see how this is going to cause an oscillation
> as
> *>the cap is effectively providing NEGATIVE feedback at anything over DC.
>
> Correct but I have to add again that relays do not switch slowly. As I
> said before the magnetic force generated by the coil depends on the
> inverse square of the gap and relays have a well known 'catch' and
> 'release' voltage, far apart from each other. Just try to make an ordinary
> relay switch 'slowly'. Use a benchtop regulated psu as accurate as you
> wish.
>
Absolutetly, I did word that rather badly.  I meant the voltage accross the
relay would fall rather slowly, obviously once the magnetic field has become
small enough the contacts will open at pretty much the same speed.

> The capacitor on CB is equivalent to a capacitor across BE where Cbe ~=
> Ccb * beta I think. This is most useful since it causes negative feedback
> in ac there where the transistor has most gain (which in this circuit
> amplifies noise). The beta is relatively low when the transistor is off or
> saturated and there the capacitor plays a smaller role. So entering and
> leaving saturation and off state is less slowed down than with a Cbe cap.
> Also a 1uF plastic cap will have significantly lower leakage than a 100uF
> electrolytic.
>
Yep, that's how I see it.  I did simulate the circuit with the cap accross
CB just to make sure and couldn't persuade it to oscillate at all.

Mike

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