Michael Rigby-Jones wrote:

> > Wrong again, it's an NPN transistor driving a
> > relay with it's collector, therefore when the
> > relay turns off the collector goes high (to
> > about 1v higher to turn on the catch diode) and
> > WHEN the collector goes high your suggestion of
> > a cap to the base WILL turn the transistor back
> > on, giving exactly the vibration he complained of.

> 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.


The relay won't switch off slowly as such, what
happens is it drops out when it reaches about
4v, and won't pull in again until about 9v.
All that is required is a cap to slow any change
in the transistor because it is B-E and changes
there must happen slowly.

Peter's suggestion of putting the cap C-B is bad
as when the relay rutns off the collector goes to
PSU+1v, the cap couples this sudden rise directly
back to the base and it makes a good oscillator.
B-E makes it oscillate less, C-B makes it oscillate
more. :o)
-Roman

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.