10 Jul
2007
10 Jul
'07
11:45 a.m.
Cameron Kaiser said:
> > You're making me feel very old. When I
was a kid, I used to make a
> > neon
> > bulb blink using one resistor and one capacitor.
>
> Relaxation oscillators are fun. I like the UJT version as well,
> though UJTs are getting harder to find than they once were. I have
> two or three 2N2646 UJTs left.
A quick look here at a few distributors here lists at least old stock
availability, at a wide price range. Problem is IMHO that such old parts
are not ROHS conforming; thus they are delisted, e.g. at Digikey. Farnell
still offers them, at an obscene price, Reichelt in Germany here has a
fair price.
But UJTs are mainly one-trick ponys which is why they are primarily found
in basic courses in electronics :-) They had their time in the 60s and
70s.
Relaxation oscillator?
Tony may feel very old, but I'm just feeling very dumb. ;)
Now that is exactly the work principle that Tony has described with his
neon sawtooth oscillator: a resistor charges a capacitor in series. The
load to this voltage divider is the UJT or the neon bulb which have a
characteristics with a negative resistance at some point - once some peak
voltage is reached resistance will drop very fast and will discharge the
cap. Result is the sawtooth wave.
There are other circuits with a comparable characteristic, like the PUT
which is basically a SCR with an anode gate - you can make one with
_parallel_ coupled PNP and NPN transistors. Or with a bit more effort you
can utilize the same effect with a NIC or a gyrator.
--
Dr.-Ing. Holger Veit
IT-Management
Fraunhofer IAIS
Institut f?r Intelligente Analyse- und Informationssysteme
Schloss Birlinghoven
D-53757 Sankt Augustin
e-mail: holger.veit at iais.fraunhofer.de
Tel. +49 2241 14 2448
Fax. +49 2241 14 2342