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From: lagasse@biomed.UUCP (Robert C. Lagasse)
Newsgroups: net.analog
Subject: strobe oscillator
Message-ID: <151@biomed.UUCP>
Date: Tue, 4-Feb-86 14:18:56 EST
Article-I.D.: biomed.151
Posted: Tue Feb  4 14:18:56 1986
Date-Received: Fri, 7-Feb-86 20:17:19 EST
Organization: Mass. Gen. Hosp. - Dept. Biomed. Engineering
Lines: 30

Dear gentlemen:  
     Are there a few simple rules of thumb for designing an oscillator of
the type used for camera strobe flash units.  The type I am talking about
typically use a "simple" transistor oscillator feeding a subminiature
transformer.  It operates usually from 3-9V DC and charges a large (about
300 uF) capacitor to about 300 V in a few seconds between flashes.  The
oscillator operates in the audio range as the transformer can be heard
sweeping a range of frequencies as the capacitor charges.  The transformer
used is of conventional (laminated frame) or toroidal (even more fun)
design.  The circuit typically derives a feedback from the transformer,
sometimes by a separate winding.  Here come the how-do's:

HOW DO YOU  1) Easily determine the primary and secondary impedences of the
transformer you want.  If you want to wind your own ,  say on a toroid
ferrite, how do you select wire gage and # of turns.
2) How do you pick the frequency at which it will oscillate?
3) How do you select capacitor values to start experimenting with?
4) How do you insure that an oscillator like this will self-start and not
just sit there and pour DC into the transformer, ultimately burning out the
transistors?

   What I am looking for are practical trade shortcuts and practices.
Please don't tell me to go read a text on oscillators.  Reference to an old
magazine article would be nice.  Thanks in advance.
Bob Lagasse @ Mass. General Hospital



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