> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of cdb
> Sent: 16 February 2011 10:15
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE]DC fluro inverter problem
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> :: Are your TIP41Cs TIP41Cs  ?
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> Definitely TIP41C's.
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> :: How old are the cores?
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> At least 5 years old.
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> :: What are they made of?
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> Ferrite.
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> :: Try swapping cores (whole inductors) between a good and a bad one
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> Tried this, it appears to solve the immediate problem, but not the
cause
> or
> possible prevention.

Then Russells theory is almost likely correct.  Powered iron cores
suffer an irreversible shift in core loss when heated for long periods
of time.  If the design was marginal at start of life, then the
increasing core loss could reduce the feedback signal to the two drive
transistors to the extent that both transistors could be biased on,
which would obviously let the smoke out very quickly.

Have you checked the waveforms on the transistor bases between good/bad
units?  Would it be possible to try adding some more turns to the
feedback winding on a bad unit, or is the transformer encapsulated?

Regards

Mike

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