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PICList Thread
'[PICLIST] Re [EE]: Design Challenge - low power st'
2001\09\02@031749 by Russell McMahon

picon face
Russell wrote -
>...It is now in limited production and so far 2000+ of these have
>been built, production tested and accepted by the Customer...

& Dave responded
/ This makes everything MUCH more clear and I can now understand a
/ little better why every suggestion made to you so far has been met by
/ obdurate ** resistance and long-winded self-justification.
/ I've pointed out a number of things in your design that warrant close
/ attention.  Whether or not you attend to them is your business.
/ It is certainly no concern of mine and, so long as it appears that
/ whatever I say might just as productively be said to a brick wall,
/ neither is any continuation of this discussion.
/ Best of luck with your product, and have a nice day.

                I'm somewhat nonplussed by Dave's antipathy,
                     or why he responds as he does unto me
       or why he deems my explanations, to be, rather, obfuscations,
            I'm flummoxed by such floccinaucinihilipilification *
                                 and from Olin as well,
                          - but he's known to be grumpy
                             as and when the mood takes him
                                 so I'll try not to be jumpy.

I'm not completely sure why "works faultlessly in practice over a sample of
several thousand products and with a few units "torture tested" (Customers
terminology) for over 1000 operating hours so far" should invite such

It's clear (to me anyway) that Dave & I are suffering from typical doses of
human nature and have strong mental filters in place.
He's quite certain that I ignore all questions and guidance and waffle on
interminably in self justification.
Conversely, I'm quite certain that, if he actually reads my replies, then he
certainly doesn't notice that I have agreed with a significant number of the
points raised, admitted various limitations and tried to give technical
responses to some points raised where I considered it useful.

Dave & Olin & others have made a number of good points and I've agreed with
many either partially or completely. Where I disagreed I tried to explain
why in most cases. In a number of instances the
nature of the probable applications mean that some points are less critical
than they may be in demanding applications. As noted on several occasions ,
the circuit is not meant to solve all needs - it is cheap and cheerful and
has limitations - it also works in the real world. Points on aspects such as
zener current, hysteresis, switching speed and many more have been replied
to and in many cases agreed with. Short of saying (untruthfully in all
cases) that it is a terrible design, doesn't meet any needs, doesn't work in
practice, blows up in use, won't oscillate, drops out of operation
unexpectedly, is unstable, generates excess EMC and is an utter disaster all
round I don't know how I can make Dave happy. Changing the whole basis of
operation and recanting my claim to its original source just might do it.
Anyone else got any ideas?

I considered posting a summary of useful points raised and my responses to
them, all  drawn entirely from past posts but decided the game is unlikely
to be worth the candle. As noted recently, when I get time I intend to
produce a low current demonstrator of the circuit. To make more people, if
not everybody, happy I'll look at adding explicit static feedback *** (as
suggested by Olin) to complement the existing dynamic hysteresis. This will
almost certainly not alter the core nature of the
design and should be doable at minimum cost.

Stay tuned ....


               Russell McMahon

* - you'll need at least a Shorter Oxford Dictionary for that one if you
don't have it in your quiver already. Websters doesn't hack it.

** - I liked Dave's "obdurate" here  - it was very well done in the context

Interestingly, just adding a feedback resistor as suggested by Olin (QBUK2
collector to QBUK1 base) would provide static hysteresis but also would
introduce exactly the same incorrect-trending feed-forward as occurred in
the elegant 2 transistor circuit. Just as was done in the original relay
driver, splitting the feedback resistor and adding an intermediate clamp
zener to restrict the change in feedback voltage with Vin would have the
same beneficial effect.


           Russell McMahon

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