Russell,

I would love to see that circuit you're discussing below.
Could you post that design?

Thanks!

Daniel...


> This design is crying out for a buck regulator solution.
> Sometimes the cry can be hard to hear :-)
> Using a high side discrete P Channel FET or PNP transistor will
allow you to
> easily address the 80 volt requirement.
>
> I can show you a design which uses 3 transistors (including the high
side
> switch), an inductor, 5 resistors (possibly 4), a zener and a
capacitor.
> Voltage withstand is very largely a function of the rating of the
high side
> transistor although two resistors need to have their dissipation
designed
> for worst case input voltage. I have a version operating with a 10.5
volt
> output and a 0.1 volt output change for input voltages from 12 volts
to 200
> volts. (It fails gracefully below 12 volts in and output tracks Vin
less a
> volt or so).
>
> Using surface mount components such as eg SOT-23 transistors
(possibly a
> SOT223 for the high side transistor) & 0805 resistors and capacitor
this
> would "probably" be able to be sneaked into a corner of your PCB.
>
>
> If using a linear solution you could easily meet the 80 volt two
minute test
> with an input transistor that turned off when Vin rose too high.
This
> transistor could replace your input dropping resistor during normal
> operation.
>
>
>
>
>
>
>
>       Russell McMahon
> _____________________________
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