> Can anyone suggest a good way of creating a 5V supply (max 250mA) from
> a 40V supply.

Sounds like you need a simple buck regulator.  Otherwise, a linear
regulator will dissipate (40V - 5V) * 250mA = 8.75 watts.  You can make
the buck regulator fairly rough with an output voltage from 7-8V, then use
a 7805 or something to make a nice clean 5V.


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