Russell McMahon <apptech@paradise.net.nz> wrote:
> ******************   BUT   *******************
> as shown this circuit has a major problem.
> 
> Vgs_max for the IRF7416 is -20 volts and when supply voltage is much 
> above 20 volts the FET will be driven into a potentially (pun almost 
> intended) fatal state.

Don't you see that the Vgs on Q3 is limited by the ratio of R4 to R8?

Q6 is a current sink driven by the regulated 5.0V supply to the
PIC10F204, and the drop across R4 will be controlled by that current.

With the component values shown, the current through Q6 will be
4.3V / 750ohms = 5.733 mA, and this produces a drop across R4 of
5.733 mA * 2000 ohms = 11.5 V, which is replicated by the emitter
follower driver stage. The excess supply voltage, if any, appears
across Q6 and Q4. No problem at all.

-- Dave Tweed
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