fred jones wrote:
>
> Thanks to all of you for so many good suggestions.  I appreciate everyone
> taking the trouble to answer.  What I'm doing is building a metal detector.
> The 24volts is being supplied by 2 12volt gel cells in series.  The 24 volts
> is used to pulse the coil (the more, the better the depth) this is used to
> supply a 7905 for -5v to the negative supply on the analog components such
> as op amps.  The negative supply is then stepped up with a couple of charge
> pumps to provide the positive 5v supply for powering the PIC16f877 as well
> as the analog electronics such as the positive rail for the op amps.  I had
> heck trying to get a clean level out of the charge pumps.  That is why I'm a
> little gun shy on another switcher.  The heat coming off of the 7905 may
> make it worth the effort though, let alone the wasted energy.


Maybe a re-design is best? If using dual rail
opamps why not put the Gnd at mid point between the
2 batteries, then generate the +5v and -5v with a
7805 and 7905, easier and less heat for the regulators.
Your only problem is to drive the 24v "pulser"
transistor but that should be easily fixed.
-Roman

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