9V DC Supply

Hi,
I have this particular problem of needing a stable 9V DC (max 500mA).
Input voltage is variable from 8 to 24V DC and or AC.
I checked the many manufacturers of Step-up or -down converters but none could give a solution with the input voltages I have.
Can a good soul get me on track as I can't see a solution.
Maybe a combination of a Step-up and step down but the parts count would be a lot higher.
Thanks,

Does your input need to have the same 'ground' as the output? If not, consider an "inverting buck" converter. 8-24V in, -9V out. Should be able to find one at National, Linear, or Maxim.

Yep; ground (negative) is the same.
I was using a backup battery of 9V but this isn't reliable enough.
When the external psu fails the batteriy soon will go below the minimum voltage of 8.75V.
The voltage needed by the application is exact 8.75V to make measurements consistant reliable. Very akward but that's why the psu is so complicated.
And it needs some sort of battery backup but do to the small size of the remote sencor the unit can only harbour one 9V cell and a small space left over for some psu regulation.
AC and or DC up to 24V is available but power failures are very common and can last for days.
Hence the battery back-up.

Problems, problems, problems all the time (Everly Brothers)