Absolutely brilliant. Thanks for the answer and the plot.
The only lasting query I have is that the diode has a small variation in voltage due to changes in current so this looks like an RC circuit but very small, does it in fact have any characteristics - even small that could be useful. Looking at the plot I suspect not but just have to ask?
It is a 12v car like circuit and the main power supply has lots of intermittent demands - not engine starting but lights, pumps, etc that can pull 20 or 30 amps from the supply - so are you suggesting a larger cap to stop these spikes and surges from coming through to the instrument circuit?
Thanks
Q