[PhilBot]

Originally Posted by dcbrown

Many regulator circuits worry about sudden current discharges and transient spikes delivered back into the output pin of the regulator and put a protection diode across the input/output pins. D1 in essense is across the pins but in the reverse direction of what is needed so the regulator isn't protected. I guess if you burn out the chip you might not notice because you still have R1 delivering 100ma+ charging current?

For me, D1 is the real fly in the ointment. It's hard for me to tell what it is doing to the current coming directly from the 12V battery.

If the regulator voltage is at 8.3V and the battery is below this, then I guess D1 is reverse biased, and has no effect. The only way D1 comes into play is if you disconnect the backup battery.... If this happens it shunts the 12V to the regulator output... which is innecessary since the D2 would shunt it to ground anyway.

IMHO D1 would be much better served protecting the backup battery from being discharged when the 12V breaker is off.

Phil.