Jinx wrote:

>Hi Aaron, the circuit you have there limits you to a small range
>of input voltages
>
>About 9VDC at the lower end for good 5V regulation (is a 7805
>really needed or would a 100mA 78L05 or 5V1 zener do ?) and
>11VDC at the upper end to prevent over-voltage at the A/D pin
>  
>


Load varies from 100 to 200ish mA.  I figured I would adjust the voltage 
divider based on whatever wallwart I found in my junk drawer.  Or as you 
suggested in your other message just get a single 'power good' signal 
from the anode of D1.

>As the i/p note says AC/DC, it's possible that one day a 12V
>unregulated supply could be plugged in. That may be 20V or
>more at less than its rated load (=10V-ish at the PIC pin, bye-
>bye pin), so you'll have to make sure the regulator can dissipate
>the wattage from the voltage drop * I across it. 
>

I was figuring on a heatsink.  Hmm, if I measure the incoming voltage, I 
could actually dynamically adjust the load to reduce heat dissipation.  
The load is primarilly a multiplexed LED matrix.  I could do less 'on' 
time and make the LEDs dimmer but keep the 7805 dissipation lower.

>When I make
>something like this I'll add a 7812 or series resistor (if I'm sure
>about the transformer voltage) to the front end to take some of
>the sting out of the i/p voltage for the 7805
>
>If you want to just detect (rather than measure) outside voltage
>presence I'd suggest replacing the lower 10k with a 4V7 or 5V1
>zener
>
>BAT46-type Schottkys are much smaller and cheaper than
>1N5819. ISTR they're about 150mA, which is plenty. The
>PIC should also run OK with a 1N914 from the 7805 rather
>than the 1N5819 (I'm assuming the whole circuit requires less
>than ~ 100mA)
>  
>

See above.

>You might want to look at the leakage current for C3. It will have
>a bearing on the battery life. You could probably remove it without
>affecting performance
>
>  
>

Thanks for the comments.

Aaron
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