Pic Dude wrote:
>
> > If you MUST have them on the same heatsink, use a load
> > resistor after the 7812, then put the 2 diodes on its
> > OUTPUT with a series resitor to supply volts. For low
> > currents this will work ok and give you 12v + 1.4v etc.
> > Just allow a little more current through the 12v load
> > resistor than through the diodes.
>
> You've lost me here.  Load resistor "after" the 7812 =
> resistor from output to ground or in series w/app ckt?
> Now, if I add the diodes on the output, it'll DROP the
> voltage by 1.4v.  Not sure what arrangement could cause
> it to RAISE the voltage by 1.4V.
>
> Care you do some more of your awesome ASCII art?


Here it is, not a great circuit but it will do
what you wanted with the same parts count you
already have.

 Vin------*-----------------------------,
+20v?     |                             |
          |                             R  +13.4v
          |                             *--------------
          |                             |          vout
          |                             D
          |          ,--------,         D
          |          |        |         |
          '----------| 7812   |---------* +12v
                 in  |        | out     |
                     '--------'         |
                         |              |
                         |              |
                         |              RL
                         |              |
                         |              |
                         |              |
                         |              |
Gnd ---------------------*--------------*-------


I think a much better circuit is just using one R and
one zener diode, the Vpp volts you need are VERY low
current. I'm sure you could even use a 2-resistor
voltage divider as the PIC only needs a signal >13v
to activate Vpp.
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads