Wojtek Zabolotny wrote:

>I'd suggest connecting the 7812 in the following way:
>
>                        +--------+
> > +16 (unstab.)        |        |           ca. +13.5 (stab.)
>------+-----------------+  7812  +----+-----+----
>      |                 |        |    |     |
>    --+--               +----+---+    |   --+--
>    --+--                    |        |   --+--
>      |          +--|<|-|<|--+-+===+--+     |
>     -+-         |  2 diodes    3k         -+-
>            GND -+-
>
>That way you get almost stable 13.5V. It should be much safer solution than
>trying to program 16F877 with 11.9V.


This ckt is good. If you want to be able to adjust Vpp, use a
little potentiometer instead of the diodes, a .5-1K pot should
fine work with the 3K resistor shown.

Less safe would be to replace both 3K and diodes with a 5K pot
[less safe in that it is more easy to fry something in case where
the pot is adjusted to the wrong position].

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