Here's my favorite.  Limited only by the forward voltage of the power
transistor, and the accuracy of zener you can find.


-------+-----------C   E----
       |             B
       +---/\/\/\/---+
       |             |
      ---           ---
      /Z\ Vz = 5.x  \ / SCR
      ---           ---
       |            /|
       |           | |
       +-----------+ |
       |             |
       /             |
       \             |
       /             |
       \             |
       /             |
       |             |
-------+-------------+-----

If I need to explain what any of that is, just tell me.

--Brendan

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