[sdiy] Voltage regs + diode drops.

[Scott Gravenhorst]

Dave Kendall <davekendall at ntlworld.com> wrote:
>Hi all.
>
>Another one I can't find a good answer to....
>
>Is there any good reason not to use diodes in series after the output 
>terminal of a positive voltage regulator to get odd voltages, eg. 2 x 
>1N4001 after a LM7809 to get just below +8V?
>
>It seems it *should* work, but I haven't seen it done anywhere yet. Are 
>there any potential problems that anyone can think of?
>FWIW, It would be for a CMOS logic circuit, and with a single OP-amp or 
>transistor level shifter on the output.
>

http://www.kpsec.freeuk.com/components/diode.htm

This shows a nice graphic of a diode's voltage drop versus current through it.  As
you can see, it is not a vertical line even in what looks like the linear portion of
the curve.  As the current increases, so does the drop with more error in the lower
current range than at the higher end.  This would be enough to put me off doing this.

I've found this technique useful, however, to remove or subtract a diode drop of
voltage from a _signal_ that has a diode voltage offset.  I just wouldn't do it in a
power supply.

ADDITIONALLY in second reply: I've seen schematics for 78xx and 79xx regulators that
show how to add some additional voltage by modifying the circuitry at the ground of a
3 terminal regulator turning it into a variable voltage regulator.  I'm sure that the
range is limited.

All in all, what you want is some sort of variable circuit, either by using the
technique I mentioned above or a different IC that is designed to be voltage adjustable.

-- ScottG
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