Olin Lathrop wrote:
>
> > I need an adjustable negative supply voltage in the -13 to -16 volt range
> > for the contrast control of an LCD module.  I would like to generate this
> > supply from a 9 volt battery that is also used for the positive supply of
> > the rest of the circuit.  I realize that I could just use a couple more 9
> > volt batteries and a pot to do it, but I would like it to be a little more
> > compact than that.  Can someone suggest some kind of simple charge pump
> that
> > will generate the voltage from the normal supply voltage, without using
> any
> > transformers?  I am quite weak when it comes to "funky" analog
> electronics.
> > ;-)
>
> Like you said, a charge pump, especially since your current requirements are
> very low.  However, you will need two charge pumps to go from +9 to -16,
> although that's still probably the best way to go.  Another alternative
> would require a single inductor.


This was done in a rush, but I think something
like this would work?

Super cheap -15v smps psu.

   +9v------------------------*----*---------------------
   In                         |    |
                              R    |  BC337??
                              |    C  any 200ma NPN small
        L is a small 1500uH   *---B   transistor  (x2)
        (1.5mH) prewound RF   |    E
        inductor.             '----|------Rx------,
                                   |              |
                                   |              C
                                   *----L--*-*---B
                                   |       | |    E
                                   |       | |    |
                           1N4148  - D1    C R1   |
                        or 1N914   ^       | |    |
                                   |  0.1uF| |    |
                                   |       | |    |
   Gnd-----------------------------|-------*-*----*------
                                   |       |      |
           (Hopefully R1 sets the  |       |      | 15v 400mW?
           constant current, maybe |       C2     ~ Zener
           5 or 10mA ??)           |  10uF |      ^ diode
                                   |  25v  |      |
                                   |       |      |
                                   |       |      |
  -15v-----------------------------*-------*------*------
   Out

I think Rx may not be needed, depending how much
you need to keep the parts down. These inductors
are only slightly are only about 10mm x 5mm round
and not much bigger than a resistor. :o)
-Roman

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