9,12,15,18 Power Box

[s.r.v.]

Hello,
I'd like to make a little box that takes 2 9V inputs, and has 1 output with a rotary switch to go between 9,12,15, or 18 volts. I have a few ideas about how to do it, but how do I cut the voltage? Also, should I add a electrolytic cap somewhere in there? I'm a little lost on power supplies...Thanks for any help

[asfastasdark]

Cut the voltage using voltage dividers.

[calpolyengineer]

or zener diodes

-Joe

[Paul Perry (Frostwave)]

I don't think voltage dividers or zeners are going to be easy - the voltage divider approach gives too much series resiatance.
And the zener is going to only work over a certain range of current.

The way I would do this, is to use an adjustable regulator like a LM317 & switch the resistors that do the setting.

Alternatively, and easier, get a multiple voltage plug pack & put it in a box with a proper switch!

[davent]

Hi,
Couldn't you just set up a 317 voltage regulator with different resistor values switched to give you the required voltages.

LM317 Datasheet  http://www.st.com/stonline/products/literature/ds/2154/lm217.pdf  See the schematic on page 9 , switch fixed resistors for R2 to give you the required voltages. The formula to calculate  the needed R2 values is at the top of page 9.

dave