Switching voltages on a power supply

If you're using an adjustable voltage regulator to control the output from a power supply, you need to switch in the right resistor to pick the right voltage. There's a table of resistor values for the LM317 voltage regulator here; below, I describe several ways to accomplish the switching itself.

There are a lot of ways to do it; here are a few:

• The simplest to make is to put the voltage regulator in a little box, with 2 binding posts inside. When you need to change voltage, pop open the case, and clip in the right resistor. Keep various resistors in the box, and label them with the voltage you'll get if you clip them in.
• Far better is to put all the resistors on a little circuit board, each connected to one switch on a DIP switch (such as Radio Shack part number 275-1301). Then open the box and click the right switch (or mount it so you can change it from the outside). If you turn more than one switch on, you'll get unexpected voltages (you can calculate what they'll be eaisly enough).
• You can use a variable resistor (potentiometer, pot, or volume control) so you can have any voltage you want. But then you need a meter (at least to calibrate and mark the thing in the beginning) to know what you're getting, and a slight jar will throw it off. You can get pots that take several full turns, which would be more accurate but harder to mark.
• Use a rotary selector switch to switch in the right resistor as needed. Radio Shack at least once had a 12-position switch, part number 275-1385. All I find now is a 2 pole 6-throw version, part number 275-1386. You could use this to get 12 voltages by adding a SPDT switch to swap between the 2 poles of the selector -- make one the 'low range' and the other the 'high range'. Ocean State Electronics, however, does have a 12-position rotary switch.
• Use one or more slide switches. It's hard to find any with more than 5 positions (see vendor list below), though I believe they are made.

Automating your voltage selection

If you use a 4-pin connector for power out, you can use two pins for the usual power and ground, and put the adjustment resistor across the other 2 pins. Then you can either

• have a cable for each voltage you want, and put any AdaptaPlug on the end; or
• have a dedicated cable and tip for each device, that does the right tip shape, polarity, and voltage control (this functions much like the Targus supply, but using completely different technology).

If you do decide to go with a 4-pin connector arrangement, I'd recommend avoiding any type of connector that's commonly used for something else. For example, I think mini-DIN-4 was used for Apple ADB connections; a 4-pin RJ11 is a typical pone jack; a 4-pin 2.5mm phone jack is a Nokia headset connector (on some models); and so on.

Some relevant links

• There is a list of switch manufacturers here.
• A nice online switch catalog is ElectroStock. They even have radio-button style gang switches, apparently available with as many as you want in a row.
• Ocean State Electronics also has a nice selection.
• Lamb Industries has a large selection as well, including mulit-position slide switches up to 5 throws and miniature rotaries up to 16 positions.

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