Universal 3-way power supply

Time for something simple and useful. Ever wanted a number of pre-set voltages easy to access for Your projects? Tired of always making regulator setups at the lab deck? Now, there is a easy change on this! Build this tiny regulator board that allows You to use up to three different voltage outputs, each of maximum 2 A (depending of regulator choise). Everything is built, using standard surface mount (SMD) components.

The project itself isn't much to write home about, but to be able to build one, a decent documentation is needed. And that is what this page is all about. I will, in some easy steps, help You out with the design of the power supply board. To start, You will need some basic skills to pull this off. First, soldering is needed. Some SMD experience may be useful, but this board aren't tricky to solder even for someone who has never seen SMD before. The smallest parts are of size 1206, that are still rather large. Second, if you plan to make your own board, be prepared for some chemical action. I will not guide you in the art of making PCB's - there are enough of that around the web. Use a suitable search engine to find it. Just be careful when etching the board - the chemicals could be rather nasty.

The Features
Making the PCB

There are two ways to obtain this PCB for the power supply:

1. Make it yourself. This can be done by traditional methods as the board is rather simple with friendly tolerances. You have either the oiginal Protel-file to start from or a PDF. The board is designed to work with only the components side. The solder side does not hold any wires. So, if you do this at home, make it as a single side board with a component side only. If you use the PDF, please verify that the printout is 147.6 x 43.4 mm in size.

2. Contact me and I can sell You a professionally made board, double sided and with solder mask and component print. Use the mail address at this page. A board will go for 10 + shipping and handling.

The Parts

This is what You need to build this thing:

Value Position Type Note
R1a, R1b, R1c R1, R4, R6 1206 Values depends on output voltage
R2a, R2b, R2c R2, R5, R7 1206 Values depends on output voltage
R2a, R2b, R2c R8, R9, R10 1206 Use these together with trimmers
Rled R3 1206 Depends on voltage from U1 and LED type
R2a", R2b", R2c" P1, P2, P3 Horisontal trimmer Value depends on output voltage
100 nF C1, C2, C4, C5, C7, C8, C10 1206  
1 F C3, C6, C9 1206 Used for improved regulation. Not mandatory
10 F/50 V C13, C14, C15, C16 Elyt ø6.3  
1000 F/50 V C11, C12 Elyt ø16  
LM317S or LM1117S U1, U2, U3 TO263  
STPS340U D1 - D10 DO-214AA 3 A Schottky diodes.
5 mm LED D11   Choose any color you like...

Of course, this list isn't mandatory. The component selection need to be adapted to the wanted result. Here are some suggestions:
Counting on it...

To decide the output voltage for each regulator at the board, the classical formula stated in the data sheet of LM317 should be used:

Vout = 1.25 * (1 + R2/R1)

There is a almost theoretical current draw from the regulation loop through R2 too, but in practical use, this can be ignored. My schematics have been designed to match the R1 & R2 marking of the resistors. Note that the PCB markings are not the same! As there are alternative ways to select R2, I have used the and " to mark these positions. To indicate the tree different regulators, the letters "a", "b" and "c" have been added.

This board allows four different ways to combine resistors to set the output voltage. The entire design assumes a fixed value for R1 (that must be used in all cases). The adjustments are done with R2. The pictures below are generic - all three regulator sections are identical. Grayed out parts are not used.

Fixed output using R2 only. Fixed output by paralleling R2 and R2. The trimmer (R2") is replaced with a wire.

The variable version: Trimmer (P2") in series with R2. This makes it possible to adjust the output between two levels. If trimmer only is wanted, use a zero ohm for R2. All combined together: use the trimmer section parallel with the fixed R2 if all other options can't give you the desired output voltage.

If you want to use panel potentiometers instead of trimmers, there are no problems with that. Just solder short wires to the trimmer holes and connect them to the potentiometer. Note that the center tab (the moving one) must be connected to the single hole of the trimmer footprint.

Also, the protection diodes around the regulators are optional as long as the output voltage is less than 25 V and the output electrolytic is 10 F or less. Higher voltages and/or capacitor values requires the diodes to prevent a destroying current rush in case of a shorting of the regulator inputs (the output side capacitors will discharge through the regulator in this case if the diodes aren't used).

Connecting to a power source

Of course, this thing needs power from somewhere. The board has been designed to operate from both AC and DC power sources. For the AC option, there is a full bridge rectifier build of four discrete diodes (D2, D3, D5, D6). This works fine with any AC source, either an external "wall wart" type or a plain transformer (single winding). If you prefer to run the board from DC instead, there are two options:

1. Connect the power source to the "Unregulated" connector. Positive should go to "V+" and negative to "GND" (there is no polarity protection here). In this case, the rectifier diodes aren't needed. The filter capacitors (C11 and C12) may be needed, depending on the quality of the DC source.

2. Connect the power via the rectifier part. In this case, the "AC input" is used and the diodes must be mounted. Doing this, you will have full polarity protection (it doesn't matter what way the positive and negative are connected) but you get some voltage drop over the diodes instead. You must also count for the maximum current taken from the source as the diodes does have a limit for that (the suggested schottky diodes can take 3 A maximum).

The mandatory pictures:

The layout of the board.

An example of assembly - one fixed and two adjustable outputs.