Mini Dual Voltage Power Supply

Switching ‘buck converter’ power supply modules are fantastic, efficient, and can be bought for a few dollars (if not cents!). One drawback for me is that sometimes I need to have both positive and negative voltage supplies, and the buck converter/wall wart combination is not easily adapted to suit.

For these situations I have developed a reusable basic design using the LM317/LM337 3-Terminal Adjustable Voltage Regulators.
The LM317 positive voltage regulator, and its negative voltage counterpart the LM337, are capable of supplying 1.5A over a 1.2V to 37V output range. They require only two external resistors to set the output voltage. These devices also have just 3 pins (Vin, Ground and Vout), which greatly simplifies the design of the power supply circuit.

Power Supply Design

The circuit is relatively straightforward and follows the ‘typical application’ circuits found in the LM317/337 data sheets. Eagle CAD files for both the schematic and board layout are available from here.
The design starts from an input AC voltage generated by a center-tapped transformer being rectified through a 4 diode full-wave rectifier. The center tap of the transformer becomes the ground for the circuit, splitting the voltage so that we end up getting a V+ and V- DC voltage. The AC wiring before the transformer is not covered here as it depends on your country and your own skill level, but the usual warnings apply – AC mains power can kill so do not work with AC unless you are confident and have the right tools.
The transformer output AC voltage should be suitable for the required output DC voltage. Linear Regulators will drop about 1.5V during operation, and the AC sinusiodal voltage is related to the regulator input voltage (see this tutorial for details). In all cases the transformer should also be rated for the appropriate for output current requirements (ie, it needs to be less than or equal to 1.5A, the max rated output of the regulators).
For example, to get regulated 5V DC output, the input to the regulator needs to be at least 6.5V, which means that the peak AC voltage should be at least 6.5V/0.637, or at least 10.2V AC. Similarly, for regulated 12V DC output, the transformer needs to supply at least 21.2V AC.

The polarised electrolytic capacitors need to be rated for double the output voltage (factor of safety). Select the rectifier diodes to suit supply current. The 1N4004 diodes are nominal and probably suitable for most applications.
The output voltage is adjusted using the multi-turn trimpots R1 and R4. The output voltages are V+ = 1.25 * (1 + R1/R2) and V- = -1.25 * (1 + R3/R4). In practice, it is easiest to simply adjust the pots while reading the output on a multimeter.
For single sided supply, you can omit the omit top (V+) or bottom section (V-) of the schematic. Also note that with the two sides being independently adjustable, V+ and V- do not need to be symmetrical.
The design fits comfortably on a 2″ square single sided PCB. The layout exploits the inherent symmetry of the design to simplify the layout.