MiniPow LM317/337 Bipolar Power Supply

Introduction

Completed MiniPow board

MiniPow is a high quality bipolar power supply, designed for use in preamps and other audio applications requiring low-noise ±15V supplies.

It is based on the ubiquitous and high quality LM317 and LM337 positive and negative linear regulators. A large amount of high-quality rail capacitance is present as well; 3600µF Panasonic FM for each rail. The circuit used is based on the "Adjustable Regulator with Improved Ripple Rejection" on page 15 of the LM317 datasheet (National). This circuit is also used in other popular projects like the TREAD and STEPS from tangent. This design deviates in that it is a bipolar supply (includes both positive and negative regulators for a ± supply), and thanks to the almost-exclusive use of surface mount parts, manages in a very small board (63mm x 35mm). Also note that this supply is not tweakable or adjustable; select the correct resistors and output voltage should be within a couple percent.

This project started from a need for a power supply for the MiniVol volume control. I had a ±12V supply on hand, but the PGA2320 used demands ±15V, so this design was born. There aren't a lot of bipolar supply projects out there, and even fewer with size as a main goal, so hopefully this information is useful. The design is nothing special, but may be useful if looking for a small, high-quality bipolar supply.

The supply can be built with the recommended parts for under $20, not including the PCB.

Schematic & Circuit Description

MiniPow schematic diagram

The circuit is a fairly simple implementation of the LM317/337 linear supplies. It follows the "Adjustable Regulator with Improved Ripple Rejection" example on page 15 of the LM317 datasheet (National) with few changes. Input filter capacitance is increased a great deal, and a rectifier and some protection have been added.

PCB Layout

MiniPow PCB layout

In the image, blue areas are copper in the bottom layer, red areas are copper in the top layer, purple areas have copper in both layers, white areas are the top layer silkscreen and yellow areas are bottom layer silkscreen. The PCB design is also fairly simple, with the main goal being to minimize the size of the board while still keeping a decent amount of onboard rail capacitance. The rectifiers and their snubbers are on the top layer, forming the bridge rectifier, while the regulators and their related capacitors are on the bottom layer, taking up nearly the same amount of space. The majority of board space is taken with the 4 bulk rail capacitors and input/output terminals. Four mounting holes are provided.

Parts

The table below lists parts for the MiniPow. Substitutions may be made at the builder's discretion. Detailed descriptions and substitutes are below the table.

Designator(s)QuantityDescriptionDigiKey #
Required onboard
C1-1010100nF X7R ceramic capacitor399-1249-1-ND
C11-1441800µF 35V electrolytic capacitorP12410-ND
C15-16210µF 35V tantalum capacitor718-1052-1-ND
C17-18233µF 35V electrolytic capacitorP12925-ND
R1, R3, R4, R641.33KΩ 1% resistorRHM1.33KFCT-ND
R2, R52120Ω 1% resistorRHM120FCT-ND
D1-8860V 1A Schottky rectifier568-4138-1-ND
D9-124200V general purpose rectifierBAV20WSTPMSCT-ND
U11LM317 1A positive adjustable regulatorLM317AEMPCT-ND
U21LM337 1A negative adjustable regulatorLM337IMPCT-ND
Optional & Offboard
D131green LED160-1169-1-ND
D141red LED160-1167-1-ND
IN/OUT24 position 0.2" terminal block277-1249-ND

Capacitors

Resistors

Semiconductors

Optional Components

Build Notes

Changelog

Files

minipow.pdfHigh quality board layout and schematic
minipow_altium.zipAltium Designer 6 design files
minipow_gerber.zipGerber CAM outputs for PCB fabrication
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