Dual Polarity Power Supply

This project involves working with line voltages (typically 120 or 240VAC) and therefore should only be built by those comfortable working with potentially lethal voltages/currents. The circuitry confined to the circuit board is not particularly dangerous, however connecting it through the requisite step-down transformer presents a risk of severe electrical shock. I can assume no responsibility for anyone injured in attempting to build this power supply. A thorough understanding of electrical safety practices and a healthy dose of respect will help keep you building electronics for a long time!

This document describes how to construct a dual polarity linear power supply, which can be configured for any positive or negative voltage between 1.2-35V. A power supply is the fundamental building block of all but the simplest of electronic devices. It converts the alternating current (AC) from our wall outlets into direct current (DC) at some specified voltage. Because so many electronic devices need DC to function properly, a versatile power supply is the perfect addition to a hobbyist's collection of test equipment. The supply is based on National Semiconductor's LM317 and LM337 variable voltage regulators, which have become very popular because they provide impressive regulation characteristics and relatively high power output from a small TO-220 package. These regulators are specified to deliver up to 1.5A (output voltage dependent) if provided adequate heatsinking, which is sufficient for testing or powering a wide variety of everyday circuits.

There are two versions of the power supply, yet both are fundamentally the same. The first is a compact fixed-voltage supply measuring 2.9" x 2.2", which is well suited for applications requiring small size and modest current capacity such as active crossover networks and other op-amp based projects. The second is a larger (4.6" x 2.25") variable supply sporting more robust heatsinking, and the regulators are positioned such that they may be mounted to a large chassis heatsink for even greater heat dissipation. And since this version of the supply is variable, it is ideal for experimenting. The regulators are fully short-circuit and thermally protected, so there's no risk of destroying the supply if you hook something up incorrectly or accidentally short the outputs together.


Circuit Description

The circuit is fairly standard of a linear supply, and the function of each part can be described as follows:

How about an example? Let's say you want to get +15VDC from the power supply. You will probably want to use a 24VCT (12-0-12) transformer, which will supply a peak voltage of 12(1.414)=16.97V to the the rectifiers. The slight overhead (~2V) will account for the voltage drop across the diodes and regulator dropout. However, the greater the output load, the more voltage overhead required due to increased regulator dropout. The idea is to select a transformer whose peak output voltage is slightly higher than the desired regulated voltage (enough to compensate for diode drop and regulator dropout), all of which will minimize heat dissipation and generally improve the life of the power supply. Similarly, if you are building the variable supply and expect to utilize the full range of voltage variability, you will probably want to use a 48VCT transformer.

Table 1. Parts list for variable power suply

Ref. Des.
Part Type
Lead Spacing
TX1 transformer 48VCT max     n/a
SW1 switch SPST     n/a
F1 fuse 2A slow     n/a
D1 - D6 1N4003 diode   1A 200PIV A 400mils
C1a & C2a cap polarized
2200uF 50V R 300mils
C1b & C2b cap film 0.1-1.0uF 50V min R 590;490;390mils
U1 LM317 volt reg +1.2-+37V 1.5A   TO-220 package
U2 LM337 volt reg -37--1.2V 1.5A   TO-220 package
C3-C4 cap polarized
10uF 50V R 80mils
R1 & R3 resistor
metal film
120ohm 1/2W A 500mils
R2 & R4 potentiometer
linear trim
4.7k 1/2W   400 x 500mils
(10 x 12.7mm)
C5a & C6a cap polarized
470uF 50V R 200mils
C5b & C6b cap film 0.1-1.0uF 50V min R 740;620;500mils
HS1 - HS2 heat sink       1000mil

Notes on parts selection include those above, plus:



Ferric Chloride

Green-black solution in water with a mild acid-like odor. May cause eye and skin burns. May be harmful if swallowed. Ingestion may cause rapid heartbeat, low blood pressure, shock, and possible coma. May cause kidney and liver damage. Inhalation of vapor may cause severe respiratory tract irritation. Substance has caused adverse reproductive effects in animals.

Target Organs: Kidneys, liver, cardiovascular system.

Adverse Health Effects


Notes to Physician:



OSHA Vacated PELs:

Ferric Chloride: No OSHA Vacated PELs are listed for this chemical.

Physical Properties: