Linear Regulated Dual Polarity Power Supply


WARNING:  This project involves working with line voltages (typically 120VAC) and therefore should only be built by those comfortable working with potentially lethal voltages and 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.  This 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.  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.

There are two versions of the power supply which are fundamentally very similar.  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 like TTL logic circuits or op-amp based projects such as active crossover networks.  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.  See the photographs near the bottom where I built one into an ATX power supply chassis.



Circuit Description

Power supply schematic

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

1A 48VCT (24-0-24) transformer is about the practical upper limit of commonly available models.  This limit is set by the maximum input voltage of the regulators, and also by the bulk filter capacitors which are rated at 35VDC.  Actually, these are floating regulators so they can withstand considerably higher input voltages as long as the input-output differential does not exceed 40V.  Select a transformer which will output a slightly higher peak AC voltage than the DC voltage you expect to obtain from the power supply.  Recall that peak AC voltage is 1.414 times higher than the RMS value specified by the transformer.

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.  Or if you are building the variable supply and expect to utilize the full range of voltage variability, you should use a 48VCT transformer.

Printed circuit boards are not available for these power supply designs (as opposed to the mic preamp).  However, the printed circuit artwork is presented for each design so builders may etch their own boards.  Please see this page for some information on etching methods (note that some of the content is specific to the microphone preamplifier project found elsewhere on this site).



Version #1:  Fixed-voltage compact power supply
Click for a larger image
  schematic:   psu1_sch.gif
  copper layer:   psu1_copper.gif
  silkscreen overlay:   psu1_silk.gif

  All the above in Adobe Acrobat:
  psu1.pdf

Table 1.  Parts list for fixed voltage power supply
Ref. Des.
Part Type
Value
Capacity
Form
Lead Spacing
TX1 transformer 48VCT max     n/a
SW1 switch SPST     n/a
F1 fuse 1A slow     n/a
D1 - D6 1N4003 diode   1A 200PIV A 400mils
(10mm)
C1a & C2a cap polarized
electrolytic
2200µF 35V R 300mils
(7.5mm)
C1b & C2b cap film 0.1-1.0µF 50V min R 400;330;260mils
(10;8.5;6.5mm)
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
electrolytic
10µF 35V R 80mils
(2mm)
R1 & R3 resistor
metal film
120ohm 1/2W A 500mils
(12.7mm)
R2 & R4 resistor
metal film
calculated
see text
1/2W A 500mils
(12.7mm)
C5a & C6a cap polarized
electrolytic
470µF 35V R 200mils
(5mm)
C5b & C6b cap film 0.1-1.0µF 50V min R 490;390;290mils
(12.5;10;7.5mm)
HS1 - HS2 heat sink       700mil
(17.8mm)

Some notes on parts selection:



Version #2:  Variable power supply
Click for a larger image
  schematic:   psu2_sch.gif
  copper layer:   psu2_copper.gif
  silkscreen overlay:   psu2_silk.gif

  All the above in Adobe Acrobat:
  psu2.pdf

Table 2.  Parts list for variable power supply
Ref. Des.
Part Type
Value
Capacity
Form
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
(10mm)
C1a & C2a cap polarized
electrolytic
2200µF 35V R 300mils
(7.5mm)
C1b & C2b cap film 0.1-1.0µF 50V min R 590;490;390mils
(15;12.5;10mm)
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
electrolytic
10µF 35V R 80mils
(2mm)
R1 & R3 resistor
metal film
120ohm 1/2W A 500mils
(12.7mm)
R2 & R4 potentiometer
linear trim
4.7k 1/2W   400 x 500mils
(10 x 12.7mm)
C5a & C6a cap polarized
electrolytic
470µF 35V R 200mils
(5mm)
C5b & C6b cap film 0.1-1.0µF 50V min R 740;620;500mils
(18.8;15.7;12.7mm)
HS1 - HS2 heat sink       1000mil
(25.4mm)

Notes on parts selection include those above, plus:

Miscellaneous items needed for assembly:
  1. thermal grease for junction between regulators and heat sinks
  2. #6 x 1/2" screws, nuts, and flat washers for mounting heatsinks to regulators
  3. Optional:  hot glue to secure capacitors to PCB


Project Photos

Full view of the variable supply built into the chassis of a failed ATX power supply.  This box makes an ideal home because it has a handy IEC connector for AC input and a place for a cooling fan.  A dedicated multimeter sits atop to monitor output voltage because it was cheaper than any panel-mount voltage meters I could find...as in $3 from Harbor Freight Tools, SKU 90899.  And it checks out favorably against our calibrated meters at work.  Unreal.  Who knows if its accuracy will hold over time, but it's plenty good for my needs and it's easy enough to spot-check periodically if something looks awry.
Front view of chassis showing binding post output jacks, potentiometer knobs for adjusting output, and power LED.

Side view showing AC input, power switch, fuseholder, and cooling fan.



Here are the innards with the chassis open
Inside closeup of the printed circuit board and transformer.  I used individual heatsinks on the two regulators because they had been in my toolbox for about 5 years and they needed a happy home.
I also installed a separate 5V regulator board cobbled together from parts removed from a dead ATX power supply (a simple LM7805 fixed-voltage regulator).  It lights the power LED and spins the cooling fan quietly.



Revised:  January 27, 2006
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