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!

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.

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

• TX1 is a center-tapped transformer which converts the line voltage from 120 or 240VAC to a lower value which can be managed by the filter circuitry.  See note 1 for more details.
• Diodes D1-D4 rectify the AC from the transformer output into a pulsing DC waveform.
• Capacitors C1a and C2a are large electrolytics which perform bulk filtering duties of the pulsing DC waveform into a raw DC voltage.  C1b and C2b are small film capacitors bypassing the electrolytics to improve transient response and filter high frequency line noise.
• U1 and U2 are the complementary adjustable voltage regulators LM317 (+) & LM337 (-).  The output voltage may be programmed easily with two external resistors.
• Resistors R1-R4 set the regulated output voltage.  The LM317 & 337 regulators develop 1.25V across R1 and R3, respectively.  This reference voltage causes a constant current to flow through the voltage-setting resistors R2 and R4 which generates an output voltage approximately equal to the following relationship:

• Capacitors C3 and C4 improve the regulators' ripple rejection from 65dB to 80dB (typical) by preventing ripple voltage from being amplified at the output of the regulator.
• Capacitors C5a and C6a are electrolytics that stiffen the output voltage and reduce output impedance.  Capacitors C5b and C6b are small film bypass caps (similar to those at the regulator input) to filter any high frequency noise present at the output.  The size of the four bypass capacitors is not particularly critical--the most important quality being they have reasonably low impedance at high frequencies, which is true of virtually all modern polyester, polypropylene, polystyrene, or mylar film capacitors.
• Finally, D5 and D6 protect the regulator from damage in the event the input is shorted.  Lacking these, the output capacitance can dump its charge instantaneously through low impedance paths internal to the regulators.  The diodes bypass the regulator and allow the charge to safely dissipate.

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).

schematic:   psu1_sch.gif   copper layer:   psu1_copper.gif   silkscreen overlay:   psu1_silk.gif   All the above in Adobe Acrobat:   psu1.pdf

Some notes on parts selection:

• All electrolytics are rated at 35V, and those by different manufacturers (of a given capacitance) will generally have the same footprint & lead spacing.  Selection and purchasing is therefore simplified.
• The printed circuit board can accommodate several different sizes of film bypass capacitors.  The footprint for each has multiple solder pads that may be selected based on the individual capacitor you're using.
• Be sure to obtain the 1.5A-rated regulators in a TO-220 package.  They are offered in several different varieties including low power T-92 packages, low power/low dropout TO-220 packages, and high power TO-3 packages.
• Suitable heat sinks:  Mouser #532-592502B34     MECI #310-0088

schematic:   psu2_sch.gif   copper layer:   psu2_copper.gif   silkscreen overlay:   psu2_silk.gif   All the above in Adobe Acrobat:   psu2.pdf

Notes on parts selection include those above, plus:

• Layout of PCB enables regulators to be commonly mounted to a heavy chassis heatsink to improve power dissipation.  If using this configuration, you must isolate the devices from the heatsink with mica insulators & nylon screws because the metal tab of each regulator is internally connected to its output pin.
• Suitable potentiometer:  MECI # 450-0138
• Suitable extruded aluminum heat sinks:  Mouser #532-531102B00; 532-531202B00     MECI #320-0185

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