A low cost, compact 4 amp regulated supply


Owners of Elecraft K3 and other rigs built for truly portable use are always looking for a power source that provides enough wattage to run the system, good voltage regulation,  and is light weight and compact. Low cost is a bonus. I believe the project described here meets all these criteria.

It’s certainly tempting to employ a surplus laptop computer “brick” power supply, but generally speaking they are not fully regulated.  A supply rated at 13.8 VDC could well put out 16 VDC or more at rig startup – enough to fry the innards of a K3, which is rated for a maximum of 15.0 VDC. To provide voltage stability, I decided to pair up a 15 VDC  4 amp switching power supply (Allelectronics.com #PS-154. $16.00) with a home-brew variable voltage regulator.  The specified output is sufficient to handle the needs of my Elecraft K3-10 (12 watts max output, with a supply draw of 3.6 amps at 13.8 VDC input).

For regulation, I chose to use a circuit design published in the technical specifications for the LM-317 variable voltage regulator, which can handle 1.5 amp continuous load.  The schematic (at right) runs three LM-317s in parallel, with output adjustment voltage  buffered through a high-performance LM-308 op-amp.  The design is rated for 4.0 amps load, nicely matching the max output of the power supply.

After building a prototype on a 3” x 3” section of pre-drilled PCB, I decided to see if the physical size of the board could be shrunk to 2” x 2” form factor so it could either be mounted in a custom enclosure with the power supply (also 2” wide) or in a 2” x 2” metal enclosure. This was feasible, even using very large (3 watt) output resistors, which were the only size I cold obtain from my local components dealer.  They should be 1/2 watt or larger resistors to carry the output load. (1.33 amps max current per LM318 produces .26 volt drop across the resistor, and thus  about 0.35 watt dissipated at max load.)  1 watt resistors would be more conservative. The PCB layout (at left, a PDF file that can be printed at 100% onto transfer paper to etch a board) will accommodate 3 watt or smaller resistors.

I designed this board with the idea that the voltage regulator could be used in two ways.  First, as I chose, it could be installed into a custom-made case fabricated to the precise dimensions of the power supply plus te regulator board.  Second, for someone who didn’t want to fabricate an enclosure, it could be installed in a commercially fabricated box measuring 2 x 2 x 1 inches or larger. 

My project encloses all the circuitry in an enclosure fabricated from .032” brass stock. (I prefer brass over aluminum mostly for esthetic reasons.) It measures 7.0 x 2.5 x 1.25 inches, and weighs only 15 ounces, excluding the power cord. The DC output is through a pair of Anderson PowerPole connectors mounted on one end. These are my shack’s standard connector. An unexpected advantage of this arrangement is that I can plug the power supply directly onto the PowerPole connectors on the back of my K3,eliminating the need for a power cord.

A few construction notes about the regulator circuit board. 

First, this MUST be fabricated using 2 ounce single sided PCB material. The input and output traces can be called upon to carry up to 4.0 amps, and you have to use traces 100 mm wide to carry that load if you use the more common 1 ounce PCB material. (This classification system is based on the number of ounces of copper required for a PCB that is 12 x 12 inches.  One ounce copper is 1.4 mils thick.  Two ounce copper is 2.8 mils thick.  If you can’t obtain a 2 ounce board locally, they’re sold on eBay.

The 13.8VDC output from this power supply is with Anderson PowerPole connectors. They are configured so that (if desired) the power supply can be plugged directly into the power input of the K3, without use of a power cord. (If I were to build this unit with the voltage regulator circuitry in a separate 2 x 2 enclosure, I would probably use PowerPole connectors for the output, with a coaxial power jack on the input, to accept the output plug from the computer power supply.)


When traveling light, don’t let your power supply bog you down; build this simple 13.8 VDC Unit

A highly regulated 4 amp portable power supply for the Elecraft K3-10 or other QRP transceivers. The “guts” of the power supply are a commercially available computer “brick” supply and a home-brew voltage regulator circuit.