13.8 Volt 4 Amp DC Power Supply

(C) 2012, G. Forrest Cook

Photo of 12.8V 4A DC Power Supply

Schematic of 12.8V 4A DC Power Supply


This low voltage supply is useful for powering a variety of 12V (nominal) DC devices. It is perfect for powering Ham Radio QRP rigs, VHF radios, CB radios and car stereos. It can provide up to 4 Amps of current at 13.8V for a total of 55 Watts. The output voltage is kept steady with an adjustable regulator circuit.

This design started with an old dead power supply I had in my junk box, it can be built from new parts if you can locate a suitably rated power transformer. The circuit uses few parts and is fairly easy to build.


The power transformer converts 120VAC into 38VAC center-tapped. This is fed to the two diodes to full-wave rectify the AC into DC. The 12,000uF capacitor filters the ripple from the DC supply and the 100nF capacitor improves the high-frequency bypass characteristics of the 12,000uF capacitor.

The regulator configuration was inspired by the applications notes for the LM309 regulator in the 1979 Motorola linear circuits databook. By adding a high-current pass transistor, a voltage regulator's current capacity can be greatly boosted. In this circuit, the regulator was changed to an adjustable type (LM317L) and a high-gain PNP darlington pass transistor (2N6052) was used. The LM317L voltage regulator changes the base current on the 2N6052 in order to keep the output voltage at the regulated voltage setting.

The 10uF, 35V tantalum capacitor filters the input to the LM317L regulator, the value was selected to produce fast respose to large changes in the output current, while preventing oscillations in the regulator. The two 100nF capacitors on the output terminals bypass any stray radio frequency energy to the chassis ground.

Note that there are two grounds in this device and they are isolated from each other. The ground from the AC input is connected to the chassis and the output bypass capacitors. The DC negative lead is also considered a ground and is isolated from the AC ground. The two grounds may optionally be combined by connecting the negative output lead to the chassis ground.


This circuit was built on the chassis of a Standard Power Supplies model SPS40-12 which had a defective regulator circuit board. The chassis, heat sink, power transformer and 12,000 uF capacitor were all were salvaged from the original device, everything else was removed. The rebuilt power supply chassis was mounted in a metal box. The AC input connector, AC fuse, DC output terminals and chassis ground connection are monted on the box.

The 2N6052 transistor was mounted on an aluminum bracket that thermally couples to the larger chassis heat sink, it was attached with screws and thermal grease was used on all of the metal junctions. The 2N6052 should be electrically isolated from the heat sink using a mica insulator and plastic shoulder washers.

Most of the small parts were wired point-to-point under the 2N6052 transistor, the circuitry could also be built on a perforated prototype board. The 10uF, 35V tantalum capacitor on the input side of the LM317L is critical for preventing oscillations, if you can't find a tantalum part, use a 10uF 35V electrolytic capacitor in parallel with a 100nF monolythic or ceramic capacitor.


With no load attached, monitor the output voltage with a DC voltmeter and adjust the 5K potentiometer until the output reaches the desired voltage. When the load is connected to the supply, the output voltage should be fairly stable as long as the current is kept under 4 Amps.

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