12 Volt 4 Amp Fixed/Adjustable DC Power Supply

(C) 2012-2024, G. Forrest Cook


This low voltage suppliy is useful for powering a variety of 12V (nominal) DC devices. It can also be used as an adjustable DC power supply with a range of 1.25 to 17V. The power supply is useful for powering lower power Ham Radio rigs, VHF radios, CB radios and car stereos. It can provide regulated 13.5VDC power at just under 4 Amps or 12.2VDC at 4 Amps. The output voltage is kept steady with an adjustable regulator circuit. Older first and second generation power supply projects are shown below, the new third generation version is recommended for new builds.

Version 3 of the 12V Fixed/Adjustable 4A Power Supply

Photo of 13.5V 4A DC Power Supply V2

An earlier version of the Fixed/Adjustable DC power supply

Schematic of 13.5V 4A DC Power Supply V3

Version 3 schematic for the Fixed/Adjustable DC power supply


This is the new and improved third version of this supply, it is much more rugged and stable than the first two versions. Note that for continous use above 2A, a much larger heat sink than the one shown in the above photo should be used, see the photo below for an example. This project is a junk box special that was mostly built from surplus parts, new parts were used for the bridge rectifier and PNP Darlington pass transistor.

120VAC power is fed to the supply through an optional IEC plug power line filter. The filter can be helpful for reducing noise from the AC line and is recommended if the supply is to be used with radio equipment. The line power is controled with an SPST toggle switch and a 2 Amp fuse protects the circuitry from overload conditions.

The above photo and schematic show the regulated DC power supply. The photo shows an earlier version, a few additional parts have been added including the variable voltage control. The output voltage is selected by a panel-mounted SPDT switch. In the fixed 13.5V setting, the voltage is set by the 1K trimmer pot. In the variable setting, the voltage is set by the 5K 10 turn potentiometer that is mounted on the front panel. The variable output voltage range is from 1.25V to 17V.

The power supply uses a single-ended power transformer that feeds a Schottky diode bridge rectifier which converts the AC into pulsating DC. After rectification, the DC power is filtered by a 10,000uF electrolytic capacitor. The supply's maximum output current is limited to 4 Amps by the capabilities of the power transformer. Schottky diodes are not absolutely required, regular silicon diodes will drop more voltage and produce more heat, they will also require a slightly higher input voltage to produce the same output voltage.

The power transformer used in this supply was a surplus part. Two additional taps were added to the transformer's output winding by winding two turns each of 14 gauge insulated copper wire around the existing transformer windings. The extra windings boost the transformer voltage up by around 1/2 Volt per tap. This increases the maximum regulated voltage that the supply can produce. The 15.5V tap allows the maximum output voltage to go up to 17V, the lower voltage taps can be used if the maximum regulated output voltage is below 17V.

The LM317T variable voltage regulator provides the main voltage regulation for the supply. The 2N6052 PNP darlington transistor increases the current handling capacity of the regulator above the 1.5A maximum that the LM317T can provide. The 1N4002 diode protects the LM317T from any voltage spikes that may show up on the adjust pin, it is an optional part since there is no capacitor on the adjust pin. The 6AO5 diode protects the LM317T in in case the output terminals are connected to a voltage that is higher than the LM317T input voltage. This can prevent problems if the supply is connected to a battery while it is powered off. The LED and 1K current limiting resistor acts as a primitive voltage indicator. An optional analog meter can also be added across the supply, a common 1 milliamp meter with a series current limit/calibration resistor would work.

The 47uF and 100nF capacitors on the output of the regulator provide a capacitive load that prevents spurious oscillations. The 10nF capacitor on the output pins bypasses radio frequency energy that may be picked up by the output wires and the 10nF capacitor between the negative side of the circuit and the AC ground/chassis ground bypasses RF on the DC side of the supply to the AC ground.


Details coming soon.

Previous Versions of the Supply

Version 2 notes

Note that the LM338K IC used in the version 2 circuit is now considered an obsolete part and prices have gone up while availability has gone down. The LM338K is also a very fragile part and is more suited for use in applications that have a fixed load resistance. The LM338K is not recommended for a variable voltage bench supply. Shorting the output of the supply will destroy the LM338K. If you build this version of the circuit, be sure to never short the output, this includes connecting it directly to a lead-acid battery that is discharged to zero volts.

Version 1 notes

This version of the supply was very similar to the latest version. A lower current LM317L adjustable regulator was used instead of the LM317T and the 2N6052 base to emitter resistor was 150 ohms, this has been reduced to 5.1 ohms in the new version. The transformer in this version had a center-tapped output and used a dual Schottky full wave rectifier.

The version 1 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 power supply chassis was mounted in a metal box. The AC input connector, AC fuse, power switch, DC output terminals and chassis ground connection were monted on the outside of the metal box.

Photo of the Version 1 13.5V 4A DC Power Supply

The first version of the Fixed/Adjustable DC power supply

Back to FC's Ham Radio Circuits page.