A increasing number of radios, and power amplifiers, are requiring the use of 28 Volts DC for operation.  This little project will make a portable 28 Volt DC power supply using two 12 Volt, 4.5 Ah rechargeable lead-acid batteries from Radio Shack (part #23-289), a Maxim IC MAX471 current-sense amplifier and a surplus TDK RMD-A-24 DC-DC converter from Mendelson Electronics (part #500-0122).  A simple 1 mA current meter is used to display both the power supply's output voltage and current.

The entire assembly is housed in a surplus military ammo box for protection and easy transportation.

Schematic

• GBPPR Portable 28 Volt DC Power Supply  (44k PNG)

Pictures

• GBPPR Portable 28 Volt DC Power Supply - Picture 1  Outside case view, it's an old, waterproof ammo box with a short quick disconnect DC output jack on the side.
• GBPPR Portable 28 Volt DC Power Supply - Picture 2  Inside case view, the two 12 volt batteries are supported to absorb any shocks.  The DC-DC converter is mounted under the control panel.  The meter is reading 28 Volts.
• GBPPR Portable 28 Volt DC Power Supply - Picture 3  Closeup of the DC-DC converter and the control panel.

Datasheets

• Maxim MAX471 Current-Sense Amplifier  (124k PDF)