Spring 2007 Issue
Photo A. Drilling out the rivets to remove old
components from a typical “battery booster.” The new electrolytic cap with
its home-brew mounting strap, meter, rectifier, and regulator in its heat
sink are in the foreground. (Photos by Jack Chandler Studio, St. Joseph,
Remodeling an Old
Auto Battery “Booster”
into a 12-volt Power Supply
One of the best ways to rekindle your interest
and enthusiasm for amateur radio is to build a
piece of equipment for your station. Here’s a
4-amp “12-volt” power supply built from an
old automotive 6-volt battery booster that’s
an easy, inexpensive weekend project.
If you’re concerned that the project appears
“home-brewed,” it can be placed out-of-sight
behind your rig, yet provide excellent service.
By Tom Turner, K8VBL/VP2VEL
Two-meter FM transceivers of older vintage are available at hamfests for bargain prices. Used with a simple home-brew J-pole antenna, an old crystal-controlled rig is useful for monitoring the local FM repeaters. However, the “12-volt” 4-amp power supplies for these rigs usually are not bargains, because they can be used with the newer rigs as well.
A handy and inexpensive 12-volt power supply
can be built from an old 6-volt automotive battery booster. Available at
garage sales and farm sales, an otherwise worthless, old battery booster
provides a transformer, on/off switch, and cabinet. Additional major parts
required are a high-capacitance ”computer” electrolytic capacitor,
obtainable at most ham/computer fests, and a bridge rectifier and
three-terminal regulator that are inexpensive over-the-counter items at
electronics stores. If you are unable to scrounge a suitable battery
booster to remodel, a transformer may be purchased along with the other
parts, and the power supply may be built up in a suitable enclosure, such
as a school lunch box.
While cleaning out the tractor shed, I came
across two old 6-volt battery boosters that had been used in the 1950s and
’60s to trickle-charge auto and tractor batteries. Their name-plate
ratings are 7 volts, 6 amps DC output at 110 volts 60 Hz AC input. It
occurred to me that they could be remodeled into regulated “12-volt”
supplies that could power 2-meter, 20-watt FM rigs, which require about 4
amps in the transmit mode.
A typical battery booster consists of a 100-VA continuous-duty step-down transformer and a pair of old-time selenium rectifier diodes in a full-wave center-tapped circuit. An 8-amp circuit breaker in one DC lead offers overload protection, and a current indicator shows charging amps. Would the transformer secondary voltage be high enough to provide at least 14 volts regulated DC output?
To investigate, I blew most of the dust and
chaff out of the booster’s enclosure, disconnected the long-defunct
selenium rectifiers, and connected an AC voltmeter across the transformer
secondary. Cautiously I plugged the unit into 120 VAC. The indicated
voltage was 18 VRMS. Eighteen volts RMS times 1.41 equals 25.5 volts peak.
Subtracting 1.2 volts forward voltage drop through a pair of silicon
rectifiers in a bridge circuit leaves 24.3 volts peak. This voltage could
be filtered by a surplus high-capacitance 30/40-volt computer electrolytic
capacitor with adequate safety margin. A 1.2– 32-volt, 5-ampere regulator
then could be set to provide regulated DC output voltage of 13.8 volts as
required by most 20-watt 2-meter FM rigs. A rheostat in the regulator’s
voltage divider would provide any DC output from 1.2 to about 20 volts, so
the power supply could also be used for experiments with 5- or 12-volt
logic, or perhaps power a vintage 6-volt radio.
To remodel an old 6-volt battery booster into a modern, adjustable “12-volt” power supply, drill out the mounting rivets and remove all components from the chassis (photo A). Retain only the transformer, on/off switch, chassis, and enclosure. The component locations will probably have to be changed to accommodate the new filter cap and solid-state DC modules. Sand any rust spots off the sheet-metal parts and spray paint them to match the rig with which the power supply will be used—generally gray or flat black.
© Copyright 2007, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.