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, Michigan)
 

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.

Birth of a Notion

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.

What Have We Here?

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.
Of course, the remodeled battery booster will still provide its original function—trickle charging 6- or 12-volt batteries. This little goodie definitely has possibilities!

Remodeling the Old Booster

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.

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