Battery Substituter Has Muscle

IT WOULD SEEM THAT BUILDING A LITTLE power supply to substitute for four flash light (D cell) batteries would be a simple “handbook” job. Not so! The application in question is powering a widely-sold toy pinball machine.

The problem is that the bright bulb and electromagnetic counter draw an initial surge of about 4 amperes when the steel ball activates a scoring bumper. There are, no doubt, other toys that suffer from the same problem. The pinball machine used up alkaline batteries at a rate that began to cost a significant part of the entire house electrical bill.

The problem has been solved, and 1 set up the following criteria for the design:

1.    The supply should use a safe (U-L recognized) line-plug mod ule as calculators or small tape recorders using rechargable batteries do. The idea was not to shock too many of our children, ages 4 through 14.

2.    The supply should use an inexpensive and easily available regulator like the National LM34OT5, or the 7805-series voltage regulator available from several sources.

3.    Despite using the commonly available 5-volt logic regulators, the supply should provide 6.3 to 6.5 volts.

To make the little, 100 mA-rated, calculator/charger supply provide 4-ampere surges required a minor reversal of design philosophy. Not only is the little charger supply limited in power, but the plastic packaged regulators that are easily avail able, are only rated for 1 to 1.5 amperes. Three design tricks provide the solution.

1.    Only moderate filtering on the regulator input (see the schematic) is needed, and fast recovery results.

2.    Massive filtering on the output of the regulator is used (not the usual technique) to supply the surges. The three-terminal IC regulators are stable under these conditions.

3.    A LED is inserted in the ground lead of the 5-volt IC regulator. This boosts its output voltage by about 1.5 volts, and provides a nice pilot-light as well.

Parts are not critical. The line-plug supply (transformer is part of plug) that I used had an open-circuit voltage of about 10 volts and could supply a little over 100 mA at 8 volts. [A power converter designed to supply 9 volts to small radios and calculators will do nicely. Editor

The capacitors are also uncritical since none of the ripple currents are very high. I happened to use two 100,000-microfarad computer-type capacitors on the output because I could get them inexpensively. A number of smaller paralleled capacitors rated at 8 volts or more would certainly work well also. Surplus or junk-box devices would be quite suitable. The IC regulators must have current-limiting to avoid damaging the line-plug type supply with surges.

The battery-eliminator components can be placed wherever you can find the space. The location for the charger is obvious—in the wall receptacle. I distributed the other parts around the pinball machine. The small capacitor and regulator are on a bracket on the back of the machine. The LED is glued into the front panel and the two 100,000uF capacitors are carefully insulated and then taped to the machine’s back legs. — Peter Lefferts