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Reducing charging circuit interference

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If you are beset by regulator hash in your mobile station, the following method will in many instances remove most, if not all, of the offending interference. I incorporated the method in my automobile which uses a Leece-Neville alternator but it should also be equally effective in a conventional charging system.

Construct two filters by connecting a 10 ohm resistor and a 500 nF capacitor as shown in Fig. 1. The capacitor can be the type normally sold by automobile parts suppliers as a generator bypass capacitor. The resistors should be connected as close as possible to the capacitor, taped at the junction and secured to the capacitor with plastic tape. Anchor the filters to any metallic portion of the car body, preferably as close to the regulator as possible. The two filter leads are fed into the regulator box where one is connected to the uppe section of the voltage regulator bracket and thy other is connected to the upper portion of the current regulator bracket. These filters will not affect the functioning of the regulator.

Fig 1
Fig. 1. Filters on the voltage and current regulators to reduce radio interference.

In addition to the regulator filters, a tuned filter in the field lead will aid in the reduction of interference. It is constructed by winding a self-supporting coil, L1, of 8 turns of No. 10 enameled wire about 1 inch in diameter. Connect a 100 pF variable capacitor across the coil and bypass the filter with a 5 nF capacitor and a 10 ohm resistor. Insert the filter in series with the generator field lead and tune the capacitor for minimum interference.

Fig 2
Fig. 2 Tuned filter placed in the generator field lead. See text for information on L1.

Other schemes can be used to reduce any remaining residual ignition of regulator hash, such as inserting 100 nF disc ceramic capacitors from the tail lights, from the license plate lights and from the dome lights to ground. Also, grounding the muffler and tail pipes in several spots will sometimes help. If the receiver's antenna or power leads pass in close proximity to the regulator, generator or high-voltage ignition components, try moving them away or encasing them in a grounded shielded jacket.

Maurice I. Sasson, M.D., W2JAJ.