Aviation Electronics Technician 1 - Historic Naval Ships Association

Figure 10-4.-Internal construction of feedthrough and conventional capacitor.constructions of feedthroughcapacitors are shown in figuredifferences in the two types.Selection of Capacitorsand conventional10-4. Notice theCapacitors used for filtering circuits in aircraftshould be selected for characteristics such as physicalsize, high temperature and humidity tolerances, andphysical ruggedness. The capacitors should have anadequate voltage rating (at least twice that of thecircuit to be filtered), and should be installed withminimum lead length.Application of Capacitive FiltersEvery circuit carrying an unintentionally varyingvoltage or current capable of causing radiointerference should be bypassed to ground by suitablecapacitors. When the nature of the variations are suchthat interference is caused at both high and lowfrequencies, a capacitor should be chosen andinstalled to provide an adequate insertion loss at thelowest frequency where interference exists. Whenthe overall capacitance required at low frequencyprovides inadequate insertion loss at high frequencies,it should be bridged in the shortest and most directmanner possible by a second capacitor.A capacitive filter should be installed as near aspossible to the actual source of interference. Leadlength should be held to an absolute minimum for tworeasons. First, the lead to the capacitor carriesinterference that must not be allowed to radiate.Second, the lead has inductance that tends to lowerthe maximum frequency for which the capacitor is aneffective bypass.To the extent possible, a filter capacitor should beinstalled to make use of any element of the filteredcircuit that provides a better filtering action. Figures10-5, 10-6, and 10-7 illustrate proper use of filtercapacitors.Capacitive Filtering in an AC CircuitThe radio interference generated in slip ring acmotors and generators is a transient caused bysliding contacts plus high-frequency energy fromother internal sources. For this reason, filteringshould be aimed at reducing high-frequency andvery-high-frequency noise components with the useof low-capacitance, high-grade capacitors. Whereverpossible, feedthrough capacitors should be used.Capacitances should be chosen low enough in valueto represent a high impedance at the power frequencyand to avoid resonance with the internal inductancesof the filtered unit. Voltage ratings should be at leasttwice the peak voltage across the capacitors.In a four-wire electrical system, the neutral leadcarries all three phases; a large quantity of the thirdharmonic of the power frequency is present. ThisFigure 10-5.-Capacitive filtering of a reversible dc seriesmotor.10-10