Aviation Electronics Technician 1 - Historic Naval Ships Association

Figure 10-9.-Methods for using RC filters in relay circuits.Figure 10-9 shows two RC filters used to absorbthe transient interference resulting from the openingof a relay field. In circuit A, the value of Ra should below enough to provide a resistance path to ground lessthan the line impedance and high enough to lower theQ sufficiently. The capacitor should be at least 0.25µF with a voltage rating several times the lonevoltage. Circuit B has the advantages of reducing thecapacitor and coil leads to absolute minimum andreducing the relay field current. It has thedisadvantage of carrying the dc coil current. Normalvalues of each resistance (Rb) in circuit B is 5 percentof the dc resistance of the coil. The capacitor isnormally 0.25 µF. Circuit B serves as both a dampingload and a high-loss transmission line.INDUCTIVE-CAPACITIVE FILTERSFiltering of radio interference is done by means ofan inductor inserted in series with the ac powersource. The inductor offers negligible impedance tothe ac or power-line frequency and an increasinglyhigh impedance to transient interference as frequencyis increased. Combinations of inductance andcapacitance are widely used to reduce both broadbandand narrow-band interference.filter prevents the transmission of interferencevoltages into the wiring harness, and blockstransmission or reception of radio-frequency energyabove a specified frequency.The ideal low-pass filter has no insertion loss atfrequencies below its cutoff frequency, but has aninfinite insertion loss at all higher frequencies.Practical filters fall short of the ideal in three ways.First, a filter of acceptable physical size and weighthas some insertion loss, even under dc conditions.Second, because of the lack of a pure inductor, thetransition from low to high impedance is gradualinstead of abrupt. Third, the impedance is held to afinite value for the same reason. Figure 10-10compares the insertion loss of a typical low-pass filterwith that of the hypothetical ideal filter.Figure 10-11 shows the arrangement and typicalparameters of a low-pass filter that has a design cutofffrequency of 100 kHz. Inductor L must carry loadcurrent. It must be wound of wire large enough thatIts dc insertion loss is negligible. Therefore, filtersare rated as to maximum current. The capacitors C1Filters used to reduce radio interference transmissionsare available in the Navy supply system.The filters come in a large variety of types and sizes.Filters are classified as to their frequency characteristics-namely,low-pass, high-pass, bandpass, andband-reject filters.Filters are also classified as to their applications-namely,power-line, antenna, and audiofilters. The type most often used in aircraft is thelow-pass, power-line filter.Low-Pass FiltersA low-pass filter is used in an aircraft to filterpower leads coming from interference sources. TheFigure 10-10.-Insertion loss curve of a low-pass power-linefilter.10-12