Figure 10-9.-Methods for using RC filters in relay circuits.
Figure 10-9 shows two RC filters used to absorb
the transient interference resulting from the opening
of a relay field. In circuit A, the value of Ra should be
low enough to provide a resistance path to ground less
than the line impedance and high enough to lower the
Q sufficiently. The capacitor should be at least 0.25
µF with a voltage rating several times the lone
voltage. Circuit B has the advantages of reducing the
capacitor and coil leads to absolute minimum and
reducing the relay field current. It has the
disadvantage of carrying the dc coil current. Normal
values of each resistance (Rb) in circuit B is 5 percent
of the dc resistance of the coil. The capacitor is
normally 0.25 µF. Circuit B serves as both a damping
load and a high-loss transmission line.
INDUCTIVE-CAPACITIVE FILTERS
Filtering of radio interference is done by means of
an inductor inserted in series with the ac power
source. The inductor offers negligible impedance to
the ac or power-line frequency and an increasingly
high impedance to transient interference as frequency
is increased. Combinations of inductance and
capacitance are widely used to reduce both broadband
and narrow-band interference.
Filters used to reduce radio interference trans-
missions are available in the Navy supply system.
The filters come in a large variety of types and sizes.
Filters are classified as to their frequency charac-
teristics-namely, low-pass, high-pass, bandpass, and
band-reject filters.
Filters are also classified as to their appli-
cations-namely, power-line, antenna, and audio
filters. The type most often used in aircraft is the
low-pass, power-line filter.
Low-Pass Filters
A low-pass filter is used in an aircraft to
power leads coming from interference sources.
filter
The
10-12
filter prevents the transmission of interference
voltages into the wiring harness, and blocks
transmission or reception of radio-frequency energy
above a specified frequency.
The ideal low-pass filter has no insertion loss at
frequencies below its cutoff frequency, but has an
infinite insertion loss at all higher frequencies.
Practical filters fall short of the ideal in three ways.
First, a filter of acceptable physical size and weight
has some insertion loss, even under dc conditions.
Second, because of the lack of a pure inductor, the
transition from low to high impedance is gradual
instead of abrupt. Third, the impedance is held to a
finite value for the same reason.
Figure 10-10
compares the insertion loss of a typical low-pass filter
with that of the hypothetical ideal filter.
Figure 10-11 shows the arrangement and typical
parameters of a low-pass filter that has a design cutoff
frequency of 100 kHz. Inductor L must carry load
current. It must be wound of wire large enough that
Its dc insertion loss is negligible. Therefore, filters
are rated as to maximum current. The capacitors C1
Figure 10-10.-Insertion loss curve of a low-pass power-line
filter.
