18WuD2x

capacitive diaphragm • capacitive window 97
capacitive diaphragm A metal plate deliberately
placed in a waveguide to introduce capacitive reactance
and, thereby, cancel an inductive reactance.
capacitive-discharge ignition An electronic ignition
system for automotive engines. Provides
nearly constant high voltage, regardless of engine
speed. A dc-to-dc step-up converter charges a
large capacitor (typically to 300 volts) when the
distributor breaker points are closed; when they
are open, the capacitor discharges through the
ignition coil, thereby generating an ignition pulse
of several thousand volts.
capacitive division Reduction of an ac voltage by
a capacitive voltage divider.
capacitive feedback Feeding energy back from the
output to the input of an amplifier or oscillator
through a capacitor.
capacitive-input filter A smoothing filter for ac
power supplies, in which the element closest to
the rectifier is a capacitor, regardless of the components
or circuits placed subsequently.
capacitive load A load consisting of a capacitor or
a predominantly capacitive circuit.
capacitive loading In an antenna, the addition of
capacitance in series with the element(s). This
raises the resonant frequency for a radiator having
a given physical length. It can also serve to increase
the physical length required for a radiator
having a specified resonant frequency. Compare
INDUCTIVE LOADING.
capacitive post A protrusion inside a waveguide
for the purpose of introducing capacitive reactance
to cancel an inductive reactance.
capacitive potentiometer See CAPACITIVE VOL-
TAGE DIVIDER.
capacitive pressure sensor A pressure sensor
that uses a radio-frequency oscillator and a pair
of metal plates separated by dielectric foam. The
circuit is designed so a change in the capacitance
between the plates causes the oscillator frequency
to change. This change is sensed. A signal
is sent to an analog-to-digital converter (ADC)
and then to a microcomputer that calculates the
extent of the pressure.
capacitive proximity sensor A transducer used in
mobile robots that detects the presence of certain
kinds of objects. It consists of an oscillator whose
frequency is determined by an inductancecapacitance
(LC) circuit to which a metal plate
is connected. When a conducting or partially conducting
object comes near the plate, the mutual
capacitance changes the oscillator frequency.
This change is detected and sent to the robot
controller.
capacitive reactance Symbol, X C . Unit, ohm. The
reactance exhibited by an ideal capacitor, considered
as a negative pure-imaginary quantity;
X C = –j/(6.28fC), where f is the frequency in
hertz, C is the capacitance in farads, and j is the
unit imaginary number (the square root of –1).
Alternatively, f can be specified in megahertz
and C in microfarads. In a pure capacitive reactance,
current leads voltage by 90 degrees. Also
see CAPACITANCE, CAPACITOR, and REAC-
TANCE.
capacitive speaker See ELECTROSTATIC SPEAKER.
capacitive transducer A transducer consisting essentially
of a refined variable capacitor whose
value is varied by a quantity under test, such as
pressure, temperature, liquid level, etc.
capacitive tuning Variable-capacitor tuning of a
circuit.
capacitive voltage divider A capacitive attenuator
usually consisting of two series-connected capacitors
whose values are such that an applied ac
voltage is divided across them in the desired ratio.
capacitive welding An electronic welding system
in which energy stored in a capacitor is discharged
through the joint to be welded. This develops
the heat necessary for the operation.
capacitive window A pair of capacitive diaphragms
used in a waveguide to introduce capacitive
reactance.