Page 192 - Electrical Engineering Dictionary

in the machine by developing an induction- leader typically propagates down the residual
type torque that opposes any change in speed. channel of the previous stroke.

damping a characteristic built into electri-
dashpot timer a fluid time-on timer that
cal circuits and mechanical systems that pre-
can be used in definite time DC motor accel-
vents rapid or excessive corrections that may
eration starters and controllers. The dashpot
lead to instability or oscillatory conditions.
timer functions where a magnetic field forces
a piston to move within a cylinder when the
damping coefficient electrical torque
coilisenergized. Themovementofthepiston
component in phase with the rotor speed.
is limited by fluid passing through an orifice
on the piston. The amount of fluid passing
damping factor a measure of the ability
through the orifice is controlled by a throttle
of the PLL to track an input signal step. Usu-
value, which in turn determines the amount
ally used to indicate the amount of overshoot
of time delay. After the fluid pressure equal-
present in the output to a step perturbation in
izes across the piston, movement stops and
the input.
contacts change state. The fluid can either
be air (pneumatic dashpot) or oil (hydraulic
damping ratio the ratio of the real part of
dashpot). When the timer is deenergized, a
the resonant frequency to the undamped res-
check valve allows the pressure to equalize
onant frequency, in a second-order system.
across the piston rapidly, thereby causing the
contacts to change state “immediately.”
dark current a noise source in photode-
tectors, corresponding to undesired output
signals in the absence of light. The main data any information, represented in bi-
source of dark currents are ohmic leakage nary, that a computer receives, processes, or
due to imperfect insulation and thermionic outputs.
emission.
data access fault a fault, signaled in the
darkfield performance the amount of processor, related to an abnormal condition
light that is blocked in a light valve projector detected during data operand fetch or store.
when a totally dark scene is being displayed.
This parameter is critical to good contrast ra-
data acquisition (1) method used for cap-
tio.
turing information and converting it to a form
suitable for computer use.
Darlington bipolar junction transistor
a combination of two bipolar junction tran- (2) process of measuring real-world quan-
sistors (BJT) where the emitter current of tities and bringing the information into a
one transistor drives the base of the second computer for storage or processing.
transistor. The arrangement reduces the cur-
rent required from the base driver circuit, and
data bottleneck a computer calculation
the effective current gain of the combination
in which the speed of calculation is limited
is approximately the product of individual
by the rate at which data is presented to the
gains. The configuration can be made from
processor rather than by the intrinsic speed of
two discrete transistors or can be obtained as
the processor itself. Ultra high speed parallel
a single integrated device.
processors are very frequently limited in this
way.
dart leader in lightning, a continuously
moving leader lowering charge preceding a
return stroke subsequent to the first. A dart data buffer See buffer.

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2000 by CRC Press LLC