EASA Module 4 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

N-type semiconductor

Silicon doped with a pentavalent impurity such as phosphorus or arsenic. Each donor atom supplies one free electron, so electrons are the majority carriers and holes are the minority carriers.

P-type semiconductor

Silicon doped with a trivalent impurity such as boron or aluminium. Each acceptor atom creates a hole, so holes are the majority carriers and electrons are the minority carriers.

Depletion region

The narrow zone each side of a PN junction that is emptied of free charge carriers. Fixed ionised dopant atoms remain, producing a potential barrier that opposes further diffusion.

Forward bias of a junction

Connection in which the P region is made positive relative to the N region. The barrier is reduced, majority carriers cross, and current flows above about 0.7 V for silicon.

Zener diode

A diode designed to operate in reverse breakdown, where the voltage across it stays nearly constant. A series resistor is required to limit the current through it.

Light emitting diode

A forward-biased PN junction that emits light when electrons recombine with holes. Colour depends on the semiconductor band gap, and typical forward voltages are 1.8 to 3.3 V.

Photodiode

A diode operated in reverse bias whose leakage current increases with incident light. It is used in optical sensing and fibre-optic receivers, usually followed by an amplifier stage.

Transistor current gain beta

The ratio of collector current to base current in a bipolar transistor, also written hFE. It typically ranges from 50 to 500 and varies with temperature and collector current.

Thyristor (SCR)

A four-layer PNPN device that latches into conduction when a gate pulse is applied. It keeps conducting until the anode current falls below the holding current.

Ripple factor

The ratio of AC ripple voltage to DC output voltage of a rectifier supply. It is about 1.21 for a half-wave and 0.48 for a full-wave circuit.

Thermal runaway

A condition in which rising junction temperature increases current, which raises temperature further. In bipolar transistors it can destroy the device unless current is limited or heatsinking is adequate.

FR-4 laminate

A flame-retardant woven glass fabric bonded with epoxy resin, the commonest circuit board base material. Relative permittivity is about 4.2 to 4.7 and glass transition temperature 130 to 180 degrees Celsius.

One ounce copper

A copper foil weight of one ounce per square foot, corresponding to a thickness of about 35 micrometres. It is the standard cladding thickness for ordinary printed circuit boards.

Solder mask

A patterned polymer coating applied over the copper, leaving only pads and test points exposed. It prevents solder bridging between adjacent pads and protects tracks from contamination.

Fiducial mark

A small copper or gold reference dot placed on the board surface. Vision systems in pick-and-place machines use it to align component placement with the board pattern.

Annular ring

The width of copper remaining between the edge of a drilled hole and the edge of its pad. Too small a ring risks an open circuit after thermal cycling or drilling.

Blind via

A plated hole that connects an outer layer to one or more inner layers but does not pass through the whole board. It increases routing density but complicates manufacture.

Buried via

A plated hole that connects only inner layers and is invisible from either surface. It is formed during lamination of the multilayer stack-up, which raises cost considerably.

Ground plane

A continuous copper area on a board layer, usually connected to zero volts. It provides a low-impedance return path, reduces loop area and suppresses electromagnetic interference.

HASL finish

Hot air solder levelling, in which the board is dipped in molten solder and excess is blown off. It is cheap and easily soldered but leaves an uneven surface.

ENIG finish

Electroless nickel immersion gold, a flat finish with a nickel barrier under a thin gold layer. It suits fine-pitch components and long storage but costs more than HASL.

Thermal relief

A pad connected to a large copper plane by a few narrow spokes instead of solid copper. The spokes reduce heat conduction so the joint can be soldered easily.

Servomechanism

A closed-loop control system in which output position or speed is measured and compared with the demand. The resulting error drives the motor until output matches command.

Error signal

The difference between the demanded input and the measured feedback in a closed-loop system. Its magnitude and sign determine the direction and effort applied by the actuator.

Tachogenerator

A small generator coupled to a servo motor shaft that produces a voltage proportional to speed. Its signal is fed back to provide rate feedback and hence damping.

Resolver

A rotary transformer whose two stator windings produce voltages proportional to the sine and cosine of shaft angle. It gives accurate absolute position and is mechanically robust.

Synchro control transformer

A synchro used as an error detector. Its rotor voltage is proportional to the sine of the angular difference between the transmitter and receiver shaft positions.

Absolute encoder

A position sensor whose output code identifies the shaft angle uniquely at every point. Position is known immediately after power-up, without a homing or reference run.

Incremental encoder

A sensor that outputs a pulse train as the shaft rotates, giving speed and relative position. An index pulse or separate reference is needed to establish the absolute datum.

Stepper step angle

The angular movement produced by one input pulse, equal to 360 degrees divided by the number of steps per revolution. Typical values are 1.8 or 7.5 degrees.

Integral control action

Control action whose output is proportional to the accumulated error over time. It eliminates steady-state error but reduces stability and slows the response if too strong.

Backlash

Lost motion between the teeth of gears or within a linkage when the direction of drive reverses. In a servo loop it causes a dead band and can produce limit-cycle oscillation.