EM5 Practice Exams: Soldering

EM5 Practice Exams: Chapter 1 - Soldering

• Traditional solder is made of tin and lead.
• Eutectic solder is a tin-lead alloy that melts at the lowest temperature.
• The composition of eutectic solder is 63% tin and 37% lead.
• Traditional solder in electronics consists of 60% tin and 40% lead.
• When cooling a liquid solder joint, it must be held still.
• Lead-free solder is mostly tin.
• Before using a soldering tip for the first time, it must be tinned.
• During soldering, rosin (or equivalent) flux removes oxide.
• In a proper solder joint, the solder wets and alloys with the metal parts of the joint.
• A solder joint where the solder does not wet all surfaces is a cold solder joint.
• A soldering iron holder should enclose the iron and be tip-resistant.
• With time and use, a copper tip will pit.
• An iron-coated tip doesn't pit and lasts longer.
• Most soldering guns use high current at the tip to produce heat.
• The most important soldering gun maintenance task is tightening the tip connections.
• When soldering, solder should first be applied to the work.
• Solder is not needed during soldering if all parts are heavily pre-tinned.
• A clip-on heat sink protects parts from excessive heat.
• Aluminum cannot be soldered using the usual techniques.
• A precaution when soldering CMOS components is grounding the iron.
• SMT components are typically soldered using solder paste.
• Bumping a PC board on the bench top should never be used as a desoldering method.
• If a flammable solvent must be near a soldering iron, it should be in a metal container.
• A soldering tip is cleaned by wiping it on a damp sponge.

EM5 Practice Exams: Chapter 2 - Electrical Conductors

• The reason there's a limit to maximum current through a given wire is that the wire has resistance.
• Resistivity is the per-cubic-meter resistance of a material.
• Voltage drop is the voltage difference between the ends of a wire.
• Voltage drop is determined by the wire's resistance and the current it's carrying.
• Power loss in a wire is voltage drop multiplied by current.
• The most common wire materials are copper and aluminum.
• Aluminum corrodes easily, limiting its use in wiring.
• For long-distance, high-voltage power transmission, aluminum conductor with steel reinforcement is used.
• A wire's resistance and length are directly proportional.
• The relationship between a wire's resistance and its cross-sectional area is inversely proportional.
• In American Wire Gauge (AWG), a larger gauge number means a smaller diameter wire.
• The fire code requires No. 12 wire for a 20-amp AC branch circuit.
• A 250kcmil wire is equivalent to 250,000 1-mil wires in parallel.
• Aluminum has about 1.6 times the resistivity of copper.
• The resistivity of copper wire rises by about 0.22% per degree F when temperature rises.
• Stranded wire should be used where the wire will be flexed a lot.
• Rubber insulation is used where the wire will be flexed a lot.
• PTFE (Teflon) insulation is used where the insulation must be slippery for pulling thru conduit.
• The main limitation on the use of PVC insulation is that it burns and emits noxious fumes in a fire.
• In a commercial building, where a wire runs vertically between floors, R-rated wire should be used.
• A plenum is the space above a ceiling, when used as an air conduit.
• Skin effect causes current in a wire to be pushed toward the surface.
• The purpose of Litz wire is to prevent skin effect.
• A cable is two or more wires bundled together.
• In coaxial cable, the signal wire is surrounded by a shield.
• Crosstalk is unwanted signal coupling from one wire or cable to another.
• When UTP cable is used, crosstalk is prevented by using balanced signals.
• Twisting a pair of wires improves interference and crosstalk rejection.

EM5 Practice Exams: Chapter 3 - Generators

• "Electromagnetic induction" means voltage induced in a wire by a changing magnetic field.
• Fleming's rule states that the thumb represents thrust, the first finger represents force/field, and the center finger represents current.
• For generators, the right-hand rule is used with either conventional or electron flow.
• A prime mover is the original source of energy in a generating system.
• Voltage created in a generator always starts out as AC.
• In a generator, the part that moves is called the rotor.
• In a generator, the part that does not move is called the stator.
• The mechanism that makes electrical contact with the rotor in an AC generator is the slip ring assembly.
• In a traditional DC generator, AC is turned into DC by the commutator.
• In a generator, the part where voltage is induced is the armature.
• The sliding pieces that make electrical contact with the slip rings are called brushes.
• A magneto is a generator where the field is supplied by permanent magnets.
• Excitation current is current used to magnetize the field.
• Self-excitation involves using part of the generator's own output to create the field.
• A generator where the armature and field coils are connected in series is a series-wound generator.
• When the electrical load on a series generator is increased, output voltage increases a lot.
• A generator where the armature and field coils are connected in parallel is a shunt-wound generator or a parallel-wound generator.
• When the electrical load on a parallel generator is increased, output voltage decreases a little.
• A generator using both series and parallel wiring is a compound generator.
• When the electrical load on a series/parallel generator increases, the output voltage stays fairly stable.
• An AC generator's output frequency depends on the speed and number of poles.
• A three-phase generator puts out 3 sine waves, 120° apart.
• In a delta system, each winding is connected across two phases.
• In a wye system, each winding is connected between a phase and ground.
• In a wye system, all 3 are wired in parallel
• Faraday's law states that the output voltage varies with number of windings, number of poles, speed of rotation, and strength of field.

EM5 Practice Exams: Chapter 4 - Motors

• Fleming's left hand rule for motors is used with DC motors only.
• A series-wound motor does not have a stable speed.
• In a shunt-wound motor, back voltage can limit maximum speed.
• The usual way of changing the speed of a DC motor is to change the supply voltage.
• Most DC motors can be braked by turning off the power and shorting the wires.
• A universal motor works on AC or DC.
• A brushless AC motor uses a rotating field to start the rotor turning.
• With a 60Hz AC source, a synchronous 2-pole motor will turn at 3,600 RPM.
• A typical induction (non-synchronous) 2-pole motor running on 60Hz current and fully loaded typically turns at 3,500 RPM.
• In an induction motor, the rotor is magnetized by being inside the field.
• The difference between synchronous speed and actual speed in an induction motor is called slip.
• In a single-phase motor, the rotor is made to turn by creating a second phase inside the motor.
• In a capacitor-run motor, the capacitor is there to create a continuous 90° phase shift.
• In a shaded pole motor, phase shift is created by placing a shorting ring around part of each pole.
• A split-phase motor creates a rotating field using two different kinds of coils positioned at right angles.
• A 4-pole motor will run at half the speed of a 2-pole motor.
• One electrical horsepower is 746 watts.
• When an overloaded motor slows down dramatically, that's called pullout.
• When a motor is stalled or just starting, the current draw is typically 6 to 10 times its running current.
• For most AC motors, the only practical way to vary the speed is to vary the AC power frequency.
• "Dynamic" braking of an AC motor is done by putting DC across the power wires.
• A stepper motor can stop and freeze in a particular position.
• A permanent-magnet DC motor rotation can be reversed by reversing the DC polarity.
• A series, shunt, compound or universal motor can be reversed by swapping armature and stator wires(not both).
• A 3-phase motor can be reversed by swapping any two power leads.

EM5 Practice Exams: Chapter 5 - AC Power Grid

• Long distance power transmission is usually done at several hundred kV.
• A motor-generator set is usually used to provide isolation from lightning that may hit the high-voltage transmission lines.
• Delta voltages (across phases) are about 1.7 times wye voltages (between each phase and common).
• A system with 480V delta is about 277V wye.
• In the delta design, the phases do have a common point that may be grounded.
• The "secondary transmission" means the voltage that comes out of the transformer secondary at the power plant.
• Power drops are wires feeding individual homes and businesses.
• 120/240V residential power is commonly called secondary transmission.
• An advantage of DC ultra-high voltage transmission is easy conversion to 120VAC.
• When the 3 phases are not exactly 120° apart or exactly the same voltages, motors run hot and produce less torque.
• "Single phasing" occurs when one phase drops out.
• A transient is a spike of much higher voltage than the normal AC voltage on a wire.
• "Base load" is the normal load on a power plant during a given period.
• "Peak load" is the short-term heaviest load on a power plant during a given period.
• A useful method of load shaving is shifting some use to normally low-load times.
• When a load has a poor power factor, voltage and current are out of step.
• The usual method of improving power factor is connecting capacitors or inductors depending on the situation across the power line.
• The power density of sunlight at noon on a clear day is around 1kW per square meter.
• A good solar panel turns about 25% of sunlight into electricity.
• Microgeneration means generating in very high number of locations.

EM5 Practice Exams: Chapter 6 - Circuit Controls and Protection

• The switch illustrated is SPDT.
• Switches in high-current circuits should be physically large.
• If a high-current switch is used in a low-current circuit, contacts may oxidize and become unreliable.
• A pushbutton labeled "NC" is a closed circuit when you're not pushing it.
• Another name for alternating pushbutton action is push-on, push-off.
• A momentary NO 2-position SPST switch would be labeled (OFF)-ON.
• Using a large, stiff switch is not an effective way to prevent accidental operation.
• When installing crimp connections, the most important consideration is to crimp hard enough to create a cold weld.
• The part of a relay that moves is called the armature.
• A contactor is a multipole relay.
• A limitation of solid-state relays is the "contacts" may be AC or DC only.
• A fuse blows due to excessive current.
• When a unit is working normally, the voltage drop across its fuse is almost zero.
• A fuse's voltage rating identifies the highest-voltage circuit the fuse can safely be used in.
• When a fuse is enclosed in a cartridge holder, its rated current is reduced to about 75% of the current marked on the fuse.
• After a fuse blows, zero volts appears across it.
• A fuse labeled F6.25AL250V would be 6.25A, low breaking capacity, 250VAC maximum, fast blow.
• The main difference between a fuse and a breaker is a breaker can be reset and used again.
• Breakers in AC breaker boxes are usually thermal.
• A breaker that uses a bimetallic strip or disc is a thermal breaker.
• Breakers are sometimes ganged together to break 2 or 3 phases at the same time.
• The symbol identifies an arc-detecting breaker.
• A ground-fault-interrupt trips when outgoing current is different from the incoming current.
• An arc-sensing breaker breaks when it detects high frequently energy on the wires.
• A current-limited source cannot put out enough current to start a fire.

EM5 Practice Exams: Chapter 7 - Interfacing Electronic Products - Levels and Impedances

• A signal getting weaker as it moves through a cable is called attenuation.
• High frequencies suffer the greatest loss in a cable.
• Wavelength = speed / frequency.
• In air and space, electrical signals travel at about 3 X 108 meters per second.
• "Impedance matching" means the impedances of sending unit, cable and receiving unit are all the same.
• Impedances are usually not matched in master antenna systems.
• Impedance conversion is usually done using a transformer.
• A transformer that converts between a balanced signal and an unbalanced one is usually called a balun.
• A cable shield should be connected to equipment chassis.
• If a signal's strength normally varies, as in audio, the range of variation is called the dynamic range.
• A reference level is a specific level that other levels are compared against.
• The professional reference line level in audio is 0dBm, one milliwatt.
• In audio, "line level" is generally in the range of one volt AC.
• Microphone level is usually much lower than line level.
• "Pad" is another name for attenuator.
• A condenser microphone has a built-in preamplifier and requires DC power.
• Headroom is the difference between the normal/maximum signal level and the clipping or overload level.
• AGC means automatic gain control.
• In analog, "compression" means reducing a signal's dynamic range.
• The standard baseband video level is approximately 1V P-P.
• Another name for 'balanced pair" is a differential pair.
• In RS232, voltage levels for "1" and "0" may be as high as + and -9V.
• The four wires of a USB cable are +5V, ground, differential pair for data.
• The unusual aspect of MIDI is both send and receive units are electrically isolated.
• MIDI and LVDS signals exist as current levels.
• 4- to 25-mA current loops are most often used by sensors in industrial systems.
• The level of electrical noise that's always present in every system is called the noise floor.
• White noise has equal energy in every frequency span.
• Pink noise has equal energy in every proportional (logarithmic) frequency span.
• In a 50Ω system, the noise contained in each Hz of spectrum is roughly 1nV.

EM5 Practice Exams: Chapter 8 - Interfacing Electronic Products - Cables and Connectors

• When stripping a wire end, the most important thing is not to nick the conductor.
• When stripping varnish off a wire, tilt the knife away from the wire end.
• The only acceptable kind of flux that is acceptable in electronics is rosin.
• Stranded wire ends should not be tinned (other than factory tinning) if they're going to be clamped or squeezed.
• When connecting a braided shield, there can't be any flyaway strands.
• The purpose of a drain wire is so you don't have to con