EASA Module 4 common mistakes
EASA Module 4 · Understand the traps and how to avoid them.
States that a zener diode regulates by being forward biased, or treats its reverse breakdown as destructive in the same way as an ordinary rectifier diode's breakdown.
Zeners are taught alongside rectifiers, so the rule 'reverse bias beyond the rating destroys the diode' is carried over unchanged and the distinction between controlled zener/avalanche breakdown and irreversible breakdown is never made.
What to do instead
Learn it as a paired rule: forward biased it is just a 0.7 V diode and regulates nothing; reverse biased beyond Vz it enters a non-destructive breakdown and holds the voltage near constant. Vz is fixed at manufacture by doping level (low Vz = heavy doping, zener effect; higher Vz = avalanche effect), and the current must be limited by a series resistor.
On an NPN transistor in the active region, writes that the base-emitter junction is reverse biased, or draws the emitter arrow pointing inwards.
The transistor is memorised as a symbol rather than as two junctions of opposite doping, so under exam pressure the polarity and current directions of NPN and PNP are swapped.
What to do instead
Anchor on 'NPN = Not Pointing iN': the emitter arrow points outwards, conventional current enters at the base and the collector and leaves at the emitter. Active region: base-emitter forward biased (about 0.7 V silicon), base-collector reverse biased. Reverse both statements for a PNP, where current enters at the emitter.
Claims that after a thyristor has fired, the gate current can be increased or reduced to modulate the anode current.
Candidates generalise from the bipolar transistor, where base current controls collector current continuously, and do not recognise the SCR's regenerative latching as a different mechanism.
What to do instead
Treat the gate as a trigger, not a tap. Once anode current exceeds the latching current the gate loses control and conduction continues until anode current falls below the holding current (natural or forced commutation). The gate can only control the firing angle, i.e. delay switch-on in a phase-controlled rectifier, never vary the current once conducting.
Uses 0.7 V as the forward drop for every diode, or states that the forward voltage drop increases as junction temperature rises.
A single '0.7 V' figure is over-learned early and never revised, and the temperature coefficient is a small number that is easily remembered with the wrong sign.
What to do instead
Silicon diode forward drop is about 0.6–0.7 V; germanium about 0.2–0.3 V. The forward drop falls by roughly 2 mV per degree Celsius rise (about −2.5 mV/°C for a silicon junction), which is why a diode can be used as a temperature sensor. Reject any option presenting the drop as fixed regardless of current and temperature.
Describes a plated-through-hole board as being drilled after the copper has been etched, or says the hole barrel is only mechanical and provides no electrical connection between layers.
The fabrication sequence is abstract and learned from flow diagrams, so the order of the steps collapses on recall, and the word 'plated' suggests surface finish rather than interconnect.
What to do instead
Chain the process in order: laminate the copper-clad board, drill, activate and electroless-copper the barrel, pattern plate and etch, apply solder mask and surface finish. State explicitly that the plated barrel is the electrical interconnect between top, bottom and inner-layer pads and that it carries the component current, which is why barrel cracking is a failure mode.
Accepts a domed or ball-shaped, dull solder joint as satisfactory, or blames visible flux residue for a joint that is actually a cold joint.
Joints are judged by overall appearance without linking the shape to the wetting mechanism, and flux residue is visible while insufficient heat is not.
What to do instead
Apply the acceptance criterion: a good joint is a smooth concave fillet wetted to both pad and lead, shiny for tin-lead solder (slightly dull is normal for lead-free). A cold or dry joint is rounded, grainy and poorly wetted, caused by insufficient heat, contaminated or oxidised surfaces, or movement during cooling; it is remade with flux and correct heat. Flux residue is cosmetic and cleanable and does not cause the joint to be weak.
Confuses solder mask with conformal coating, for example by saying conformal coating is applied before soldering in order to define the pads.
Both are described as protective coatings over the board, so they are merged into a single idea and their different stages in the build sequence are lost.
What to do instead
Separate them by purpose and timing: solder mask is permanent, applied during board fabrication, and defines the pads and prevents solder bridging between tracks. Conformal coating is applied after assembly and test, protects against moisture, fungus, corrosion and contamination; connectors, test points and heatsink contact areas must be masked before application, and the coating must remain repairable.
Writes the error signal as input plus feedback, or claims that increasing negative feedback increases both the overall gain and the steady-state error of a servo.
The block diagram is memorised as 'the output feeds back' without the comparator sign, and the words 'feedback' and 'gain' are assumed to move together rather than in opposite directions.
What to do instead
Fix the comparator first: error = demand (input) minus feedback. Negative feedback reduces overall gain by the factor 1/(1 + GH) but reduces steady-state error, distortion and sensitivity to component drift, widens bandwidth and improves stability; the cost is the reduced forward gain. Positive feedback does the opposite and drives the system towards runaway.
Describes a resolver as having three stator windings at 120 degrees giving one output, or states that a synchro control transformer output has constant magnitude with the phase alone carrying the angle.
Both devices are rotary transformers built from a rotor and stator windings, so their winding geometry and output formats blur together in recall.
What to do instead
Key them by geometry and output: synchro has three stator windings at 120 degrees and a single-phase rotor, used in torque transmitter/receiver chains; a control transformer produces an error signal that is null when the rotor is at 90 degrees to the electrical zero. A resolver has two stator windings at 90 degrees, producing two outputs whose amplitudes are proportional to the sine and cosine of rotor angle, used for angle-to-digital conversion.
Says servo hunting or oscillation is cured by increasing amplifier gain, or claims that tachogenerator feedback raises the effective loop gain.
Gain is assumed to make the response 'sharper', and velocity feedback is confused with position feedback because both are negative feedback loops around the same amplifier.
What to do instead
Hunting is over-oscillation, so treat its causes: excessive loop gain, excessive lag in the amplifier or motor, or mechanical backlash and dead band. Remedies are to reduce gain and add velocity (tachogenerator) feedback, which is derivative feedback; it damps overshoot and shortens settling time but lowers the effective loop gain, so it must be balanced against the resulting steady-state accuracy.