Electronic Components, Semiconductors and Basic Electronic Circuits
What the exam tests
Passive components, diodes, transistors, simple circuits and safe handling.
Core
- Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Fourth band is tolerance (gold ±5%, silver ±10%).
- Capacitors store charge and block DC; they filter supply rails and decouple ICs. Discharge before testing. A capacitance meter is needed for value; an ohmmeter only shows charging.
- Silicon diodes drop about 0.6–0.7 V forward and block reverse. Zener diodes regulate in reverse breakdown. LEDs need a series resistor to limit current.
- NPN transistors conduct when base-emitter is forward biased (about 0.7 V); PNP conduct with base below emitter. MOSFETs are voltage-driven and common in ECU outputs.
- A 7805-style regulator needs input and output capacitors; logic gates and op-amps process signals.
- Observe ESD precautions and heat limits when soldering.
Worked example
An LED with a 2.0 V forward drop is to run at 20 mA from 12.6 V. Resistor voltage = 12.6 − 2.0 = 10.6 V. R = V/I = 10.6/0.020 = 530 Ω, so use the nearest preferred value, 510 Ω or 560 Ω. Resistor power = I²R = 0.0004 × 530 = 0.21 W, so a 0.25 W part is marginal; choose 0.5 W. Check the current with the actual resistor value.
Traps
Reading resistor bands from the wrong end; testing a diode in-circuit with power on; assuming a capacitor is discharged; using an LED without a resistor; damaging a MOSFET with static.