Basic Electronics and Control
Basic Electronics and Control tests semiconductor devices, rectifiers, relay drivers, sensors and simple control logic used in vocational panels.
Core rules
- A diode conducts in forward bias with about 0.7 V drop for silicon; it blocks reverse voltage up to its rating.
- Half-wave rectifier: V_dc ≈ V_peak/π; full-wave: V_dc ≈ 2V_peak/π. V_peak = √2 × V_rms.
- A transistor can switch a relay or contactor; the base resistor limits base current, and a freewheeling diode protects against the coil's inductive kick.
- Sensors may give digital (on/off) or analogue (4–20 mA, 0–10 V) signals; PLC inputs are typically 24 V DC.
- Logic functions: AND, OR, NOT, NAND; a truth table is the quickest way to check a control sequence.
Worked example
A 12 V relay coil has 120 Ω resistance, so coil current = 12/120 = 0.1 A. An NPN transistor with hFE = 100 needs base current of at least 0.1/100 = 1 mA; use 2 mA for saturation. With a 5 V logic output, base resistor = (5 − 0.7)/0.002 = 2150 Ω, so a 2.2 kΩ resistor is selected. A 1N4007 diode is connected across the coil, cathode to +12 V, to clamp the turn-off spike.
Traps
Do not use V_rms where V_peak is required; do not omit the freewheeling diode; do not reverse an electrolytic capacitor; and do not connect a PNP transistor as if it were NPN.