Electrical and Electronic Fundamentals and Control
Electrical and Electronic Fundamentals and Control
This domain tests Ohm's law, series and parallel circuits, power, simple components and control devices such as switches, relays, sensors and logic.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- Series: current is common; resistances add; voltage divides. Parallel: voltage is common; 1/Rtotal = 1/R1 + 1/R2 + ...; current divides.
- A diode conducts one way; an LED needs a series resistor. A transistor can act as a switch; a relay uses a small coil current to switch a larger load.
- Control logic: a normally open pushbutton must be held; a latching circuit keeps a relay energised. Sensors include thermistors, LDRs and limit switches.
- Mains work requires isolation, correct fusing and enclosure; low-voltage control does not remove the need for care.
Worked example
A 100 Ω resistor and a 150 Ω resistor are connected in parallel across 12 V. 1/Rp = 1/100 + 1/150 = 3/300 + 2/300 = 5/300, so Rp = 60 Ω. Total current I = V/R = 12/60 = 0.2 A. Total power P = V × I = 12 × 0.2 = 2.4 W. Reasoning: parallel resistance is always less than the smallest branch resistance.
Traps
Adding parallel resistances directly; mixing mA and A; assuming a relay coil and contact are the same circuit; forgetting a diode's polarity; working on live mains.