Electrical Principles and Circuit Fundamentals
Electrical principles underpin every test on this exam. Expect questions on Ohm's law, series and parallel circuits, voltage drop, power, magnetism, and how capacitance and inductance affect DC circuits.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- In a series circuit current is common, voltage divides in proportion to resistance, and total resistance sums.
- In a parallel circuit voltage is common, current divides inversely with resistance, and total resistance is less than the smallest branch.
- Kirchhoff's laws: current into a node equals current out; voltage around a closed loop sums to zero.
- Conductor resistance rises with length and temperature and falls with cross-sectional area. Chassis ground is a conductor with resistance; it is not a perfect zero-volt reference.
- Voltage drop is the difference between source and load voltage and must be measured while the circuit is under load.
Worked example
A 12.6 V battery feeds a 4 m supply wire of 0.01 Ω/m, a relay contact of 0.02 Ω, and a 6 A load. Ground return adds 0.05 Ω. Total conductor and contact resistance = (4 × 0.01) + 0.02 + 0.05 = 0.11 Ω. Drop = 6 A × 0.11 Ω = 0.66 V. Load voltage = 12.6 − 0.66 = 11.94 V. If the specification allows 0.5 V total drop, the circuit fails and the high-resistance point must be located by measuring drop across each segment under load, not by measuring resistance with the circuit off.
Traps
Measuring voltage drop with the load disconnected reads near zero and proves nothing. Assuming a good ground because the meter reads 0 V at the battery negative ignores ground-side drop. Adding resistance in parallel across a sensor skews its signal. Confusing available voltage with loaded voltage hides bad connections. Remember that an open circuit shows full source voltage at the open point, which can mislead you into thinking the circuit is healthy.