Electrical and Electronic Systems
Circuits, testing and networks
Almost every electrical question reduces to Ohm's law and a voltage drop. Voltage is pressure, current is flow, resistance is restriction: E = I × R. Series circuits share voltage and carry the same current; parallel circuits share current and see the same voltage. A fully charged 12 V battery rests near 12.6 V after surface charge is removed (about 12.2 V is roughly 50 % state of charge), and a charging system typically holds 13.8–14.8 V.
Test with a load, not just a meter. Voltage drop testing under current flow is the professional method: measure from the battery positive post to the component, then from the component ground back to the battery negative post. A good high-current path drops very little (commonly ≤ 0.2–0.5 V); 1 V or more means resistance, usually at a connection, ground or cable end. Never use an ohmmeter on a live circuit, and remember a battery can show good open-circuit voltage and still fail a load or conductance test.
Electronics and networks: 5 V reference sensors, PWM and duty-cycle signals, relays and drivers, and CAN buses. A healthy CAN pair is biased near 2.5 V each, and two 120 Ω terminators measured in parallel give about 60 Ω across the bus with power off.
Worked example. A 12 V circuit with 3 Ω of resistance draws I = 12 ÷ 3 = 4 A. If the same circuit should draw 4 A but draws 2 A, resistance has roughly doubled - find the corrosion rather than replacing the load.