Electronics Foundations

Everything in troubleshooting reduces to predicting what voltage, current, or resistance should be at a point, measuring what it is, and explaining the difference. This module gives you the prediction tools.

1. The three quantities

Key mental model: voltage is always between two points (it's a difference, like height). Current is through one point (like water flow in a pipe). Saying "the voltage at TP3" really means "voltage between TP3 and ground."

2. Ohm's Law and Power

V = I × R        I = V / R        R = V / I
P = V × I        P = I² × R       P = V² / R

• 5V across 1kΩ → I = 5/1000 = 5mA. This is the calculation you'll do most in your life.
• A part dissipating more power than rated gets hot, drifts, then dies. Discolored board = chronic overheating = look for what's drawing too much current.

Unit prefixes you must know cold: M (mega, ×1,000,000) · k (kilo, ×1,000) · m (milli, ÷1,000) · µ (micro, ÷1,000,000) · n (nano, ÷10⁹) · p (pico, ÷10¹²)

So 4.7kΩ = 4,700Ω; 100nF = 0.1µF; 2.2mA = 0.0022A.

3. Series and parallel

Series (one path, components in a chain):

• Same current through everything.
• Resistances add: R_total = R1 + R2 + …
• Voltage divides proportionally to resistance — this is the voltage divider, the single most common analog building block: Vout = Vin × R2 / (R1 + R2) (R2 is the one Vout is taken across)

Parallel (components side by side between the same two nodes):

• Same voltage across everything.
• Current divides; total resistance drops below the smallest: Two resistors: R_total = (R1 × R2) / (R1 + R2). Two equal resistors → half.

Why this matters for repair: every component on a board sits in a web of parallel paths. Measure a 10kΩ resistor in-circuit and you may read 3kΩ because other parts are in parallel with it. In-circuit ohms can read low but never legitimately read high — a high in-circuit reading means the part (or its solder joint) really is open. (See 04 — DMM Mastery.)

4. DC vs AC

• DC — constant polarity. Power rails (3.3V, 5V, ±15V, 28V in aerospace) are DC. 28VDC is the classic aircraft bus voltage; 115VAC 400Hz is the classic aircraft AC supply (400Hz, not 60Hz — transformers can be smaller).
• AC — polarity alternates. Described by frequency (Hz) and amplitude. Amplitude has three common expressions: peak (Vp), peak-to-peak (Vpp), and RMS (the "DC-equivalent heating value"; for a sine wave, RMS = Vp ÷ √2 ≈ 0.707 × Vp).
• A DMM in V~ mode reads RMS. Cheap meters are only accurate on sine waves; a true-RMS meter reads any waveshape correctly — you want true-RMS for work.
• Real signals are usually DC with AC riding on it — e.g., a 5V rail with 50mV of switching ripple. The scope's AC coupling lets you see the small AC part without the big DC offset (see 05 — Oscilloscope Mastery).

5. The passive components' jobs

• Resistor — sets current, divides voltage, pulls a line up or down, terminates a bus, senses current (tiny "shunt" values like 0.01Ω).
• Capacitor — stores charge. Blocks DC, passes AC. Its AC opposition (reactance) falls with frequency: Xc = 1/(2πfC). Jobs: decoupling/bypass (a local energy reservoir next to every IC's power pin — those little 100nF caps everywhere), bulk filtering (big electrolytics smoothing a supply), coupling (passing signal while blocking DC), timing.
• Inductor — stores energy in a magnetic field. Passes DC, opposes changing current: Xl = 2πfL. Jobs: filters, energy storage in switching supplies, ferrite beads to choke high-frequency noise.
• Time constant: an RC pair charges/discharges with τ = R×C seconds (63% per τ, ~fully in 5τ). This rules reset circuits, debouncing, and filters.

6. The semiconductors' jobs

• Diode — one-way valve. Conducts when forward-biased (anode ~0.6V above cathode for silicon), blocks reversed. Jobs: rectification, reverse-polarity protection, clamping, steering.
• Zener diode — deliberately operated in reverse breakdown at a precise voltage. Jobs: voltage reference, protection clamp.
• BJT transistor (NPN/PNP) — current-controlled switch/amplifier. Small base current controls large collector current. Quick health intuition: it's two diode junctions (B-E and B-C) back-to-back — testable with DMM diode mode.
• MOSFET (N/P-channel) — voltage-controlled switch. Gate voltage opens the drain-source channel. Dominant in power switching. Fails shorted D-S more often than open.
• Op-amp — high-gain differential amplifier; with negative feedback it does math: amplify, buffer, filter, compare. Golden rules under feedback: inputs draw no current, and the output does whatever it must to make the two inputs equal.
• IC — anything from a 4-gate logic chip to a billion-transistor processor. To a repair tech an IC is: power pins, ground pins, inputs, outputs — verify the first two, stimulate the third, watch the fourth.

7. Grounds and returns

"Ground" is just the agreed 0V reference node. Boards often have multiple grounds (analog ground, digital ground, chassis ground) joined at one deliberate point to control noise. Two practical consequences:

1. Always know which ground your black probe is on.
2. A "ground" that has resistance to the real ground (cracked joint, corroded standoff) creates bizarre, intermittent symptoms — measurable as a few ohms or as voltage appearing "on ground" under load.

8. Self-check (answer without notes)

1. A 330Ω resistor across 3.3V — how much current? How much power? Reveal answer
2. Two 10kΩ resistors in parallel? In series? Reveal answer
3. A voltage divider: 12V in, R1=10kΩ on top, R2=2kΩ on bottom. Vout? Reveal answer
4. Why does a 100nF cap sit next to every IC power pin? Reveal answer
5. Why can an in-circuit resistance reading be lower than the part's marked value but not legitimately higher? Reveal answer

Next: 02 — Reading Schematics