Component Identification & How Each Part Fails

Open visuals/02-component-gallery.html in a browser — it's the picture book for this module (drawn images of every package, color-code chart, polarity table). This module is the text: what each part is, how to read it, and how it fails, because in repair, knowing the failure habits of each species is half the job.

1. Resistors

Through-hole: tan/blue cylinders with color bands. 4-band = value(2) + multiplier + tolerance. 5-band (precision) = value(3) + multiplier + tolerance. Colors: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9; multiplier gold ×0.1, silver ×0.01; tolerance brown ±1%, gold ±5%, silver ±10%. Example: yellow-violet-red-gold = 47 × 100 = 4.7kΩ ±5%.

SMD chip resistors: marked with codes — 472 = 47×10² = 4.7kΩ; 4702 = 47kΩ (4-digit, 1%); 0 or 000 = zero-ohm jumper; R10 = 0.10Ω. Tiny 0201/0402 parts are unmarked — use the BOM.

Failure habits: resistors mostly fail open (or drift high), usually from overload — and an overloaded resistor is often the victim of a short elsewhere. A burnt resistor answers "what died?" but you must still answer "what killed it?" before powering up a new one. Current-sense and fusible resistors are designed to be the sacrifice.

2. Capacitors

Marking code: 104 = 10 × 10⁴ pF = 100nF. 223 = 22nF. Letters after = tolerance.

DMM checks: capacitance mode out of circuit; in-circuit, a cap should not be a short (charging behavior in ohms mode: reading climbs as it charges). High ESR needs an ESR meter or scope (excess ripple).

3. Inductors, beads, transformers

• Inductors — coils, sometimes look like fat resistors (with color bands) or little cubes. Fail open (broken wire) or short turns (hard to detect; usually found by circuit misbehavior). DMM: should read near-zero ohms (continuity) — open = dead.
• Ferrite beads (FB) — look like SMD resistors, marked in Ω at 100MHz. They are continuity at DC. An open bead silently kills the rail it feeds.
• Transformers — check each winding for continuity; isolation between windings should be open (megohms).

4. Diodes

• Rectifier (DO-41 etc.) — black cylinder, band = cathode, matching the bar on the symbol.
• SMD diodes (SOD-123, SMA/SMB/SMC) — band = cathode. Marked with cryptic codes; check BOM.
• Zener — often small glass body. Tests like a normal diode forward; its reverse breakdown (the useful spec) needs applied voltage above its rating to verify.
• LED — flat side / shorter lead = cathode. DMM diode mode lights it dimly (Vf 1.6–3.3V; blue/white may exceed some meters' test voltage).
• TVS — protection part across inputs/rails; fails short when it has done its job absorbing a transient. A shorted TVS saved the board — replace it and find what transient hit.
• Failure habits: diodes fail short more often than open. Diode test both ways: ~0.5–0.7V one way, OL the other = healthy silicon. Low both ways = shorted. Schottky reads 0.15–0.45V — that's normal, not a defect.

5. Transistors

• BJT (TO-92, SOT-23, TO-220...): two junctions. Diode-test B-E and B-C: both should read like diodes one way (NPN: + on base). C-E should read open both ways (except a body-diode reading in some). Shorted C-E is the common death.
• MOSFET: gate-to-anything should be open (it's a capacitor); D-S shows the body diode one way. Fails D-S short typically. Gate oxide is ESD-fragile — handle with discipline.
• Pinouts vary by part number — never assume; check the datasheet. TO-220 tabs are usually electrically connected to the middle pin (drain/collector) — they are live.

6. ICs and packages

Identification = package + topside marking → datasheet. Packages to know on sight (drawn in the gallery): DIP, SOIC, TSSOP, QFP, QFN, BGA, SOT-23, SOT-223, DPAK/D2PAK, TO-220/247.

• Pin 1: dot/notch; numbering counterclockwise from top view.
• QFN/BGA: joints partly or fully hidden → visual inspection can't clear them; X-ray or boundary scan territory. BGA failure habit: cracked solder balls from thermal cycling → intermittent, flex-sensitive faults.
• IC failure habits: ESD/overvoltage on I/O pins (dead input/output, rest works), latch-up, internally shorted rails (chip gets hot fast — your finger or an IR camera finds it), cracked internal bond wires (intermittent with temperature).

7. Electromechanical & hardware

• Connectors — the #1 field-failure site on aircraft equipment: bent/recessed pins, fretting corrosion, cracked solder tails from cable strain. Always inspect connectors early. Aerospace circulars (MIL-DTL-38999 family) are keyed and torqued — note insert arrangement and pin gauges.
• Relays (K) — coil resistance per datasheet (typically tens–hundreds of Ω); contacts pit and stick. Click ≠ contact closed; verify with continuity.
• Switches — verify contact resistance near 0Ω; oxidation causes ohms and intermittents.
• Crystals/oscillators (Y) — crystals are mechanically fragile: a dropped board with a dead clock = suspect the crystal. Can't fully test with DMM; scope the oscillation (see 07 — Digital Board Troubleshooting).
• Fuses — verify with continuity, not eyeballs (the element can fail invisibly). A blown fuse is a symptom; find the load that blew it.

8. Board-level structures (faults that aren't "components")

• Vias — plated barrels connecting layers. Cracked via barrels from thermal cycling are a classic aerospace intermittent: works cold, fails hot (or vice versa), responds to board flex.
• Traces — can crack at flex points or burn open (a "fuse" you didn't order). Look for the subtle brown line.
• Solder joints — cold joints (gray, lumpy), cracked joints (annular ring around a lead, visible under magnification), insufficient/excess solder, tombstoned SMDs. Statistically the most common assembly defect class.
• Conformal coating — the clear protective layer over aerospace boards. It hides defects, blocks probes (probe at test points or pierce per approved procedure), and must be repaired after rework per IPC-7711/7721 (10 — Aerospace Standards, ESD, and Workmanship).
• Contamination/corrosion — flux residue (white haze), electrolyte, salt/humidity dendrites that grow conductive shorts between traces.

9. The polarity table (recite it cold)

Backwards installation of any of these is both a thing you must never do in rework and a thing to check for on a failed unit — assembly errors escape into the field.

10. Drill (daily, week 3)

1. Decode 10 random resistor color codes and 5 SMD codes per day until instant.
2. From the gallery HTML, cover captions and name each package.
3. Recite the polarity table and the "failure habits" of: electrolytic, tantalum, MLCC, MOSFET, TVS, via.

Next: 04 — DMM Mastery