3.6 Resistor Color Codes, Semiconductors, Digital Logic & Electrostatic Discharge

3.6 Resistor Color Codes, Semiconductors, Digital Logic & Electrostatic Discharge

Fundamentals of Electricity and Electronics (AM.I.A) carries 27 knowledge elements — more than any other General subject. Sections 3.1 through 3.5 covered circuits, AC theory, batteries, rotating machines, and wiring. This section closes the electronics end of the subject: resistor types and color coding (AM.I.A.K20), semiconductors (K21), digital logic (K22), binary numbers (K23), and electrostatic discharge (K24).

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1. Resistor Types and the Color Code

Fixed resistors in avionics are carbon-composition, carbon-film, metal-film, or wire-wound. Wire-wound resistors handle the highest power and are the ones you find in dimming circuits and load banks; they are usually stamped with their value rather than color-banded. Variable resistors come in two flavours worth distinguishing: a rheostat is a two-terminal variable resistor wired in series to control current, while a potentiometer is a three-terminal device wired across a voltage to produce a variable voltage output.

The four-band EIA color code is a memorization item that appears on the AMG test with a figure from the testing supplement.

Worked example. A resistor is banded yellow – violet – orange – gold. Bands 1 and 2 give the digits 4 and 7, so the significant figures are 47. Band 3 (orange) multiplies by 1,000. The nominal value is 47 x 1,000 = 47,000 ohms = 47 kilohms. The gold tolerance band means ±5%, so the acceptable range is 47,000 ± 2,350 ohms, or 44,650 to 49,350 ohms. A meter reading of 50,100 ohms would fail this resistor.

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2. Semiconductors (AM.I.A.K21)

Semiconductor materials — silicon and germanium — are neither good conductors nor good insulators until they are doped. Adding a pentavalent impurity produces N-type material with free electrons as the majority carrier; adding a trivalent impurity produces P-type material with holes as the majority carrier. Joining the two creates a PN junction, which is the basis of every solid-state device on the aircraft.

• Diode. A one-way valve for current. Forward biased (positive to the anode, negative to the cathode) it conducts after overcoming a barrier potential of roughly 0.7 V in silicon or 0.3 V in germanium. Reverse biased, it blocks. The cathode end carries a painted band. In aircraft systems diodes appear as rectifiers converting alternator AC to DC, as blocking diodes preventing bus back-feed, and as flyback (freewheeling) diodes across relay coils to absorb the inductive spike when the coil is de-energized.
• Zener diode. Deliberately operated in reverse breakdown at a precise voltage; used as a voltage reference or simple regulator.
• Transistor. A three-terminal device — emitter, base, collector — built as NPN or PNP. A small base current controls a much larger collector current, so a transistor works as either an amplifier or a solid-state switch. Solid-state switching has no contacts to pit or arc, which is why transistorized voltage regulators displaced vibrating-contact regulators.
• Integrated circuit (IC). Thousands to billions of transistors, diodes, and resistors on one silicon die. ICs are not field-repairable — the maintenance action is always replacement of the line-replaceable unit or card.
• Rectifier circuits. A half-wave rectifier uses one diode and passes only one half of each AC cycle. A full-wave bridge uses four diodes and passes both halves, producing a smoother output with double the ripple frequency for the same input.

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3. Digital Logic and Binary (AM.I.A.K22 and K23)

Binary is base 2: each place value doubles going left — 1, 2, 4, 8, 16, 32, 64, 128. To convert binary to decimal, add the place values wherever a 1 appears. Binary 1011 0010 is 128 + 32 + 16 + 2 = 178. To convert decimal to binary, subtract the largest place value that fits and repeat.

+-------------------------------------------------------------------------+
|                    LOGIC GATE TRUTH SUMMARY (2 inputs)                   |
+-------------------------------------------------------------------------+
|  GATE    | OUTPUT IS 1 WHEN...                    | 0 0 | 0 1 | 1 0 | 1 1 |
|----------|----------------------------------------|-----|-----|-----|-----|
|  AND     | ALL inputs are 1                       |  0  |  0  |  0  |  1  |
|  OR      | ANY input is 1                         |  0  |  1  |  1  |  1  |
|  NAND    | NOT all inputs are 1                   |  1  |  1  |  1  |  0  |
|  NOR     | NO input is 1                          |  1  |  0  |  0  |  0  |
|  XOR     | inputs DIFFER                          |  0  |  1  |  1  |  0  |
|  NOT     | (1 input) input is 0 -- the INVERTER   |  1  |  0  |  --  | -- |
+-------------------------------------------------------------------------+

• Inverter (NOT gate). Single input, single output; the output is always the opposite of the input. On a schematic it is a triangle with a small bubble at the tip. That bubble means inversion wherever it appears — an AND gate with an output bubble is a NAND gate.
• Flip-flop. A bistable circuit that holds one bit until it is told to change. Flip-flops are the building block of counters, registers, and short-term memory.
• Memory types. RAM is read/write and volatile — its contents vanish when power is removed. ROM is read-only and non-volatile; its contents survive power loss and cannot be rewritten in service. NVRAM combines both properties: it can be written in service and it retains data without power, which is why it holds fault histories and configuration data in modern avionics boxes.

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4. Electrostatic Discharge (AM.I.A.K24)

A human being does not feel a static discharge until roughly 3,000 volts, and does not see a spark until roughly 5,000 volts. Many MOS and CMOS integrated circuits are damaged by discharges of only a few tens or hundreds of volts. The consequence is the defining hazard of ESD work: you can destroy a circuit card and never know it happened. Worse, an ESD event often causes latent damage — the card works on the bench, then fails weeks later in flight.

Required practice when handling ESD-sensitive devices:

• Work at a grounded ESD mat wearing a wrist strap with a current-limiting resistor (typically 1 megohm) connected to the mat's ground point. The resistor protects the technician if the bench becomes energized.
• Transport and store cards only in static-shielding bags — the metallized bags — not in ordinary polyethylene, bubble wrap, or foam packing peanuts, which are strong charge generators.
• Handle cards by the edges and the connector shell, never by the pins or the components.
• Keep common plastics, styrofoam cups, and untreated clothing off the workbench.
• Remember that low humidity dramatically increases charge generation; a dry winter hangar is the worst-case ESD environment.
• Look for the reaching-hand-in-a-triangle ESD symbol on the unit, the shipping bag, and the maintenance manual task; when you see it, the ESD procedure is mandatory, not advisory.