Resistors

Resistors are the most common component in electronics — passive, two-terminal, and indifferent to polarity. They resist current flow, and by controlling current you control voltage drops, set bias points, protect other components, and build voltage dividers. There is almost no circuit that doesn't have at least one. Understanding resistors well means understanding a large portion of circuit behaviour.

Types

Type	Adjustable?	Notes	Typical use
Carbon film	No	Cheap, ±5–10% tolerance, noisy	Non-critical applications, rough prototyping
Metal film	No	Better tolerance (±1%), lower noise, stable with temperature	Default for most signal work
Wirewound	No	High power handling, but inductive — bad for RF	Power supplies, loads, high-wattage applications
Potentiometer	Yes (continuous)	Three terminals — two ends and a wiper	Volume controls, user-adjustable bias
Trimmer (trimpot)	Yes (one-time)	Screwdriver-adjustable, not meant for frequent use	Factory calibration, offset adjustment
Thermistor (NTC)	No (temperature-dependent)	Resistance drops as temperature rises	Temperature sensing, inrush limiting
Thermistor (PTC)	No (temperature-dependent)	Resistance rises with temperature — self-limiting	Overcurrent protection, resettable fuse
LDR (photoresistor)	No (light-dependent)	Slow response, not for precision work	Simple light sensing, dusk/dawn switching

Reading the Colour Code

Through-hole resistors have coloured bands. The standard 4-band system encodes the value as two significant digits, a multiplier, and a tolerance band:

Colour	Digit	Multiplier	Tolerance
Black	0	×1	—
Brown	1	×10	±1%
Red	2	×100	±2%
Orange	3	×1,000	—
Yellow	4	×10,000	—
Green	5	×100,000	±0.5%
Blue	6	×1,000,000	±0.25%
Violet	7	—	±0.1%
Grey	8	—	±0.05%
White	9	—	—
Gold	—	×0.1	±5%
Silver	—	×0.01	±10%

Example: Brown–Black–Red–Gold = 1, 0, ×100, ±5% = 1,000 Ω (1 kΩ) ±5%

In practice, just use a multimeter. Reading bands under poor lighting with colour-blind eyes is how mistakes happen. Measure first, trust the bands second.

SMD Resistor Codes

Surface-mount resistors use a different system — numbers printed directly on the package:

3-digit code: first two digits are significant figures, third is the multiplier exponent
  102 → 10 × 10² = 1,000 Ω (1 kΩ)
  471 → 47 × 10¹ = 470 Ω
  330 → 33 × 10⁰ = 33 Ω

4-digit code (±1% precision parts):
  1002 → 100 × 10² = 10,000 Ω (10 kΩ)
  4701 → 470 × 10¹ = 4,700 Ω (4.7 kΩ)

EIA-96 code (used on 0402/0201 parts — letter + two digits):
  01C → value 100 × multiplier C (×10) = 1,000 Ω

Standard E-Series Values

Resistors don't come in every value. The E-series defines a set of standard preferred values per decade:

Series	Steps/decade	Tolerance	Values (×1 decade)
E6	6	±20%	1.0, 1.5, 2.2, 3.3, 4.7, 6.8
E12	12	±10%	1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
E24	24	±5%	Includes E12 + 1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, 9.1
E96	96	±1%	Precision work — full table in datasheets
E192	192	±0.5%	High-precision instrumentation

When you calculate a needed resistance, round to the nearest standard value. If your calculation gives 237 Ω, use 240 Ω (E24) or 220 Ω depending on which direction is safer for your circuit.

Power Rating

Every resistor has a maximum power dissipation. Exceed it and the component overheats, drifts in value, and eventually fails — sometimes with smoke and a scorched PCB. The formula:

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

Rating	Physical size	Typical application
1/8 W	Small through-hole, most SMD	Signal circuits, low-current biasing
1/4 W	Standard through-hole	General purpose — the default choice
1/2 W	Larger through-hole	Moderate current, LED drivers, regulators
1 W	Large through-hole or TO-220 mount	Higher current loads, power supplies
2 W+	Wirewound or chassis-mount	Dummy loads, power dissipation, braking resistors

Rule of thumb: derate to 50%. If you calculate 0.47 W, use a 1 W rated resistor. Running at the rated maximum shortens lifespan significantly.

Common Circuit Applications

Current Limiting

The most common use — protecting an LED or other component:

Supply: 5V
LED forward voltage (Vf): 2V
Desired current: 20mA

R = (V_supply - V_LED) / I
R = (5V - 2V) / 0.02A = 150Ω → use 150Ω or 180Ω

Power: P = I² × R = (0.02)² × 150 = 0.06W → 1/8W resistor is fine

Voltage Divider

Two resistors in series divide a voltage proportionally:

       Vin
        |
       [R1]
        |
       Vout ←── measured here
        |
       [R2]
        |
       GND

Vout = Vin × R2 / (R1 + R2)

Example: Vin=12V, R1=10kΩ, R2=10kΩ
Vout = 12 × (10k / 20k) = 6V

Voltage dividers only work well when the load impedance is much higher than the divider resistance (typically 10× or more). If the load draws significant current, the output voltage drops from the calculated value.

Pull-up and Pull-down

Ensures a digital pin has a defined logic state when nothing is actively driving it:

Pull-up:   resistor from VCC to the pin → pin reads HIGH when floating
Pull-down: resistor from pin to GND    → pin reads LOW when floating

Typical values: 4.7 kΩ to 10 kΩ
Lower values = stronger pull, more current, better noise immunity
Higher values = weaker pull, less current, may be too slow for fast signals

Tolerance in Practice

Tolerance	Series	When it matters
±20%	E6	Almost never acceptable in signal work. Fine for bulk current limiting.
±10%	E12	Acceptable for non-critical applications
±5%	E24	General purpose default — what most hobby kits use
±1%	E96	Precision analog, feedback networks, reference circuits
±0.1%	E192	Instrumentation, metering, high-precision ADC input dividers

References

• Horowitz & Hill — The Art of Electronics, 3rd ed. Chapter 1
• Scherz & Monk — Practical Electronics for Inventors, 4th ed.
• ResistorGuide.com — comprehensive resistor reference ↗
• EEPower — colour code reference ↗