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Resistors
Quick reference for selecting resistors, color code decoding, and more.
1. Resistor selection
Key parameters when choosing a resistor:
- Resistance value — calculated using Ohm's law or taken from the schematic.
- Tolerance — accuracy (0.1%, 1%, 5%, 10%, etc.). Choose based on circuit requirements.
- Power rating — must be at least the power dissipated (
P = I²·RorP = U²/R). A 20–30% margin is recommended. - Temperature coefficient (TCR) — important for precision circuits.
- Package type — through-hole (axial) or SMD.
Example
Resistor for an LED: supply 5 V, LED forward voltage 2 V, current 20 mA.
Resistance: R = (5 – 2) / 0.02 = 150 Ω.
Power: P = 0.02² · 150 = 0.06 W → choose 0.125 W (next standard value).
Tolerance: 5% is enough for indication.
2. Resistor color code
For through-hole resistors with 4, 5 or 6 bands.
Color table
| Color | Digit | Multiplier | Tolerance | TCR (ppm/°C) |
|---|---|---|---|---|
| Black | 0 | 10⁰ | – | – |
| Brown | 1 | 10¹ | ±1% | 100 |
| Red | 2 | 10² | ±2% | 50 |
| Orange | 3 | 10³ | – | 15 |
| Yellow | 4 | 10⁴ | – | 25 |
| Green | 5 | 10⁵ | ±0.5% | 20 |
| Blue | 6 | 10⁶ | ±0.25% | 10 |
| Violet | 7 | 10⁷ | ±0.1% | 5 |
| Gray | 8 | 10⁸ | ±0.05% | – |
| White | 9 | 10⁹ | – | – |
| Gold | – | 10⁻¹ | ±5% | – |
| Silver | – | 10⁻² | ±10% | – |
Decoding by number of bands
- 4 bands: 1st digit, 2nd digit, multiplier, tolerance.
- 5 bands: 1st, 2nd, 3rd digits, multiplier, tolerance.
- 6 bands: same as 5 bands + 6th band – TCR.
Example
Resistor with bands: Brown Black Red Gold →
1 (brown), 0 (black), multiplier ×100 (red) → 10 × 100 = 1000 Ω = 1 kΩ, tolerance ±5% (gold).
3. Standard E‑series
Resistors are manufactured with standard values from the E‑series:
- E6 (20% tol.): 10, 15, 22, 33, 47, 68
- E12 (10%): 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
- E24 (5%): adds 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91
- E48/E96/E192 (1% and tighter) for precision.
Values are multiplied by 10ⁿ (e.g., 10, 100, 1k, 10k, 100k, 1M).
4. Power ratings
Typical power ratings for common packages:
| Package | Power (W) |
|---|---|
| SMD 0402 | 0.063 |
| SMD 0603 | 0.1 |
| SMD 0805 | 0.125 |
| SMD 1206 | 0.25 |
| SMD 1210 | 0.5 |
| SMD 2010 | 0.75 |
| SMD 2512 | 1 |
| Through‑hole (axial) 0.125 W | 0.125 |
| Through‑hole 0.25 W | 0.25 |
| Through‑hole 0.5 W | 0.5 |
| Through‑hole 1 W | 1 |
| Through‑hole 2 W | 2 |
5. SMD resistor marking
- 3 digits (e.g., 472): first two are mantissa, third is multiplier (10ⁿ). 472 = 47 × 10² = 4.7 kΩ.
- 4 digits (e.g., 1001): first three are mantissa, fourth is multiplier. 1001 = 100 × 10¹ = 1 kΩ.
- EIA-96 code (for 1%): two digits + letter. Digits are a code from a table, letter is multiplier (Z=0.001, R=0.01, S=0.1, A=1, B=10, C=100, D=1000, E=10000, F=100000). E.g., 01C = 100 × 100 = 10 kΩ.