Choosing resistors: four things that matter more than the value
It seems a resistor only needs the right value, but you also have to look at power rating, tolerance, temperature coefficient and package.
| Specification | Meaning | Guideline |
|---|---|---|
| Power rating | The heat it can withstand | At least twice the actual dissipation |
| Tolerance | Error from the marked value | 5% in general, 1% or better for dividers and sensing |
| Temperature coefficient | Change with temperature (ppm/°C) | 50 ppm or less for precise measurement |
| Maximum voltage | The voltage allowed across it | Several in series for high-voltage dividers |
Chip resistor packages
| Code | Size (mm) | Power rating (typical) |
|---|---|---|
| 0402 | 1.0 × 0.5 | 1/16 W |
| 0603 | 1.6 × 0.8 | 1/10 W |
| 0805 | 2.0 × 1.25 | 1/8 W |
| 1206 | 3.2 × 1.6 | 1/4 W |
For hand-soldered prototypes, 0603 or 0805 is comfortable.
E series
Resistors do not come in every value. Choose from E24 (10, 11, 12, 13, 15, 16, 18, 20, 22 …) for 5% and from E96 for 1%. Once you have a calculated value, pick the nearest standard value and recalculate the result.
Practical tips
Standardising on the same values and packages in the BOM as far as possible cuts assembly cost and stock management. Many companies fix defaults, such as 10 kΩ for pull-ups and 1 kΩ for LED resistors.