What it does#
A resistor obeys Ohm’s law: V = I × R. Put a voltage across it and a proportional current flows; it turns the difference into heat (P = V × I = I² × R). On a hobby board it does four jobs:
- Limit current, for example through an LED.
- Pull a signal to a known level: pull-ups and pull-downs.
- Divide a voltage, so a 4.2 V battery can be measured by a 3.3 V ADC.
- Tell something who you are: 5.1 kΩ on the CC pins of USB-C.
Key parameters#
- Value in ohms, written the engineering way: 4k7 = 4.7 kΩ, 100R = 100 Ω.
- Tolerance: 1 % is the normal SMD part today and costs the same as 5 %.
- Power rating: 0603 parts handle 1/10 W, 0805 1/8 W. A resistor that carries 5 V at 10 mA dissipates 50 mW, which is fine; check anything that carries real current.
Voltage divider#
V_out = V_in × R2 / (R1 + R2)
Two 100 kΩ resistors halve the battery voltage: 4.2 V becomes 2.1 V, safe for an ESP32 ADC pin. High values waste little battery (4.2 V / 200 kΩ = 21 µA), and a 100 nF capacitor at the middle gives the ADC a low-impedance source to sample.
Typical values#
| Job | Value |
|---|---|
| Pull-up on a button or reset line | 10 kΩ |
| I²C pull-ups (3.3 V, short bus) | 4.7 kΩ |
| USB-C CC (device / sink) | 5.1 kΩ, one per CC pin |
| WS2812 data series resistor | 330 Ω |
| MOSFET gate series / pull-down | 100 Ω / 10–100 kΩ |
Pitfalls#
- Read the code, not the colour. SMD resistors are marked with codes (103 = 10 kΩ, 01C = 10 kΩ in EIA-96) or not marked at all (0402, many 0603). Keep reels labelled.
- Dividers load the source. The ADC’s own input and sampling capacitor draw current; very high values (1 MΩ+) make readings noisy.
Footprint#
- Hand soldering: 0805 or the axial through-hole variant.
- JLCPCB assembly: 0603. Every value in the catalog is a basic part.