What it does#
A capacitor blocks steady (DC) current but passes changes. Charge flows in when the voltage rises and back out when it drops, so a capacitor across a supply acts like a tiny local battery that covers short current spikes. With a resistor it forms a timer or a filter: the time constant τ = R × C is how long the voltage takes to get about two-thirds of the way.
The SMD parts on hobby boards are multilayer ceramic capacitors (MLCC). They have no polarity. Big values in a small case are made with X5R/X7R ceramics; precise small values (crystal load, filters) use C0G/NP0.
Key parameters#
- Capacitance: 100n = 100 nF = 0.1 µF; 1u = 1 µF.
- Voltage rating: at least the rail voltage, better twice it.
- Dielectric: X7R/X5R for supply work. These lose capacitance under DC voltage (a 10 µF 0805 at 5 V may give only half), so pick generously. C0G for anything that sets a frequency.
Example: an RC reset delay#
This is exactly what Espressif recommends on the EN pin of every ESP32 module (see ESP32-C3).
Typical values#
| Job | Value |
|---|---|
| Decoupling at a chip pin | 100 nF X7R, 0603 |
| Bulk on a 3.3 V rail | 10 µF X5R, 0805 |
| Regulator in/out | 1 µF – 22 µF (follow the datasheet) |
| Crystal load | 10–33 pF C0G |
Pitfalls#
- Far from the pin = useless. A decoupling capacitor works only with a short path to the pin and to ground. See decoupling.
- Huge caps on signals. A big capacitor on a strapping pin (GPIO9 of an ESP32-C3) delays it so much the chip boots into the wrong mode.
Footprint#
- Hand soldering: 0805. Ceramics have no marking; keep them labelled.
- JLCPCB assembly: 0603 for ≤ 1 µF, 0805 for 10–22 µF. All catalog values are basic parts.