Passives
Capacitor
Stores electric charge on two plates separated by an insulator. It fills up when voltage is applied and gives the charge back later, so it blocks steady DC but lets changes pass.
Think of it as: A water tank with a rubber membrane across it: water pushes in and stretches it, but cannot flow straight through.
Pins
- Ceramic
- : No polarity. Either way round works.
- Electrolytic +
- : The longer leg. Must sit at the higher voltage.
- Electrolytic −
- : The shorter leg, on the side with the stripe of minus signs.
Small values (pF to a few µF) are usually ceramic; large values (µF to mF) are electrolytic cans. Ceramic codes work like resistors: "104" = 10·10⁴ pF = 100 nF.
What to look for inside
- The capacitor charges for 4 seconds, then discharges for 4 seconds, over and over.
- While charging, current flows in the leads and + and − charges pile up on opposite plates. No charge crosses the gap.
- The electric field between the plates (dashed lines) lines up the molecules of the insulator. That field is where the energy is stored.
- As it fills, the current slows down, because the stored voltage pushes back against the source.
The numbers that matter
| Spec | Typical |
|---|---|
Capacitance How much charge it holds per volt. 1 µF = 1000 nF = 1 000 000 pF. | 10 pF to 10 000 µF |
Voltage rating Never exceed it. Choose at least 1.5 to 2 times the highest voltage it will see. | 6.3 V to 50 V |
Dielectric / type X7R ceramics lose capacitance as the voltage rises; C0G stays stable but only comes in small values. | C0G, X7R, electrolytic |
ESR Hidden series resistance. Low ESR matters when smoothing a power supply. | 0.01 Ω to 1 Ω |
Where you’ll use it
- Decoupling: a 100 nF ceramic next to every chip’s power pin soaks up fast spikes.
- Smoothing the ripple out of a power supply (large electrolytics).
- Timing with a resistor: the RC time constant sets delays, blink rates and switch debouncing.
- Filtering: passing high frequencies and blocking low ones, or the reverse.
Common mistakes
- Fitting an electrolytic backwards. It heats up and can burst.
- Using a capacitor rated too close to the supply voltage.
- Forgetting it can stay charged after power is removed. Big ones can give a nasty shock.
Lessons that use it
- Lesson 2.2: RC Time Constant: Capacitor Charging Explained
- Lesson 2.3: RC Low-Pass & High-Pass Filters (Bode Plot)
- Lesson 4.1: Pull-Up Resistors & Button Debounce Explained
Go deeper
Charge stored equals capacitance times voltage.
Current flows only while the voltage is changing.
Charging through a resistor, and the energy held in the field.
Its opposition to AC falls as frequency rises.