part · Level 6 · Logic · passive
What a 100 nF capacitor is for
100 nF capacitor — A very small charge bucket that smooths out electrical wobbles.
What it is
This is the other kind of capacitor: a tiny ceramic disc or block with two legs and "104" printed on it. No polarity — it goes in either way. Its capacitance is 100 nanofarads, also written 0.1 µF: a hundred times smaller than the 10 µF electrolytic, and fast.
Small and fast is exactly what you want for two jobs. First, decoupling: sitting right across a chip's + and − pins, it supplies the tiny, sudden gulps of current the chip takes when it switches, so the supply voltage stays steady. Second, audio timing: swap it for the 10 µF in a 555 and the blink becomes a tone.
Almost every chip in the world has one of these next to it. Soon you will add one beside each chip without thinking.
How it works
The code: 104 means 10 followed by 4 zeros in picofarads — 100 000 pF = 100 nF = 0.1 µF. (103 would be 10 nF, 105 would be 1 µF.)
Timing with a 555, R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF:
- f = 1.44 ÷ ((1 000 + 20 000) × 0.0000001) = 1.44 ÷ 0.0021 ≈ 690 Hz
That is a clear tone from a piezo. With R2 = 100 kΩ it drops to about 72 Hz, a low buzz.
Decoupling: when a chip's output switches, it can want tens of milliamps for a few nanoseconds. Wire and battery cannot respond that fast, so the supply dips — unless a capacitor right at the pins delivers the charge locally. 100 nF is the traditional value because it is big enough to matter and small enough to be fast. Put it as close to the chip as the breadboard allows.
The numbers
| Capacitance | 100 nF = 0.1 µF — marked 104 |
|---|---|
| Polarity | none — ceramic — either way round |
| Voltage rating | 50 V typical — far above anything here |
| 555 tone with 1 k / 10 k | ≈ 690 Hz — 1.44 ÷ ((R1 + 2·R2) × C) |
| τ with 10 kΩ | 1 ms — R × C |
See it working
The 555 blinker from the library, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Confusing 104 with 10 µF. They differ by a factor of 100: a blinker becomes a buzz. Fix: read the code (104 = 100 nF).
- Decoupling cap far from the chip. Long breadboard wires undo the benefit. Fix: legs straight into the rows next to the power pins.
- Expecting a visible blink. With kilohm resistors a 100 nF cap times in milliseconds — you will hear it, not see it.
Questions people ask
- What does 104 mean on a capacitor?
- Two digits then a multiplier, in picofarads: 10 × 10⁴ pF = 100 000 pF = 100 nF = 0.1 µF.
- Why do circuits put a 0.1 µF capacitor across the power pins of a chip?
- To supply the brief current spikes the chip draws when it switches, keeping the voltage at its pins steady. It is called a decoupling or bypass capacitor.
- Does a ceramic capacitor have polarity?
- No. Ceramic and film capacitors go in either way round; only electrolytic (and tantalum) capacitors are polarised.
Projects that use this part
No project uses it yet — it arrives at Level 6, and the projects for that level are still being written and verified.
verified against: SparkFun — Capacitors · Texas Instruments — NE555 datasheet