684 capacitor value
680 nF
680,000 pF · 680 nF · 0.68 µF
| Marked code | 684 |
| Picofarads | 680,000 pF |
| Nanofarads | 680 nF |
| Microfarads | 0.68 µF |
| Farads | 6.80e-7 F |
Reading the code
The three digits are not a part number. The first two are the value in picofarads and the third says what to do with them:
68 × 104 = 680,000 pF
So 684 is 68 followed by 4 zeros: 680,000 pF. The code is always in picofarads even when the part is sold in microfarads, which is the whole difficulty with it — a capacitor marked 684 is stocked as 0.68 µF and the two numbers look nothing alike.
What 680 nF does at frequency
A capacitor's opposition to a signal is its reactance, 1 ÷ (2πfC), and it falls as the frequency rises. Over the range of frequencies a circuit can put across it, a 680 nF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 4.68 kΩ |
| 1 kHz — audio | 234.05 Ω |
| 100 kHz — switching | 2.34 Ω |
| 10 MHz — RF | 23.4 mΩ |
Ten times the frequency is a tenth of the reactance, every time — which is why choosing a decoupling capacitor is really choosing the frequency band you want it to be low-impedance across, and why boards carry several sizes in parallel rather than one large one.
The letter after the number
A code like 684K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 680 nF means |
|---|---|---|
684J | ±5% | 646 nF to 714 nF |
684K | ±10% | 612 nF to 748 nF |
684M | ±20% | 544 nF to 816 nF |
684Z | +80% / −20% | 544 nF to 1.224 µF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 684Z can legitimately be anything from 544 nF to 1.224 µF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 680 nF turns up
A hundred nanofarads is the decoupling capacitor — the one that sits beside every integrated circuit on every board, holding the supply steady through the current spike each time the chip switches. If a board has one value on it more than any other, it is this one.
Frequently asked questions
What is the value of a 684 capacitor?
680 nF — 680,000 pF · 680 nF · 0.68 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 684 is 68 followed by 4 zeros = 680,000 pF.
What is a 684 capacitor in µF?
0.68 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 680,000 pF, 680 nF and 0.68 µF are the same capacitor written three ways.
What does the letter after 684 mean?
Tolerance. 684K is ±10%, 684J is ±5% and 684M is ±20% — so a 684K is anywhere from 612 nF to 748 nF and still in specification. A Z means +80%/−20%, which is not a mistake: some ceramic dielectrics really are that loose.
What is the impedance of a 684 capacitor at 1 kHz?
234.05 Ω. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 2.34 Ω at 100 kHz and 23.4 mΩ at 10 MHz.
Nearby codes
- 824 capacitor — 820 nF
- 105 capacitor — 1 µF
- 125 capacitor — 1.2 µF
- 155 capacitor — 1.5 µF
- 185 capacitor — 1.8 µF
- 225 capacitor — 2.2 µF
- 275 capacitor — 2.7 µF
- 335 capacitor — 3.3 µF
- 395 capacitor — 3.9 µF
- 475 capacitor — 4.7 µF