392 capacitor value
3.9 nF
3,900 pF · 3.9 nF · 0.0039 µF
| Marked code | 392 |
| Picofarads | 3,900 pF |
| Nanofarads | 3.9 nF |
| Microfarads | 0.0039 µF |
| Farads | 3.90e-9 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:
39 × 102 = 3,900 pF
So 392 is 39 followed by 2 zeros: 3,900 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 392 is stocked as 0.0039 µF and the two numbers look nothing alike.
What 3.9 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 3.9 nF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 816.18 kΩ |
| 1 kHz — audio | 40.81 kΩ |
| 100 kHz — switching | 408.09 Ω |
| 10 MHz — RF | 4.08 Ω |
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 392K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 3.9 nF means |
|---|---|---|
392J | ±5% | 3.705 nF to 4.095 nF |
392K | ±10% | 3.51 nF to 4.29 nF |
392M | ±20% | 3.12 nF to 4.68 nF |
392Z | +80% / −20% | 3.12 nF to 7.02 nF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 392Z can legitimately be anything from 3.12 nF to 7.02 nF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 3.9 nF turns up
Single-figure nanofarads is audio filtering and snubbing across switch contacts. It is also the range where a capacitor stops being a precision component and starts being a bypass, depending on the dielectric it is made from.
Frequently asked questions
What is the value of a 392 capacitor?
3.9 nF — 3,900 pF · 3.9 nF · 0.0039 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 392 is 39 followed by 2 zeros = 3,900 pF.
What is a 392 capacitor in µF?
0.0039 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 3,900 pF, 3.9 nF and 0.0039 µF are the same capacitor written three ways.
What does the letter after 392 mean?
Tolerance. 392K is ±10%, 392J is ±5% and 392M is ±20% — so a 392K is anywhere from 3.51 nF to 4.29 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 392 capacitor at 1 kHz?
40.81 kΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 408.09 Ω at 100 kHz and 4.08 Ω at 10 MHz.
Nearby codes
- 472 capacitor — 4.7 nF
- 562 capacitor — 5.6 nF
- 682 capacitor — 6.8 nF
- 822 capacitor — 8.2 nF
- 103 capacitor — 10 nF
- 123 capacitor — 12 nF
- 153 capacitor — 15 nF
- 183 capacitor — 18 nF
- 223 capacitor — 22 nF
- 273 capacitor — 27 nF