394 capacitor value
390 nF
390,000 pF · 390 nF · 0.39 µF
| Marked code | 394 |
| Picofarads | 390,000 pF |
| Nanofarads | 390 nF |
| Microfarads | 0.39 µF |
| Farads | 3.90e-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:
39 × 104 = 390,000 pF
So 394 is 39 followed by 4 zeros: 390,000 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 394 is stocked as 0.39 µF and the two numbers look nothing alike.
What 390 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 390 nF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 8.16 kΩ |
| 1 kHz — audio | 408.09 Ω |
| 100 kHz — switching | 4.08 Ω |
| 10 MHz — RF | 40.8 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 394K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 390 nF means |
|---|---|---|
394J | ±5% | 370.5 nF to 409.5 nF |
394K | ±10% | 351 nF to 429 nF |
394M | ±20% | 312 nF to 468 nF |
394Z | +80% / −20% | 312 nF to 702 nF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 394Z can legitimately be anything from 312 nF to 702 nF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 390 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 394 capacitor?
390 nF — 390,000 pF · 390 nF · 0.39 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 394 is 39 followed by 4 zeros = 390,000 pF.
What is a 394 capacitor in µF?
0.39 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 390,000 pF, 390 nF and 0.39 µF are the same capacitor written three ways.
What does the letter after 394 mean?
Tolerance. 394K is ±10%, 394J is ±5% and 394M is ±20% — so a 394K is anywhere from 351 nF to 429 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 394 capacitor at 1 kHz?
408.09 Ω. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 4.08 Ω at 100 kHz and 40.8 mΩ at 10 MHz.
Nearby codes
- 474 capacitor — 470 nF
- 564 capacitor — 560 nF
- 684 capacitor — 680 nF
- 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