395 capacitor value
3.9 µF
3,900,000 pF · 3900 nF · 3.9 µF
| Marked code | 395 |
| Picofarads | 3,900,000 pF |
| Nanofarads | 3900 nF |
| Microfarads | 3.9 µF |
| Farads | 3.90e-6 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 × 105 = 3,900,000 pF
So 395 is 39 followed by 5 zeros: 3,900,000 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 395 is stocked as 3.9 µF and the two numbers look nothing alike.
What 3.9 µF 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 µF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 816.18 Ω |
| 1 kHz — audio | 40.81 Ω |
| 100 kHz — switching | 408.1 mΩ |
| 10 MHz — RF | 4.1 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 395K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 3.9 µF means |
|---|---|---|
395J | ±5% | 3.705 µF to 4.095 µF |
395K | ±10% | 3.51 µF to 4.29 µF |
395M | ±20% | 3.12 µF to 4.68 µF |
395Z | +80% / −20% | 3.12 µF to 7.02 µF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 395Z can legitimately be anything from 3.12 µF to 7.02 µF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 3.9 µF turns up
Hundreds of nanofarads up to a microfarad is bulk decoupling and audio coupling where the low end has to survive. Ceramics this large have moved to multilayer construction, and the capacitance falls off sharply with applied voltage — a part marked 1 µF can measure half that with its rated voltage across it.
Frequently asked questions
What is the value of a 395 capacitor?
3.9 µF — 3,900,000 pF · 3900 nF · 3.9 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 395 is 39 followed by 5 zeros = 3,900,000 pF.
What is a 395 capacitor in µF?
3.9 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 3,900,000 pF, 3900 nF and 3.9 µF are the same capacitor written three ways.
What does the letter after 395 mean?
Tolerance. 395K is ±10%, 395J is ±5% and 395M is ±20% — so a 395K is anywhere from 3.51 µF to 4.29 µF 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 395 capacitor at 1 kHz?
40.81 Ω. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 408.1 mΩ at 100 kHz and 4.1 mΩ at 10 MHz.
Nearby codes
- 475 capacitor — 4.7 µF
- 565 capacitor — 5.6 µF
- 685 capacitor — 6.8 µF
- 825 capacitor — 8.2 µF
- 106 capacitor — 10 µF
- 126 capacitor — 12 µF
- 156 capacitor — 15 µF
- 186 capacitor — 18 µF
- 226 capacitor — 22 µF
- 276 capacitor — 27 µF