396 capacitor value
39 µF
39,000,000 pF · 39000 nF · 39 µF
| Marked code | 396 |
| Picofarads | 39,000,000 pF |
| Nanofarads | 39000 nF |
| Microfarads | 39 µF |
| Farads | 3.90e-5 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 × 106 = 39,000,000 pF
So 396 is 39 followed by 6 zeros: 39,000,000 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 396 is stocked as 39 µF and the two numbers look nothing alike.
What 39 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 39 µF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 81.62 Ω |
| 1 kHz — audio | 4.08 Ω |
| 100 kHz — switching | 40.8 mΩ |
| 10 MHz — RF | 0.4 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 396K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 39 µF means |
|---|---|---|
396J | ±5% | 37.05 µF to 40.95 µF |
396K | ±10% | 35.1 µF to 42.9 µF |
396M | ±20% | 31.2 µF to 46.8 µF |
396Z | +80% / −20% | 31.2 µF to 70.2 µF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 396Z can legitimately be anything from 31.2 µF to 70.2 µF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 39 µF turns up
Microfarads and up is bulk energy storage: holding a rail up between switching cycles, smoothing a supply, or coupling a signal that goes down to a few hertz. Above about 10 µF ceramics give way to electrolytics and tantalums, which is why the code runs out here.
Frequently asked questions
What is the value of a 396 capacitor?
39 µF — 39,000,000 pF · 39000 nF · 39 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 396 is 39 followed by 6 zeros = 39,000,000 pF.
What is a 396 capacitor in µF?
39 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 39,000,000 pF, 39000 nF and 39 µF are the same capacitor written three ways.
What does the letter after 396 mean?
Tolerance. 396K is ±10%, 396J is ±5% and 396M is ±20% — so a 396K is anywhere from 35.1 µF to 42.9 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 396 capacitor at 1 kHz?
4.08 Ω. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 40.8 mΩ at 100 kHz and 0.4 mΩ at 10 MHz.
Nearby codes
- 476 capacitor — 47 µF
- 566 capacitor — 56 µF
- 686 capacitor — 68 µF
- 826 capacitor — 82 µF
- 108 capacitor — 0.1 pF
- 128 capacitor — 0.12 pF
- 158 capacitor — 0.15 pF
- 188 capacitor — 0.18 pF
- 228 capacitor — 0.22 pF
- 278 capacitor — 0.27 pF