393 capacitor value
39 nF
39,000 pF · 39 nF · 0.039 µF
| Marked code | 393 |
| Picofarads | 39,000 pF |
| Nanofarads | 39 nF |
| Microfarads | 0.039 µF |
| Farads | 3.90e-8 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 × 103 = 39,000 pF
So 393 is 39 followed by 3 zeros: 39,000 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 393 is stocked as 0.039 µF and the two numbers look nothing alike.
What 39 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 39 nF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 81.62 kΩ |
| 1 kHz — audio | 4.08 kΩ |
| 100 kHz — switching | 40.81 Ω |
| 10 MHz — RF | 408.1 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 393K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 39 nF means |
|---|---|---|
393J | ±5% | 37.05 nF to 40.95 nF |
393K | ±10% | 35.1 nF to 42.9 nF |
393M | ±20% | 31.2 nF to 46.8 nF |
393Z | +80% / −20% | 31.2 nF to 70.2 nF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 393Z can legitimately be anything from 31.2 nF to 70.2 nF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 39 nF turns up
Tens of nanofarads is coupling between audio stages and the smaller decoupling capacitors on a logic board. A 10 nF part alongside a 100 nF one is a common pairing, the smaller of the two handling the frequencies the larger one is too slow for.
Frequently asked questions
What is the value of a 393 capacitor?
39 nF — 39,000 pF · 39 nF · 0.039 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 393 is 39 followed by 3 zeros = 39,000 pF.
What is a 393 capacitor in µF?
0.039 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 39,000 pF, 39 nF and 0.039 µF are the same capacitor written three ways.
What does the letter after 393 mean?
Tolerance. 393K is ±10%, 393J is ±5% and 393M is ±20% — so a 393K is anywhere from 35.1 nF to 42.9 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 393 capacitor at 1 kHz?
4.08 kΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 40.81 Ω at 100 kHz and 408.1 mΩ at 10 MHz.
Nearby codes
- 473 capacitor — 47 nF
- 563 capacitor — 56 nF
- 683 capacitor — 68 nF
- 823 capacitor — 82 nF
- 104 capacitor — 100 nF
- 124 capacitor — 120 nF
- 154 capacitor — 150 nF
- 184 capacitor — 180 nF
- 224 capacitor — 220 nF
- 274 capacitor — 270 nF