390 capacitor value
39 pF
39 pF · 0.039 nF · 0.000039 µF
| Marked code | 390 |
| Picofarads | 39 pF |
| Nanofarads | 0.039 nF |
| Microfarads | 0.000039 µF |
| Farads | 3.90e-11 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 × 100 = 39 pF
So 390 is 39 followed by 0 zeros: 39 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 390 is stocked as 0.000039 µF and the two numbers look nothing alike.
What 39 pF 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 pF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 81.62 MΩ |
| 1 kHz — audio | 4.08 MΩ |
| 100 kHz — switching | 40.81 kΩ |
| 10 MHz — RF | 408.09 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 390K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 39 pF means |
|---|---|---|
390J | ±5% | 37.05 pF to 40.95 pF |
390K | ±10% | 35.1 pF to 42.9 pF |
390M | ±20% | 31.2 pF to 46.8 pF |
390Z | +80% / −20% | 31.2 pF to 70.2 pF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 390Z can legitimately be anything from 31.2 pF to 70.2 pF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 39 pF turns up
Tens of picofarads is the crystal load range: the two capacitors either side of a quartz crystal that set it oscillating at its marked frequency. Also RF tuning and the compensation capacitor round a fast op-amp.
Frequently asked questions
What is the value of a 390 capacitor?
39 pF — 39 pF · 0.039 nF · 0.000039 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 390 is 39 followed by 0 zeros = 39 pF.
What is a 390 capacitor in µF?
0.000039 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 39 pF, 0.039 nF and 0.000039 µF are the same capacitor written three ways.
What does the letter after 390 mean?
Tolerance. 390K is ±10%, 390J is ±5% and 390M is ±20% — so a 390K is anywhere from 35.1 pF to 42.9 pF 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 390 capacitor at 1 kHz?
4.08 MΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 40.81 kΩ at 100 kHz and 408.09 Ω at 10 MHz.
Nearby codes
- 470 capacitor — 47 pF
- 560 capacitor — 56 pF
- 680 capacitor — 68 pF
- 820 capacitor — 82 pF
- 101 capacitor — 100 pF
- 121 capacitor — 120 pF
- 151 capacitor — 150 pF
- 181 capacitor — 180 pF
- 221 capacitor — 220 pF
- 271 capacitor — 270 pF