Basics
A capacitor stores separated electric charge. The amount of charge stored for a given voltage is its capacitance:
- C is capacitance in farads.
- Q is charge in coulombs.
- V is voltage in volts.
Units
| Unit | Value | Example |
|---|---|---|
| farad (F) | 1 F | Large for ordinary electronics. |
| microfarad (µF) | 10−6 F | Power filtering, audio coupling. |
| nanofarad (nF) | 10−9 F | Filters and timing. |
| picofarad (pF) | 10−12 F | RF and small-signal circuits. |
Voltage rating
The voltage rating is not the voltage a capacitor automatically produces; it is a limit on the voltage that may be applied. Designers normally leave margin for supply tolerance, ripple, transients, temperature, aging, and the rules of the particular capacitor technology.
Capacitors in parallel
Parallel capacitors have the same voltage across them and their capacitances add:
Capacitors in series
Series capacitors carry equal charge. Their total capacitance is lower than the smallest individual capacitance:
For two capacitors:
Charging through a resistor
A resistor in series with a capacitor limits charging current. The product R × C is the time constant:
After one time constant, an initially discharged ideal capacitor charged from a fixed source has reached about 63% of its final voltage. After about five time constants it is very close to the final value.
τ = 10,000 × 0.0001 = 1 second
DC and AC behavior
After an ideal capacitor has finished charging from a steady DC source, no continuous DC current flows through its dielectric. A changing voltage, however, produces current. This is why capacitors can pass changing signals while blocking a steady DC level.
Capacitive reactance
As frequency rises, capacitive reactance falls. A capacitor therefore presents a lower impedance to faster changes—until real-world parasitic inductance and losses become important.
XC ≈ 159 Ω
Energy storage
Stored energy increases directly with capacitance and with the square of voltage. Doubling voltage stores four times as much energy in the same capacitance.
Common circuit jobs
Decoupling
A capacitor near an integrated circuit supplies short bursts of local current and provides a low-impedance path for high-frequency noise. Small ceramic capacitors are commonly used close to device power pins, often with larger bulk capacitors elsewhere on the rail.
Coupling
A series capacitor can transfer an AC signal between amplifier stages while blocking a difference in DC bias voltage.
Filtering
Capacitors and resistors can form low-pass and high-pass networks. Power supplies use capacitors to reduce ripple, while signal circuits use them to shape frequency response.
Timing
The predictable charge and discharge of an RC network is used in delays, oscillators, pulse shaping, debouncing, and many simple timing circuits.