A capacitor stores a little charge and lets it go fast. Here is how they work.
A capacitor stores electrical charge between two plates separated by an insulator. It charges up when voltage is applied and can release that charge quickly.
1. Units
Capacitance is measured in farads (F). Most capacitors are much smaller:
- µF (microfarad) = one millionth of a farad.
- nF (nanofarad) = one thousandth of a µF.
- pF (picofarad) = one thousandth of a nF.
2. What they're used for
- Smoothing the ripple out of a power supply.
- Filtering electrical noise.
- Timing circuits, with a resistor setting the time.
- Starting and running some AC motors.
3. Polarity
Electrolytic capacitors are polarised. The negative leg is marked with a stripe and is usually shorter. Fitted backwards they can overheat, bulge or burst. Ceramic and film capacitors aren't polarised.
4. Voltage rating
Every capacitor has a maximum voltage. Use one rated comfortably above the highest voltage it will see.
5. Time constant
With a resistor in series, a capacitor charges to about 63% in one time constant (R × C) and is close to full after about five. Work it out with the RC Time Constant Calculator.
Large capacitors can hold a dangerous charge after the power is off, especially in mains equipment, microwaves and inverters. Don't open mains equipment. Repairs to 240V AC equipment are a job for a qualified repairer.
6. Quick summary
- Capacitors store charge.
- Electrolytics have polarity. Watch the stripe.
- Use a voltage rating above the circuit voltage.
- Treat big capacitors as live until proven safe.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician. Low-voltage DC can still deliver very high current, so treat batteries with respect.
