Units

1. Units • The energy stored in a capacitor is dependant on the capacitance and the square of the voltage developed across the plates. W = ½ CV2 Where: W is energy in joules C is capacitance in farads V is voltage across the plates in volts

2. Time constant relationship • The instant a voltage is applied to a capacitor there is no potential across it. As there is no potential across is there is little or no effective resistance in the capacitor. • If there is no effective resistance in the circuit then high levels of current will flow. • As the capacitor charges up the voltage across it increases (increasing the effective resistance) until the voltage across the capacitor equals the voltage applied to the capacitor. • At this point no current flows.

3. Time constant relationship • The time that the capacitor takes to charge up can be determined by the resistance and capacitance in the circuit. τ = CR Where: τ is 1 time constant in seconds C is capacitance in farads R is resistance in ohms

4. Time constant relationship

5. Capacitor symbols

6. Hazards • The dielectric cannot withstand an infinite amount of voltage placed across it. At some stage the dielectric will break down and a discharge current will occur when a certain level is present. • This level is different for different dielectrics. • This level can be determined by: Breakdown Voltage = V in Vm-1 d

7. Hazards • Electrolytic capacitors are polarised and should only be connected to the correct d.c. polarity (+ve or –ve) a.c. should never be used on electrolytic capacitors • Before working on a capacitor, any charge on the capacitor should be removed (discharged) via a “bleeder” resistor. • Capacitors contain sufficient energy to cause an electric shock and burns. These shocks may not be fatal but may startle and cause secondary accidents.

8. Hazards • Capacitors can contain chemicals and it’s possible to develop a charge even if it’s disconnected from a circuit. • Capacitors should have there terminals shorted together when in storage.

9. Supercapacitors • Electric double-layer capacitors, also known as supercapacitors, electrochemical double layer capacitors (EDLCs), or ultracapacitors, are electrochemical capacitors that have an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high capacity electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor will have a capacitance in the range of tens of millifarads. The same size electric double-layer capacitor would have a capacitance of several farads, an improvement of about two or three orders of magnitude in capacitance, but usually at a lower working voltage. Larger, commercial electric double-layer capacitors have capacities as high as 5,000 farads. The highest energy density in production is 30 Wh/kg. • Supercapacitors aim to fill the gap between capacitors and batteries.

10. Supercapacitors • Electric double-layer capacitors have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. Some of the earliest uses were motor startup capacitors for large engines in tanks and submarines, and as the cost has fallen they have started to appear on diesel trucks and railroad locomotives.More recently they have become a topic of some interest in the green energy world, where their ability to soak up energy quickly makes them particularly suitable for regenerative braking applications, whereas batteries have difficulty in this application due to slow charging rates. New technology in development could potentially make EDLCs with high enough energy density to be an attractive replacement for batteries in all-electric cars and plug-in hybrids, as EDLCs are quick charging and exhibit temperature stability. They can also be used in PC Cards, flash photography devices in digital cameras, portable media players, and in automated meter reading. • From Wikipedia, the free encyclopedia