Electronic (also called "solid state") ballasts contain semiconductors and other electronic components. Electronic ballasts are like the switching power supplies you find in computers, creating a chopped electrical current with up to 50,000 pulses of electricity supplied to the lamps per second instead of the 100 or 120 pulses per second produced by a magnetic ballast. (In computers, that chopped power is eventually rectified and regulated to the desired voltages needed by the computer.)
When the lamp is cold and an electronic ballast is being used, the pulses of current sent to the lamps by the ballast are more frequent or last longer, and as the lamp reaches the desired operating current consumption, the electronic ballast provides shorter or more infrequent pulses of current to the lamp. Solid state ballasts are a fairly recent innovation.
As with Magnetic ballasts, the Electronic ballast is more complicated in Rapid-Start fixtures, requiring additional lower voltage outputs to keep all filaments heated at all times, and to impress a high voltage difference between lamp fixture and cathode voltages. These voltages are obtained from additional sets of windings in the switching power supply transformer, in the same way that a computer switching power supply has multiple sets of windings and each set is used to produce a specific voltage required.
Electronic ballasts are more power-efficient than magnetic ballasts, are lighter, and can typically be squeezed into smaller spaces. Almost all compact fluorescent lamps use an electronic ballast.
Electronic ballasts have a further advantage in that they are virtually silent. They do not produce that low frequency "hum" produced by magnetic ballast fixtures. All electronic ballasts have a Sound Rating of "A", but they are actually quieter than a compatible magnetic ballast that also has a Sound Rating of "A". In fact, a fixture containing an electronic ballast actually does produce some noise because there still is a transformer inside the switching power supply in the ballast, but because it and the lamps operate at higher frequencies than found in a fixture with a magnetic ballast, what sound does escape the enclosure of an electronic ballast fixture is usually well above the human hearing range.
There are some negatives to electronic ballasts. They are currently a lot more expensive, they don't survive power surges and overloads or high temperatures as well magnetic ballasts, they can interfere with radio and television equipment due to emissions from the power switching electronics, and the higher flicker rate can interfere with infra-red remote control systems used in televisions, VCRs and other devices.
