Editing Fluorescent lamp (section)

===Ballasts ===
{{main|Electrical ballast}}
[[File:LAjaHID.jpg|thumb|Different ballasts for fluorescent and discharge lamps]]
Fluorescent lamps are [[negative differential resistance]] devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing for even more current to flow. Connected directly to a [[constant-voltage power supply]], a fluorescent lamp would rapidly self-destruct because of the uncontrolled current flow. To prevent this, fluorescent lamps must use a [[electrical ballast|ballast]] to regulate the current flow through the lamp.

The terminal voltage across an operating lamp varies depending on the [[electric arc|arc]] current, tube diameter, temperature, and fill gas.  A general lighting service {{Convert|48|in|mm|adj=on|abbr=out|sigfig=4|sp=us}} T12<ref>T12 specifies the bulb's diameter in 1/8 inch units; a T12 bulb is 12×(1/8)&nbsp;inches or {{Convert|1.5|in|mm|abbr=on|sp=us}} in diameter.</ref> lamp operates at 430&nbsp;mA, with 100 volts drop. High-output lamps operate at 800 mA, and some types operate up to 1.5&nbsp;A. The power level varies from 33 to 82 watts per meter of tube length (10 to 25 W/ft) for T12 lamps.<ref name=GE1978>General Electric, ''Fluorescent Lamps Technical Bulletin TP 111R'', December 1978</ref>

The simplest ballast for [[alternating current]] (AC) use is an [[inductor]] placed in series, consisting of a winding on a laminated magnetic core. The [[inductance]] of this winding limits the flow of AC current. This type of ballast is common in 220–240V countries (And in North America, up to 30W lamps). Ballasts are rated for the size of lamp and power frequency. In North America, the AC voltage is insufficient to start long fluorescent lamps, so the ballast is often a step-up [[autotransformer]] with substantial [[leakage inductance]] (to limit current flow). Either form of inductive ballast may also include a [[capacitor]] for [[power factor]] correction.
[[File:New GE Ballast.JPG|thumb|230&nbsp;V [[electrical ballast|ballast]] for 18–20&nbsp;W]]

Fluorescent lamps can run directly from a [[direct current]] (DC) supply of sufficient voltage to strike an arc. The ballast must be resistive, and would consume about as much power as the lamp. When operated from DC, the starting switch is often arranged to reverse the polarity of the supply to the lamp each time it is started; otherwise, the mercury accumulates at one end of the tube. Fluorescent lamps are (almost) never operated directly from DC for those reasons. Instead, an [[inverter (electrical)|inverter]] converts the DC into AC and provides the current-limiting function as described below for electronic ballasts.