Editing Fluorescent lamp (section)

====Semi-resonant start====
[[File:65W SRS fluorescent lamp starting.ogv|thumb|left|A 65-watt fluorescent lamp starting on a semi-resonant start circuit]]
[[File:semi resonant start circuit.png|thumb|right|A semi-resonant start circuit diagram]]
The semi-resonant start circuit was invented by Thorn Lighting for use with [[fluorescent-lamp formats|T12]] fluorescent tubes. This method uses a double wound transformer and a capacitor. With no arc current, the transformer and capacitor [[electrical resonance|resonate]] at line frequency and generate about twice the supply voltage across the tube, and a small electrode heating current.<ref>Thorn Lighting Technical Handbook</ref> This tube voltage is too low to strike the arc with cold electrodes, but as the electrodes heat up to thermionic emission temperature, the tube striking voltage falls below that of the ringing voltage, and the arc strikes. As the electrodes heat, the lamp slowly, over three to five seconds, reaches full brightness. As the arc current increases and tube voltage drops, the circuit provides current limiting.

Semi-resonant start circuits are mainly restricted to use in commercial installations because of the higher initial cost of circuit components. However, there are no starter switches to be replaced and cathode damage is reduced during starting making lamps last longer, reducing maintenance costs. Because of the high open circuit tube voltage, this starting method is particularly good for starting tubes in cold locations. Additionally, the circuit power factor is almost 1.0, and no additional power factor correction is needed in the lighting installation. As the design requires that twice the supply voltage must be lower than the cold-cathode striking voltage (or the tubes would erroneously instant-start), this design cannot be used with {{nowrap|240 volt}} AC power unless the tubes are at least {{convert|1.2|m|abbr=on}} length.  Semi-resonant start fixtures are generally incompatible with energy saving T8 retrofit tubes, because such tubes have a higher starting voltage than T12 lamps and may not start reliably, especially in low temperatures. Recent proposals in some countries to phase out T12 tubes will reduce the application of this starting method.