Self ballasted mercury vapor lamps were introduced in the 1940s, and soon after that these lamps found some success because they were more economical; they permit a simple retrofit in an existing incandescent fixture to brighter mercury lighting with low intial costs. No expensive fixture replacements nor retrofits were required. Just a lamp change, thats all! Since all HID lamps require ballasting to limit the current to the arc tube (because when an arc gets established, and the electrical resistance goes down as the arc warms up), even self ballasted lamps also require current limitations. But this is done by connecting the arc tube in series with an incandescent ballast filament, since the incandescent filament is a pure electrical resistance. That is why self ballasted mercury vapor lamps light up with an appearance of an incandescent lamp when turned on, and about 5 minutes is required for the arc tube to warm up. As the arc tube warms up one can note the shift in color from the yellowish incandescent light to a whiter appearance. Since the ballast filament also stays on after the arc tube has warmed up, it also contributes to the total light output, color temperature, and CRI (color rendering index). For this reason, color temperature ratings of self ballasted lamps are typically lower than standard mercury vapor lamps. Typical color temperature for clear lamps is 4000K, CRI is 40, and for phosphor coated lamps it is 3300K and CRI is 50. They have declined in popularity in recent years but a number of manfacturers still make them today, and typical wattages are as follows: 160, 250, 450, and 750 watts.
Self ballasted lamps are also made to operate on several different voltages. For reasons in the voltage requirements to strike the arc in the arc tube, several differences are required for lower voltage lamps (usually 100 and 120 volts) and higher voltage lamps (typically a range from 208 to 280 volts). As you will see in the pictures below, 100-120 volt lamps have preheating filaments next to the electrodes inside the arc tube and a thermostatic switch, while higher voltage lamps have a starting probe next to the electrode(s) (like an conventional mercury lamp) and a starting resistor.
The close up photo above is a 120V version of the self ballasted mercury lamp. You can see the preheating filament coils next to the main electrodes. And the bimetallic strip right to the left of one of the electrodes (with the numbers/letters on it) is the thermostatic switch. When the lamp is turned on, those preheating coils actually light up like an incandescent filament to heat up the inside so that an arc can be established. After that the ballast filament has warmed the switch to cause it to open and shut off the preheating coils in the arc tube. By then the arc has already established and will warm up, and reach full brightness in about 5 minutes.
The close up photo below is a 230V version of the self ballasted mercury lamp. You can see the starting probe next to one of the electrodes and a starting resistor. The more complex components of the 120 volt lamps are not needed in higher voltage versions because the higher voltage is sufficent to strike an arc across the electrodes in the arc tube. Also note the twin ballast filament coils, this is only typical of this particular lamp and is NOT a trait of high voltage lamps, 120 volt lamps with twin ballast filaments also exist (see photo of Luxor 750 watt).
