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An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the negative differential resistance artifact in the tube's voltage-current characteristic. Ballasts vary in design complexity. They can be as simple as a series resistor or inductor, capacitors, or a combination thereof or as complex as electronic ballasts used with fluorescent lamps and high-intensity discharge lamps. == Current limiting == Ballasts limit the current through an electrical load. These are most often used when a load (such as an arc discharge) has its terminal voltage decline when current through the load increases. If such a device were connected to a constant-voltage power supply, it would draw an increasing amount of current until it was destroyed or caused the power supply to fail. To prevent this, a ballast provides a positive resistance or reactance that limits the current. The ballast provides for the proper operation of the negative-resistance device by limiting current. A gas-discharge lamp is an example of a device which, under certain conditions, has ''negative differential'' resistance. In such a situation (after lamp ignition), every little increase in the lamp current tends to reduce the voltage "dropped" across it (supposing the lamp to be connected in series with other circuit elements). We indicate as dI and dV the variations for the current (I) and the voltage (V), so each variation can be positive (or negative) if its variable increases (or decreases). We give the name of differential resistance to the ratio between dV and dI, so it can be either positive or negative (and sometimes even null). This is quite a different concept from the resistance, which is always considered positive. In the case of a gas-discharge lamp, the differential resistance (i.e. dV / dI) really becomes negative because the positive variation for the current (dI) causes a negative variation for the voltage (dV) across the lamp. Ballasts can also be used simply to limit the current in an ordinary, positive-resistance circuit. Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system. Series resistors are used as ballasts to control the current through LEDs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Electrical ballast」の詳細全文を読む スポンサード リンク
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