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In the magnetic ballast type fluorescent lamps (old ones), what is the need of a capacitor in the starter circuit and what determines its ratings?

If my understanding is correct, it's a bi-metallic strip opening and closing producing an inductive kick, so it should work fine without the capacitor too.

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  • \$\begingroup\$ What determines the ratings of starters is the wattage of the lamps. In the old fluorescent fixtures if you used a starter that wasn't properly rated for the lamps - the lamps would not light. \$\endgroup\$
    – Tinkerer
    Apr 24, 2016 at 11:43

2 Answers 2

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The capacitor in old Fluorescent Starters is for EMI suppression. This is typically a fairly-small value - anywhere between 1n to 100n, depending upon who made your particular starter.

The capacitor may also reduce contact erosion on the starter contacts - I honestly don't know. But I do know that in olden days when everyone had an AM radio sitting on the kitchen counter, you could immediately tell if someone turned on a Fluorescent lamp that didn't have that capacitor inside the starter.

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The capacitor is (in most common fluorescent lamp circuits) is for power factor correction. Since there is a coil in the ballast, the capacitor is used to bring the power factor back towards unity. Probably not such a big deal when you consider individual lamps in homes, but when you start looking at hundreds or thousands (aggregate of homes or a typical business), keeping a unity power factor is important. For business power, at least in my area, there is a steep penalty on power costs if the power factor is not close to unity.

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  • \$\begingroup\$ I don't think that power factor for household items was much of a concern back when these Fluorescent Starters were in common use. I'm pretty sure the value of the capacitor is so small that it won't have much effect on the power factor of the ballast. \$\endgroup\$ May 28, 2015 at 15:56
  • \$\begingroup\$ Drast is right though, in office buildings etc, fluorescent light fittings had capacitors to improve their power factor. But the question is about the tiny (10n?) capacitor in the starter, and I agree that's for EMI suppression as Dwyane says. \$\endgroup\$
    – tomnexus
    May 28, 2015 at 17:30
  • \$\begingroup\$ Gotcha. But the original question was regarding the capacitor in the starting circuit. \$\endgroup\$ May 28, 2015 at 18:54
  • \$\begingroup\$ Aye, I missed the starting circuit part.. its been a long time since I've had to deal with starters ! \$\endgroup\$
    – R Drast
    May 29, 2015 at 10:44

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