40W Magnetic Ballast With Electronic Ignition

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Here is the schematic of another fluorescent ballast circuit from a lamp I acquired along with the 80W electronic ballast. It actually contains a pair of the circuit in the diagram, one for each bulb. The ballast itself is unlabeled, but the chassis sticker calls for 25, 32, or 40 Watt 48″ type T12 bulbs.
The circuit is essentially a line frequency operated magnetic ballast with an electronic igniter circuit in place of the standard glow switch. Unfortunately, I could not measure the inductance of L1 as I cannot afford an LCR meter. Does anyone want to donate one to my cause? I can tell you the size though which really means nothing if you were to try and build this thing, because your core may have a different permeability and Q factor than the one used here, and getting it wrong will damage your bulbs. At any rate, L1 is a laminated iron core inductor with x number of turns to give a reactance limiting the current flowing through the fluorescent bulb FL1. This is necessary because when gas (in this case mercury vapor) becomes ionized, its resistance drops to near zero. Higher temperatures reduce the resistance even further. Lower resistances draw more current and more current makes more heat. Without some kind of external current limitation, this would end in an explosion, and we all know how dangerous mercury is. Current can also be limited with a resistor, but inductors are preferred because a resistor converts the energy into heat, whereas an inductor turns it mostly into magnetism. The 4u7 capacitor is matched with L1 and helps balance out its reactance to make the load appear more resistive. Across it is a 330K resistor to drain any residual voltage when the circuit is switched off for safety reasons.

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This entry was posted on Thursday, April 3rd, 2008 at 1:05 pm and is filed under Lighting. You can follow any responses to this entry through the RSS 2.0 feed.