I opened up a florescent bulb replacement for the standard 100 watt light bulb. These are cheap and common now. I had the bulb on a dimmer circuit, so it appears something overheaded.
I see it has a full wave recification on the front-end, some type of small flyback transformer, a couple of SCRs or FET power devices, etc.
Does anyone have a site or copy of a complete typical circuit?
Interested in how it works for my own amusement.
Rgrds,
pdrunen
martin
You couldn't even read the instructions of the bulb that told you not to put it on a dimmer, and we're supposed to tell you how an off-line switcher works?
Essentially, you convert the mains voltage to DC with a bridge and reservoir cap, then chop the DC at high frequency, so you can use a much smaller inductor for current control than you could at mains frequency.
A capacitor replaces the starter used for mains operation, forming a series resonant circuit with the inductor and the two heater elements in series. In the version I designed, I swept the frequency downward towards resonance, which causes the current through the heaters to increase, ionising the mercury vapour and also ramps up the voltage across the cap and thus the tube and eventually cause it to strike. Note you have to be careful at this stage that you don't allow the inductor to get close to saturation and included current limit that kicked the frequency back up away from resonance to deal with this.
Before the tube has struck, it is effectively open circuit. When the tube strikes, this places a resistive load across the cap that effectively removes it from circuit, the actual cap value being a compromise between reliable start-up and efficiency. Once the tube is illuminated, you can then dim it by increasing the frequency.
HTH
-- T If it\'s not broken, don\'t fix it.
Ian -- nice site. Thanks for the link.
James Arthur
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