![]() In idling mode, it passes ca 700V (peak/peak) over to the fluorescent, in operation it is stressed with over 200V at 12 kHz. The secondary capacitor is the most critical part. But probably 160V rating would also do it here. The capacitor 0,68uF 400V is stressed with 12 to 30 kHz and in order to avoid undue dielectric losses it is a good idea to select a generous voltage rating. Its purpose is to generate a voltage divider for high-frequency but in the first moment of start (after taking its charge) it makes the 500 Ohm resistor dominant for oscillator start. The capacitor 1uF 50V could certainly be a tantalum type, and possibly also a high-quality electrolytic one. The resistor made of 2 x 1kOhm in parallel was formerly a single one of 470 Ohm, but became a bit too hot for a long-term reliable operation, so I used 2 x 1 kOhm = 500Ohm just for the thermal capacity. Look for fast switch transistors, current 2A, able to handle high frequencies. According to data sheets, also a 2N 4923 should work. I have tested also BD249C, they work well. I happened to have some transistors, which outperformed all others, but they are very exotic PNP complementary types of an even more exotic 2SC1306 high-frequency power amplifier. This needs a feedback voltage higher than the operating voltage, and so the feedback winding has a higher number of turns than the primary winding. The transistor has a resonant circuit in the emitter line and operates in common-collector mode. It works according to the starting method of 1.7 as explained above. The advantage is a self-start action without manual pushbuttons or external special starting circuits. Disadvantage is a certain loss in efficiency, as we (very simplified) at first generate a high voltage and then use only a part of it. Ionization wires can be applied in addition. After start, the voltage collapses from several hundred volts to the operating voltage, which is ca 70V for an 8W rod. At first, the high idling frequency and voltage pass to the lamp rather directly, as it is electrically inactive. A capacitor is put in series with the secondary winding. It would need either a manual start, such as a pushbutton with opening contact (NC), or an external relay circuit performing the same action automatically.ġ.7 The secondary winding is made to produce a rather high voltage but without spikes. The well-known glow-start cartridge is not applicable for small high-frequency inverters, it is for grid frequency and inductive ballast only.ġ.6 Referring to the schematic given here, if one would switch off the capacitor 0,68uF (in parallel to the primary winding), the oscillator would be no longer in sine-mode, but in flyback mode and produce voltage spikes which immediately start the fluorescent. The oscillator is a sine-mode oscillator and the idling voltage is high enough for start, but does not contain spikes. When the lamp has started, the tetrode is switched off. He used a rather exotic tyristor tetrode BRY20 to switch the filaments in series to the whole secondary winding for effective pre-heating. Such one is described in the book of Nührmann, “Professionelle Schaltungstechnik” Issue 2. Possible, promising, but unusual.ġ.5 Starter circuit on the lamp side. The winding can be switched off in operation, or can be supplied by a separate starting oscillator, which is switched of as a whole. We can regard this measure as a cheap and easy additional trick, but it does not make a breakthrough, is not alone able to start.ġ.4 Ionization wire plus additional special high-voltage winding. This wire acts only where the potential difference is, it ionizes around the opposite voltage filament. But an additional single big voltage spike, (for example manually shortening the secondary winding just for a few milliseconds by pushbutton) will start the lamp.ġ.3 Ionization wire along the lamp. You get only either one of these two starting aids. The problem is that this heating power suppresses the voltage spikes. High level of radiation (EMC).ġ.2 Heating the filaments by suitable transformer windings. Disadvantage: One filament evaporates and blackens the lamp, makes it electrically unsymmetric and shortens the lifetime. Here are 7 starting methods:ġ.1 Pure flyback single-transistor inverter, makes voltage spikes in the kilovolt range in idling condition, so that the lamp will start. The 12V drivers for fluorescent lamps are tricky, because of the compromise between a good operating efficiency and the ability to start the lamp.
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