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Preheating circuit for self-oscillating ballast

Preheating circuit for self-oscillating ballast

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About 8 years ago I've made many experiment on electronic self-oscillating ballast found in cfls. Two main problems I had when I was reusing ballast of EOL cfl for other fluorescent lamps:
1- The absence of an EOL protection caused many ballast to fail when the lamps go EOL or when the lamps is removed and the ballast continue to oscillate (due for example to the capacitance of the wires).
2- The ptc resistor used for preheating is matched with the cfl tube, if we connect a different tube(a T8 for example) very often the preheating current is not enought and the lamp starts in cold mode.

The image shows a circuit that can be used in place of the two capacitor plus the ptc resistor. Pin1 and Pin2 should be connected like a starter in a normal magnetic ballasted system.
C3 is shunted during the preheat time, so the resulting capacity is about equal to the value of C4. After the preheating time the capacity is reduced to C3//C4. By chosing the correct value for C3 and C4 we can adjust the preheat current.
GR1 rectify the AC voltage.
C2 transform the pulsed voltage to DC.
D4 discharge C2 and C3 when the lamp is switched off.
R3-C1 set the preheat time. Z1 limits the maximum voltage on C1 and on the gate of T3, Z2 on the gate of T1 and on the drain of T3(T3 can be a low voltage mosfet).
T3 invert the the signal and drive T1 (the main switch of the preheating).
Z3 and T2 forms a constant voltage drop, needed to keep a minium of voltage on the circuit when T1 is closed (in the preheating time).

I've implemented this circuit with a few different ballast (cfl or cheap instant start cinese ballast)and it's work very well.

Recently I've opened an Osram QT for 18W lamps (it's based on a self-oscillatin circuit) and I was pleased because they used a very similar circuit both for preheating and for the EOL protection (see the other circuit).

osramdulux.jpg youtube_MAArf1G1QVA.jpg STARTE2MOSSCH.JPG CIRCOLUX_CON_PROTEZIONE.JPG

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Manufacturer:Kappa7 :-)

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Filename:STARTE2MOSSCH.JPG
Album name:Kappa7 / Electronic ballast circuits
Keywords:Gear
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Date added:Mar 19, 2011
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Medved
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Mar 19, 2011 at 05:02 PM Author: Medved
This remind me of my dimming experiments few years ago:
Similar shorting switch, but driven from a 555 circuit at ~200..300Hz with variable duty.
It or heat up electrodes or let the lamp light. After a bit of ballast fiddling (clamps across filaments) it was working well, but only with local pot control. Making the control remote was becoming too complex, so i started experimenting with ballast controllers designed for fluorescent dimming. But need to say, no one was able to beat that 555 based circuit for really wide dimming range with excellent arc stability.

No more selfballasted c***

Kappa7
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Mar 19, 2011 at 05:13 PM Author: Kappa7
Nice experiment, I think thats this is the only way to dim with an self-oscillating ballast.
Normaly dimming ballast need a control loop to operate because of the instability of the lamp arc. The current is regulated by shifting the frequency of the oscillator.
Medved
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Mar 19, 2011 at 05:41 PM Author: Medved
For dimming my present favorite is the IRS2530, as it is quite simple and very robust ballast design (does not need any accurate components, except few resistors in the level setting/feedback).

No more selfballasted c***

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