Help figure out this power supply for uv lamp from water purification system

[crossorb93]

Just for fun I thought I would figure out the workings of this power supply for a consumer water purification device which features a ~30W uv lamp.  It failed and I got a new one so I'm free to take it apart.  Most of it seems straighforward but I have a couple of questions.

The PIC microcontroller is powered by a SOT23-3 chip marked "50K".  Anyone know what this might be?  It looks like it might be a voltage regulator.  It's powered from the AC line input through a large resistor and a diode.  The IR2153 is powered in a similar way but it has an internal zener that clamps vcc to 16V.  Would this little chip have something similar or have I missed something in the circuit?

The other question relates to the 4 connections to the lamp.  One wire gets the square wave from the power mosfets after smoothing by a 0.8mH inductor.  Another is ground.   The third and fourth wires are connected by a capacitor.  I don't understand why there are two wires going through the current sense transformer.  How does this bulb work and what is the function of the third and fourth wires?

[moffy]

The other question relates to the 4 connections to the lamp.  One wire gets the square wave from the power mosfets after smoothing by a 0.8mH inductor.  Another is ground.   The third and fourth wires are connected by a capacitor.  I don't understand why there are two wires going through the current sense transformer.  How does this bulb work and what is the function of the third and fourth wires?

I would guess that the tube has heater filaments, hence the 4 wires, two through the current transformer would double its sensitivity. I would also assume that it is resonant, hence the inductor and capacitor are producing something approaching a sine wave and the resonance or Q is producing the voltage multiplication.

[crossorb93]

Thanks, that gives me some reading to do.

With no starter, how does this system produces the initial large voltage spike to strike the arc?

[IanB]

I think some lamps don't necessarily need a high voltage to start. Heated filaments can emit enough electrons to start a discharge going without a high voltage across the tube. Once the discharge is operational, the heated filaments can be disconnected.

[moffy]

With no starter, how does this system produces the initial large voltage spike to strike the arc?

As IanB said also without the discharge the Q of the circuit should be higher, producing a higher starting voltage which would drop when the discharge starts, a negative resistance region or a case of current increasing with decreasing voltage.

[moffy]

The reason for the two wires through the current transformer is, Wire A carries the full load current and Wire B carries the resonant cap current, so my guess is they are subtracting the cap current from the load current to get only the tube current.