I have recently acquired a NOS Philips SRS201 135W SOX lantern. I notice that the ballast circuit is different to that which I am used to in that there is a series capacitor (see the photo here (you might need to enlarge the photo to see the diagrams on the ballast and ignitor).

How does this circuit work compared to the usual type I have seen of either a leak transformer or a ballast in series with the lamp and a parallel ignitor..? This circuit does use a parallel ignitor, but I'm curious about the internal ballast wiring and the capacitor's function in that part of the circuit.

Your circuit appears to be totally different from my 135/180 w SOX lantern :https://www.lighting-gallery.net/gallery/displayimage.php?album=5198&pos=59&pid=158671 I know this doesn't answer your question since when it comes to SOX, you probably know 10 X more than I do !

I have recently acquired a NOS Philips SRS201 135W SOX lantern. I notice that the ballast circuit is different to that which I am used to in that there is a series capacitor (see the photo here (you might need to enlarge the photo to see the diagrams on the ballast and ignitor).

How does this circuit work compared to the usual type I have seen of either a leak transformer or a ballast in series with the lamp and a parallel ignitor..? This circuit does use a parallel ignitor, but I'm curious about the internal ballast wiring and the capacitor's function in that part of the circuit.

To me it looks more like a CWA: Beside the step up transformer function, the coil assembly provides series inductance.
This (partially) cancels out the capacitor impedance. The degree how much of the capacitive impedance is cancelled depend on the exact inductance and that depends on the passing current, higher current means greater degree of inductance saturation, so lower inductance, so less gets subtracted from the capacitive part, so the overall impedance gets higher, counteracting the higher current.
The whole setup is designed so the current matches the lamp specification.
In the coil assembly it is the magnetic shunt between primary and secondary, what saturates.
And the ignitor is just the HF generator, same as with any other SOX ballast.

Your circuit appears to be totally different from my 135/180 w SOX lantern :https://www.lighting-gallery.net/gallery/displayimage.php?album=5198&pos=59&pid=158671 I know this doesn't answer your question since when it comes to SOX, you probably know 10 X more than I do !

Your ballast is designed for both 120V and 240V supplies, so it looks a different design. Over here we're more used to simple series inductances as a ballast, or a leak transformer. The CWA design, if that's what mine is, is new to me and I'm not sure how it works.

Your lantern looks to be a slightly different model to mine, the mounting is different, but otherwise they look similar.

To me it looks more like a CWA: Beside the step up transformer function, the coil assembly provides series inductance.
This (partially) cancels out the capacitor impedance. The degree how much of the capacitive impedance is cancelled depend on the exact inductance and that depends on the passing current, higher current means greater degree of inductance saturation, so lower inductance, so less gets subtracted from the capacitive part, so the overall impedance gets higher, counteracting the higher current.
The whole setup is designed so the current matches the lamp specification.
In the coil assembly it is the magnetic shunt between primary and secondary, what saturates.
And the ignitor is just the HF generator, same as with any other SOX ballast.

Thanks, I'm not really familiar with the CWA design, and I'm wondering why it was used rather than the usual leak transformer or series inductance type ballasts we see here.

Someone has measured this kind of ballast: http://www.3d-meier.de/tut15/Seite504.html

Someone has measured this kind of ballast: http://www.3d-meier.de/tut15/Seite504.html

Thanks, my German is not what it could be, I'm going to have to work on it 

I’ve never took much notice of these non auto leak circuits before, but checking in the Philips catalogue (1991), it just shows the same circuit as the ballast, but sadly doesn’t explain how it works, what’s even more confusing though is the same circuit in the 1984 Philips catalogue shows the capacitor on the other side of the circuit (live side), this made more sense to me as then it works as a capacitor start circuit!
However, why it’s on the neutral side in the above circuit, I’ll admit I have no clue?, it’s certainly an interesting one?

I’ve never took much notice of these non auto leak circuits before, but checking in the Philips catalogue (1991), it just shows the same circuit as the ballast, but sadly doesn’t explain how it works, what’s even more confusing though is the same circuit in the 1984 Philips catalogue shows the capacitor on the other side of the circuit (live side), this made more sense to me as then it works as a capacitor start circuit!
However, why it’s on the neutral side in the above circuit, I’ll admit I have no clue?, it’s certainly an interesting one?

It is just in series with the lamp. It does not matter in which wire it is. With US HID that use Edison screw cap the safety requires the shell to be grounded, what leaves only the live side for the capacitor (although on ballasts using an ignitor the capacitor actually is on the "colder" side of the secondary (where the secondary is connected to the primary).

And it does not matter for the function, if the capacitor is upstream (towards the mains) or downstream (towards the lamp) of the ballast, it only has to match the impedance there (primary side requires higher value, but lower voltage rating). However whe  placed at the secondary, the gap in the main magnetic loop provides free power factor phase compensation, with the primary handling only real power. So the reason to put it on the primary could have been either suffice with lower voltage rating (it was a challenge, whe the PCBs needed to be avoided in the oil), or  maybe just a way around some CWA patents.


And to why the CWA? The main reason is its current regulation feature, which makes the system very robust against mains voltage fluctuation. Standard HX or series choke allow for barely 5% mains voltage variation when to stay within 10% lamp current spec range, the CWA allows more than 15% mains tolerance while still staying within the 10% lamp current limits. It depends only on how high voltage you allow across the capacitor, so how expensive, heavy and lossy your ballast could be (these things are traded off, whe  going to wider voltage tolerance design)...

Thanks for that explanation, Medved 

It all makes sense now, some of it 

Ah yes, if anyone will know, Medved will  , he’s always helped most of us on LG at one time or another!

Ah yes, if anyone will know, Medved will  , he’s always helped most of us on LG at one time or another!

Indeed, thanks again