I asked a while ago about the specifics of SRS ballasting, and got a little bit of useful info. Since then I have looked at plenty of resources, but I have more questions. Here is a schematic of an SRS fluorescent ballasting circuit:

Here are my questions about it:

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1) Construction Specifics:
The iron core shown has two isolated windings. I have heard this called a "double choke" before. In my eyes this is implying that both windings are equal. Are both windings the same? If not, what is the approximate turns ratio?

2) Working Principle:
I have tried to get a grasp on how this is supposed to work, but I am really having a problem. Can someone explain?

3) Abbreviation Meaning:
I think I heard somewhere that SRS stands for "Semi-Resonant Start", but I don't know for sure because everyone always just calls is "SRS". What does it mean?

---

Thanks!

I asked a while ago about the specifics of SRS ballasting, and got a little bit of useful info. Since then I have looked at plenty of resources, but I have more questions. Here is a schematic of an SRS fluorescent ballasting circuit:

Here are my questions about it:

---

1) Construction Specifics:
The iron core shown has two isolated windings. I have heard this called a "double choke" before. In my eyes this is implying that both windings are equal. Are both windings the same? If not, what is the approximate turns ratio?

2) Working Principle:
I have tried to get a grasp on how this is supposed to work, but I am really having a problem. Can someone explain?

3) Abbreviation Meaning:
I think I heard somewhere that SRS stands for "Semi-Resonant Start", but I don't know for sure because everyone always just calls is "SRS". What does it mean?

---

Thanks!

Deja Vu?

https://www.lighting-gallery.net/index.php?topic=17737.0

did you not read the awesome technical pocket booklet I linked in that thread? that explains the fundamental theory/working of Semi-resonant-start quite well


(tho does not go into turns ratios or things like that)

@LightBulbFun
Yea I was going to go back to that, but the website no longer works. I would like to make my own SRS setup which is why I am asking for more specific details now.

@LightBulbFun
Yea I was going to go back to that, but the website no longer works. I would like to make my own SRS setup which is why I am asking for more specific details now.

ahh well if only someone had the forsight to save such a valulable resource to their computer for safe keeping, so it could be reuploaded somewhere else should the original source go down


if only...






https://www.lighting-gallery.net/gallery/albums/userpics/10476/Atlas_Technical_Pocket_Book_1966-7.pdf



 

Thanks for sharing that @LightBulbFun!

So a series resonant circuit can have higher voltages across the inductor and the capacitor, but when because they are phased differently they add up to the voltage put across them. That makes plenty of sense. But there isn't just a capacitor across the tube or just an inductor, there are both in series, so shouldn't it add up to the supply voltage and no higher? How does it make higher voltage across the tube?

I know it works, and I know higher voltages are produced, I just don't get how. No resource that I have read has explained that.

When the lamp has no discharge, the capacitor current flows via one winding one direction and via the other the opposite direction. So their magnetic fields cancel out, except a small magnetic leakage between the windings.
So at the end it is only the small inductance, related to that leakage, which can boost the voltage at least a bit. The thing operates way below the resonance of the capacitor and the leakage inductance (which is the only inductance effectively in series with the cap), so the voltage boost barely makes for the resistive drop on the winding wire resistance. So it is not wrong to say the capacitor has just the mains voltage on it. And that includes also the state when the lamp has ignited, the coupled windings are enforcing that.

But for T12 tubes no voltage boost is actually needed, these ignite on the bare 220V (and above) mains, so the only thing needed is to heat up the cathodes. And the capacitor current does just that, until the lamp ignites.
Interesting thing happens after the lamp ignites: The capacitor has still the mains voltage on it (plus minus some small vlotage boost on leakage inductance and resistive drop), similar voltage being on the mains side. That means to quite a big extend the total lamp current splits to these two components, so big part being pushed from both ends of the filament to the arc itself, so quite reducing the resistive heating the lamp filaments experience. It does not cancel out completely, the 90deg phase shift and the different current in each branch won't allow that, but still the current on each of the filament end is less than the total arc current.
Compare to the preheat or even the very common current mode preheat HF output (mean electronic ballasts) circuit, where all the current flows from just one filament side.

Thanks for sharing that @LightBulbFun!

So a series resonant circuit can have higher voltages across the inductor and the capacitor, but when because they are phased differently they add up to the voltage put across them. That makes plenty of sense. But there isn't just a capacitor across the tube or just an inductor, there are both in series, so shouldn't it add up to the supply voltage and no higher? How does it make higher voltage across the tube?

I know it works, and I know higher voltages are produced, I just don't get how. No resource that I have read has explained that.

if I am reading the book right it mentions that effectively the voltage of the capacitor gets added onto the voltage of the secondary winding

When the lamp has no discharge, the capacitor current flows via one winding one direction and via the other the opposite direction. So their magnetic fields cancel out, except a small magnetic leakage between the windings.
So at the end it is only the small inductance, related to that leakage, which can boost the voltage at least a bit. The thing operates way below the resonance of the capacitor and the leakage inductance (which is the only inductance effectively in series with the cap), so the voltage boost barely makes for the resistive drop on the winding wire resistance. So it is not wrong to say the capacitor has just the mains voltage on it. And that includes also the state when the lamp has ignited, the coupled windings are enforcing that.

But for T12 tubes no voltage boost is actually needed, these ignite on the bare 220V (and above) mains, so the only thing needed is to heat up the cathodes. And the capacitor current does just that, until the lamp ignites.
Interesting thing happens after the lamp ignites: The capacitor has still the mains voltage on it (plus minus some small vlotage boost on leakage inductance and resistive drop), similar voltage being on the mains side. That means to quite a big extend the total lamp current splits to these two components, so big part being pushed from both ends of the filament to the arc itself, so quite reducing the resistive heating the lamp filaments experience. It does not cancel out completely, the 90deg phase shift and the different current in each branch won't allow that, but still the current on each of the filament end is less than the total arc current.
Compare to the preheat or even the very common current mode preheat HF output (mean electronic ballasts) circuit, where all the current flows from just one filament side.

there is certainly some above-mains-voltage produced by an SRS ballast on a couple 4ft 40W examples I have tested their OCV is about 280V on a Parmar 5ft-8ft 65W-85W one I tested it was about 315V

@Medved

So you have a transformer with two identical windings (X-Y and Y-Z) connected in series so that their total measured inductance is effectively zero across X-Z. When you apply current across X-Z, the center node Y will gain voltage relative to the points X and Z (assuming X and Z are at equal voltage)? That sort of makes sense, but anything after that I am lost.

I have a few Eltam SR40 on hand and could make some measurements if you would like, though you will have to wait until i have a window of free time

@Ash
Thanks so much! I am certainly not in a rush.

@Medved

So you have a transformer with two identical windings (X-Y and Y-Z) connected in series so that their total measured inductance is effectively zero across X-Z. When you apply current across X-Z, the center node Y will gain voltage relative to the points X and Z (assuming X and Z are at equal voltage)? That sort of makes sense, but anything after that I am lost.

Yes, assuming the transformer has ideal coupling, so magnetic flux is exactly the same through both windings, so none could leak between them.

In reality you get some nonzero inductance, because the magnetic flux generated by one winding can squeeze without going through the other winding, this property of a transformer uses to be called "leakage inductance". How much it is depends on the exact arrangement. It could be few percents, or even 20..30% if there is really a space between both coils and/or even a magnetic shunt (a piece of iron core designed to provide that path).
With standard transformer you want that to be as low as possible, but with SRS some leakage inductance is beneficial, as it boosts the voltage across the lamp for ignition above the mains voltage.

Same result could be achieved with some intentional assymetry between the 2 windings in a standard low-leakage transformer, wouldn't it ?

I haven't come around to checking out my SRS ballast, however i do wonder. The old Eltam SRS ballasts were based on Mini U core and winding formats. I don't know whether they even had the ability to make a high leakage transformer there, as was added in the B series

If the winding would be assymetrical, then it would matter which winding is connected where. But as I understood, the wiring diagrams do not explicitely mention that to matter, so my guess was they are symmetrical.
Plus SRS was developed in the T12 time and T12 were happy with about 200V for ignition with warm electrodes, so you don't need any voltage boost there, even with 220V nominal mains.
And I doubt the design won't prefer some extra "poka-yoke", when there isn't anything explicite to not allow it.

This damn circuit is quite hard to understand, despite being that simple. Vector diagrams in Atlas book from @LightBulbFun help a bit.

Despite a loud name, not much resonance is going on here. For a simplified variant, one can ignore filament resistance and assume the windings are the same with 100% coupling. That way, on startup, the circuit degenerates just to two filaments in series with a capacitor, powered from mains, and in working mode, (surprise!) to just a trivial lag single choke circuit with a PFC capacitor across THE MAINS (!). A classic circuit theory approach where you are allowed to mentally connect the nodes with the same potential helps with the analysis.

From Atlas diagrams, though the windings have some slight asymmetry, and sure coupling is not 100%, which will give a weak resonance kicking OCV up a little.

If the winding would be assymetrical, then it would matter which winding is connected where. But as I understood, the wiring diagrams do not explicitely mention that to matter, so my guess was they are symmetrical.
Plus SRS was developed in the T12 time and T12 were happy with about 200V for ignition with warm electrodes, so you don't need any voltage boost there, even with 220V nominal mains.
And I doubt the design won't prefer some extra "poka-yoke", when there isn't anything explicite to not allow it.

only checked one of my SRS ballasts so far, a Multi-wattage Parmar 5ft-8ft 65-85W jobby

https://www.lighting-gallery.net/gallery/displayimage.php?album=lastupby&cat=0&pos=27&pid=261057&uid=476#msg889328

and as you can see from my comment in the upload above, the windings are not of the same DC resistance so I would go out on a limb and say they are asymmetrical

This damn circuit is quite hard to understand, despite being that simple. Vector diagrams in Atlas book from @LightBulbFun help a bit.

Despite a loud name, not much resonance is going on here. For a simplified variant, one can ignore filament resistance and assume the windings are the same with 100% coupling. That way, on startup, the circuit degenerates just to two filaments in series with a capacitor, powered from mains, and in working mode, (surprise!) to just a trivial lag single choke circuit with a PFC capacitor across THE MAINS (!). A classic circuit theory approach where you are allowed to mentally connect the nodes with the same potential helps with the analysis.

From Atlas diagrams, though the windings have some slight asymmetry, and sure coupling is not 100%, which will give a weak resonance kicking OCV up a little.

I find it interesting that claims are made that there is no or little resonance going on when the atlas technical booklet explicitly mentions that they work by resonance to boost voltage for lamp starting

in particular what do you say about the comment in the technical booklet about the voltage rise coming from the adding of Vc to Vs? 



also should be noted SRS ballasts for 18W SOX lamps where quite common (as they officially require about 300V to strike, so an SRS ballast was seen as a good way to generate that in a compact low-loss-ish ignitor-less setup that also has the added benefit of inherent PFC correction)


https://www.lighting-gallery.net/gallery/displayimage.php?pos=-116224


(I was always a bit annoyed how they tied the coils together into a single terminal like that, so you cant use it with a regular 2ft tube or 18W-24W PL-L or 28W 2D if you wanted to LOL)