Author Topic: Capacitive Dropper Ballast  (Read 16864 times)
Keyless
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Re: Capacitive Dropper Ballast « Reply #15 on: July 26, 2017, 06:43:52 PM » Author: Keyless
Awesome, and epic  :circ:  ;D I can't thank you folks enough!   :)
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Re: Capacitive Dropper Ballast « Reply #16 on: August 23, 2017, 11:57:34 PM » Author: Keyless
Update: I tried the capacitor circuit and it works with a glow start ignitor. The bulbs are a bit flickery/dull, but over all I am really pleased. 240 volts gets them running every time, 120 is hit or miss with some of them. But anyways I am ready to try an electronic starter:


https://www.repairfaq.org/sam/flnl1.jpg

However, I have no idea what D6 and D5 are, or there rating. I had this light 12 years ago, but threw it out :( I see self ballasted CFLs use a cap across the heaters, but unsure how to size that. I did try one, but the lamp had a strong flickr and was 1/3 half lit- unless I removed the cap to which it would then go to full brightness.   
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Ash
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Re: Capacitive Dropper Ballast « Reply #17 on: August 24, 2017, 02:02:46 AM » Author: Ash
They are diacs. Their values are :

Breakover voltage - so that they dont fire when the lamp is lit, but do when the nightlight is plugged in and capacitor is empty. If you precharge the capacitor with half wave through a very big resistor, i think you might up their voltage a little

Current - The RMS current is probably around 2x the lamp rated current, but they handle quite bad impulses (shorting an oppositely charged capacitor to the line after every breakover), that in theory could reach Ipeak = line_voltage x 2 / resistance, where resistance is the 51 Ohm resistors and some little extra from the lamp filments

CFLs use a cap there as their ballast can change the output frequency. When preheating the frequency is high so the cap acts as a short. Then the frequency drops and so does the current in the cap. Not suitable for a circuit running at unchangeable and very low line frequency
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Keyless
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Re: Capacitive Dropper Ballast « Reply #18 on: August 24, 2017, 03:19:40 AM » Author: Keyless
So my best bet is a 100 volt diac on a 240 volt supply in terms of break down? Any reason the above schematic has them in series- or simply arbitrary? The current you are referring to- is this the current as the cathodes are heating up or are you referring to the IH, current that drops below its rated holding current causing the diac to stop conducting?


Tube starting theory is something I will admit I don't know all that much about it. My best guess is this: that once the diac is triggered by "line" voltage, the cathodes start to glow, and as they glow the tube voltage breaks down to the point it starts to suddenly conduct. As soon as the tube conducts, the low impedance causes current flow to drop through the diac thus switching it "off". Or do I need some device to open the diac after the preheat to allow striking first?

Thank you for explaining how a CFL does the job, I was not aware the open circuit frequency was higher. I know some CFLs also use a PTC (positive temperature coefficient resistor), would this be worth considering for fixed frequency application?

http://pro-radio.ru/user/uploads/239745.jpg   


One more starting idea came to mind that I might try; the rapid start approach. I could in theory place a 100k or higher value resistor from one lead of each cathode to the opposite lamp feed, this way having the electrodes always idling warm like a rapid start ballast. I would guess this is ok for T8 and T12 tubes as rapid start ballasts do it all the time, but for a T6 and under the I am not sure. What would your honest take be on such an approach? Never seen it done outside of a magnetic ballast (low volt dedicated heater windings)- but I am open to the idea.         
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Ash
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Re: Capacitive Dropper Ballast « Reply #19 on: August 24, 2017, 03:46:42 AM » Author: Ash
They used 2 to get the breakdown voltage they want..



The current at conduction. See what happens :

At some point the diac breaks over and conducts. The capacitor charges up to the positive line peak voltage

The diac shuts down

The line voltage goes down. When it reaches Vcap - Vdiacs it will conduct again. So we get : momentary voltage across resistors = Vdiacs

Momentary peak current = Vdiacs / R



If there is an interruption in the line voltage, a case might happen when the capacitor is charged to the peak line voltage in one polarity and the line is connected at the time of peak in the other polarity. When this happens, the diac breaks over immediately and 2x line peak voltage fall across the resistances in the circuit

Momentary peak current = Vline x 2 / R



The concept of the starting is indeed this, preheat till something happens

When the tube is active the voltage across it is the arc voltage. The diacs are chosen so their voltage is higher so they dont start conducting

As there is not choke there is no sense in interrupting the current at non zero point (this wont result in an impulse). So the diacs conduct till the zero crossing, the Ihold i think is insignificant (we can assume Ihold = 0)



The PTC "pins" the lamp in preheat mode for a moment (before it heats up), even when without it the lamp would be ready to strike (but not fully preheated). This might work at line frequency too



The resistor i dont think would do much : At high enough current it will cause voltage drop on the ballasting components, so lower the voltage across the lamp. At low current it won't have much effect on the cathode temperature

The idea of using a switch for the preheating is, that the cathodes can heat up at significant current (on the same order as the lamp working current) and at another moment the switch is open, the full voltage is applied when te cathode have not cooled down yet
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Keyless
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Re: Capacitive Dropper Ballast « Reply #20 on: August 24, 2017, 04:50:23 AM » Author: Keyless
Again, information that I could not find in several engineering books  ;D

The peak voltage will not damage the triac if sized for the current spike?

And given a diac vs a PTC, you would go the diac route?

FWIW I did find an example of the resistor idea:

https://www.repairfaq.org/sam/flnl2.jpg

Given the two caps and diodes, this is a voltage doubler- but could be done away with if these were for 230 volt mains?
« Last Edit: August 24, 2017, 04:54:27 AM by Keyless » Logged
Medved
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Re: Capacitive Dropper Ballast « Reply #21 on: August 24, 2017, 10:29:28 AM » Author: Medved
First a remark: The components used here are called "sidac", the difference is after activation the diac drops the voltage just by 5V, so from e.g. 35V breakover voltage to 30V holding, but the sidac has the holding voltage in the range of 1 or 2V, so practically fully ON. But I will use still the term "diac" here, just to keep the references to previous explanations

Again, information that I could not find in several engineering books  ;D

The peak voltage will not damage the triac if sized for the current spike?

The peak voltage across the "diac"  can not exceed the diac's breakover voltage

And given a diac vs a PTC, you would go the diac route?

There are big differences between just a PTC vs diacs:
When the diac activates, it turns ON and so eliminates the voltage to nearly zero. This causes a current spike, rather than a smooth current when compared to a low resistance state of the PTC. The thing is, both have about the same charge transfer (dictated by the capacitor and mains voltage), but the spike will exhibit way higher rms value just because of the presence of the high frequency components. And for heating up the resistive filaments what matters is the rms. So the "diacs" will offer way higher preheating currents than just the PTC alone (so the PTC alone may lead to an insufficient preheat current).

Second the fast switching of the diac cause there fast dV/dt, so cause extra ionization on that edge. Even when that is followed by a no voltage section (the lamp is shorted by the "diac"), some of the free charges still survive till the voltage gets build up again the next half wave, aiding the ignition. Again, this small difference may be sufficient to make the PTC not working.

The compare with HF CFL ballasts is not correct in one aspect: The HF ballast utilizes there a resonance to get the voltage very high after the PTC resistance rises, it does not need the PTC conductivity to change any fast for that.
But here the voltage is limited by the mains at low frequency, so it needs some fast switching element to aid the ionization (by the dV/dt) and that can not be offered by a device steered by a temperature change (it is too slow).


FWIW I did find an example of the resistor idea:

https://www.repairfaq.org/sam/flnl2.jpg

Given the two caps and diodes, this is a voltage doubler- but could be done away with if these were for 230 volt mains?

This "resistor idea" schematic is wrong in one aspect:
The connections at the ends of the tube do not go to the filament, but they are the main (the one with low value resistor) and an auxiliary (the one with a 33kOhm resistor) electrodes.
These nightlight tube are started like the MV lamps: First ballast OCV (here the double of the mains voltage) ignites a discharge just between the auxiliary (just a metal stud) and the main (in the form of a filament coated by an emission layer, but both ends connected to the same terminal) electrode, creating a ionization there (it is the smallest distance, so lowest breakdown voltage). And only this ionization then allows the discharge to be transferred between the main electrodes (lower impedance in the path).
So that circuit will not work with normal fluorescent - they are lacking the auxiliary starting electrodes.
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HomeBrewLamps
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SodiumVapor 105843202020668111118 UCpGClK_9OH8N4QkD1fp-jNw majorpayne1226 187567902@N04/
Re: Capacitive Dropper Ballast « Reply #22 on: August 24, 2017, 11:24:24 AM » Author: HomeBrewLamps
would wrapping a wire around the tube work?
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Keyless
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Re: Capacitive Dropper Ballast « Reply #23 on: August 24, 2017, 06:59:48 PM » Author: Keyless
@Medeved:  That makes sense now, thank you for clarifying the starting on those small night lights. Here is what appears to be a correct version:


http://www.next.gr/uploads/538-65c5e1544e.gif


If I was to go with a 240 volt supply on a US PL or F15T8 lamp (normally associated with 120 volts open circuit voltage), could a PTC be more likely to work? My theory is that less electrode heating would be needed as 240 volts would strike the tube almost by itself. I found the diagram of a German night which appears to be mimicking an instant start circuit:

https://www.romanblack.com/ftube4w.gif

A diode lets the capacitor charge one half wave, but blocks on the other half wave giving a current spike to strike the lamp.


A Sidac is a different component from a diac? FWIW the capacitive dropper night light I took apart 10 years had a component that looked identical to a transistor, but its third leg was no attached to anything, only the outer two were used. To this day I was never able to find out what that specific component was.   

In terms of the diacs break over voltage not being exceeded- I'm confused. I am looking for a break over of about 100-80 volts, however when power is applied the OCV will be about 120 or 240.


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Re: Capacitive Dropper Ballast « Reply #24 on: August 24, 2017, 07:02:09 PM » Author: Keyless
@Medeved:  That makes sense now, thank you for clarifying the starting on those small night lights. Here is what appears to be a correct version:


http://www.next.gr/uploads/538-65c5e1544e.gif


If I was to go with a 240 volt supply on a US PL or F15T8 lamp (normally associated with 120 volts open circuit voltage), could a PTC be more likely to work? My theory is that less electrode heating would be needed as 240 volts would strike the tube almost by itself. I found the diagram of a German night which appears to be mimicking an instant start circuit:

https://www.romanblack.com/ftube4w.gif

A diode lets the capacitor charge one half wave, but blocks on the other half wave giving a current spike to strike the lamp.


A Sidac is a different component from a diac? FWIW the capacitive dropper night light I took apart 10 years had a component that looked identical to a transistor, but its third leg was no attached to anything, only the outer two were used. To this day I was never able to find out what that specific component was.   

In terms of the diacs break over voltage not being exceeded- I'm confused. I am looking for a break over of about 100-80 volts, however when power is applied the OCV will be about 120 or 240.




It might  ;)

This site is not mine, but see the first pic:


https://www.romanblack.com/tube4w.htm

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Re: Capacitive Dropper Ballast « Reply #25 on: August 25, 2017, 07:08:51 AM » Author: Keyless
Ok- so I answered most of my own questions (ignore the above):

http://www.littelfuse.com/~/media/electronics/application_notes/switching_thyristors/littelfuse_thyristor_thyristors_for_ignition_of_fluorescent_lamps_application_note.pdf.pdf

SIDAC- the key word and I want to thank Medeved as well for bring that to my vocabulary.  :)


Now- Is the charging network necessary to get the sidac to conduct in the first place if the sidac is rated 130 volts on a 240 volt main? Of course I'd need to make sure the tube can strike with only 240 volt OCV. The charging circuit turns the main cap into a voltage doubler.   
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Re: Capacitive Dropper Ballast « Reply #26 on: September 01, 2017, 04:07:54 PM » Author: Medved

Now- Is the charging network necessary to get the sidac to conduct in the first place if the sidac is rated 130 volts on a 240 volt main? Of course I'd need to make sure the tube can strike with only 240 volt OCV. The charging circuit turns the main cap into a voltage doubler.   

A 130V sidac will fire, just voltage reaches the 130V level. But that means the voltage across the tube will never exceed that 130V, regardless what the mains is. And the 130V peak maybe too little to actually fire the arc in the lamp.
Therefore an opposite approach is usually used: Use sidac with higher breakover voltage than the mains (so above 320V for a 230V mains) and make sure the voltage is reached initially by the precharger and once the sidac starts firing, by the capacitor charge pump effect.
But once the sidac firing is below the mains peak (with some margin for fluctuations, so e.g. 210V till 260V type for a 230V mains), you do not need any precharge circuit at all...
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Re: Capacitive Dropper Ballast « Reply #27 on: September 03, 2017, 09:55:11 PM » Author: Keyless
Ok- that makes sense now. The charging circuit is simply a voltage doubling circuit.  

So in essence if I do not have a voltage doubler I can select a 200 volt RMS rated SIDAC, but with the doubler I would need a 400 volt RMS rated SIDAC? The prior would still need a 10k resistor across the SIDAC to let the cap charge in the first place.

In my experimentation I have discovered that it seems like most US PL and F15T8 tubes strike at 240 volts OCV, that is unless me breaking the two wires manually create a transient with a peak exceeding 340 volts peak.  

Lastly- V bo is the trigger voltage at the exact moment in the sine wave, not the RMS or peak?


http://www.jameco.com/Jameco/Products/ProdDS/1726348.pdf



            
« Last Edit: September 03, 2017, 09:59:20 PM by Keyless » Logged
Medved
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Re: Capacitive Dropper Ballast « Reply #28 on: September 03, 2017, 11:48:21 PM » Author: Medved
Ok- that makes sense now. The charging circuit is simply a voltage doubling circuit.  

So in essence if I do not have a voltage doubler I can select a 200 volt RMS rated SIDAC, but with the doubler I would need a 400 volt RMS rated SIDAC? The prior would still need a 10k resistor across the SIDAC to let the cap charge in the first place.

In my experimentation I have discovered that it seems like most US PL and F15T8 tubes strike at 240 volts OCV, that is unless me breaking the two wires manually create a transient with a peak exceeding 340 volts peak.  

Lastly- V bo is the trigger voltage at the exact moment in the sine wave, not the RMS or peak?


http://www.jameco.com/Jameco/Products/ProdDS/1726348.pdf


Tge Sidacs have specified the breakover voltage, what means they turn ON, at the instant the voltage reaches that level. Then it remains ON as long, as there is current flowing through the component.
All this is way faster than the mains sinewave and in fact the whole cycle repeats every mains half period, so nothing like rms applies there, it is really only about the instanteous voltage there.
So the relation between the supply rms and the breakover voltage co trols the phase, when the sidac turns on.

For the precharger you need to use a diode in series with the resistor, so it form a DC current (small), without that the capacitor wont charge at all...
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Re: Capacitive Dropper Ballast « Reply #29 on: September 04, 2017, 12:38:54 AM » Author: Keyless
So once they turn on at the 280 volt (example) threshold in the sine wave, they will continue to conduct as long as current passes, ie even at the 10 volt threshold the SIDAC will be "on", but shut off at the zero crossing only to start conducting at 280 volts on the negative half of the wave.

My understanding is that if I eliminate the diode, I will loose the double voltage, but the resistor will still allow current to flow through the cap and in turn voltage across the SIDAC.     
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