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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If it's just an AC LOPT you're after, I can make you a present of one if you're a pensioner or on benefit. I have a number of them knocking around from the time I used to do museum restorations. Send me your address in a PM if you think I can help.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
If it's just an AC LOPT you're after, I can make you a present of one if you're a pensioner or on benefit. I have a number of them knocking around from the time I used to do museum restorations. Send me your address in a PM if you think I can help.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
If it's just an AC LOPT you're after, I can make you a present of one if you're a pensioner or on benefit. I have a number of them knocking around from the time I used to do museum restorations. Send me your address in a PM if you think I can help.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I've just mailed your LOPT from the village Post Office, so hopefully you'll receive it tomorrow.
It is very typical of open-frame LOPTs from the pre-semiconductor era i.e. before doublers and triplers came along, but the EHT winding is potted in some kind of synthetic rubber, rather than pitch.
I don't have a datasheet for it, but from experience I'd guess its design output is 12 - 15 kV 2mA - say 30W in all.
The 405 line frequency was 10.125 kHz, so the LOPT will work best and be least likely to fail at around that frequency.
I've thrown in a card of 4 kV 10 mA 20 kHz diodes so you can make a series rectifier string with five of them if needs be.
I haven't tested it, as it has never been used, and is in excellent condition. Judging from the paxolin boards, hand soldered point-to-point tag strips, and replacement of pitch potting by synthetic rubber, I'd put the date at 1960 - 1963.
I'm sending you the Royal Mail Track and Trace number in a PM, so you can find out the status of your delivery.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks. I'll put it under oil, and put something like a 7.5M Ohm resistor on the output.
Any idea what voltage the primary is rated for? Any ideas regarding optimum pulse width? 50% duty cycle?
It will probably be simpler to drive it using transistors rather than valves, but sticking to as close as possible to the original waveform would probably be the best place to start.
EDIT: I found this article, as well as a few others I've not read yet, but they apply to 625 line LOPT's.
]transistor_line_timebase_2.pdf[/file]
The part about 3rd harmonic and 5th harmonic tuning is new to me.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Thanks. I'll put it under oil, and put something like a 7.5M Ohm resistor on the output.
Any idea what voltage the primary is rated for? Any ideas regarding optimum pulse width? 50% duty cycle?
It will probably be simpler to drive it using transistors rather than valves, but sticking to as close as possible to the original waveform would probably be the best place to start.
EDIT: I found this article, as well as a few others I've not read yet, but they apply to 625 line LOPT's.
The part about 3rd harmonic and 5th harmonic tuning is new to me.
I will try to answer your questions by comparing the operation of a car ignition coil to an LOPT:
As you know, in an old fashioned ignition coil, back EMF of say 300V is induced in the primary by the collapse of the magnetic field in the core. It is this 300V that is raised by the turns ratio of primary to secondary to produce the high voltage output.
But in valve era LOPTs, the back EMF induced in the primary is boosted by energy from the collapsing field in the CRT yoke by means of a robust thermionic diode known as the 'booster diode' or 'efficiency diode.' If you look at the explanation surrounding Fig 9 here you will see how this is brought about.
My point is that there is not enough energy stored in the field of the LOPT alone to produce sufficient back EMF to energise the EHT overwind to a sufficient degree without recovery of the energy from the yoke.
What voltage is the primary 'rated for' - well, HT+ plus Vboost was in the low kilovolt regime, so the turns ratio of the EHT overwind (almost an afterthought in LOPT design) is not as high as one might at first think.
As it's not really practicable to directly immitate the original mode of operation, I suggest you wind a new primary of say, 20 turns on the empty leg, determine its inductance, calculate its time constant, and use that figure as your pulse period. I wouldn't worry too much about the waveform, as the primary will be almost entirely reactive, so a square wave applied to it will produce a sawtooth current anway.
If it was me, I'd stick with the design ~10 kHz, but I'm sure you could raise this a bit without incurring too many core losses. 10 kHz is good because you can hear what's going on!
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
I will try to answer your questions by comparing the operation of a car ignition coil to an LOPT:
As you know, in an old fashioned ignition coil, back EMF of say 300V is induced in the primary by the collapse of the magnetic field in the core. It is this 300V that is raised by the turns ratio of primary to secondary to produce the high voltage output.
But in valve era LOPTs, the back EMF induced in the primary is boosted by energy from the collapsing field in the CRT yoke by means of a robust thermionic diode known as the 'booster diode' or 'efficiency diode.' If you look at the explanation surrounding Fig 9 here you will see how this is brought about.
My point is that there is not enough energy stored in the field of the LOPT alone to produce sufficient back EMF to energise the EHT overwind to a sufficient degree without recovery of the energy from the yoke.
What voltage is the primary 'rated for' - well, HT+ plus Vboost was in the low kilovolt regime, so the turns ratio of the EHT overwind (almost an afterthought in LOPT design) is not as high as one might at first think.
As it's not really practicable to directly immitate the original mode of operation, I suggest you wind a new primary of say, 20 turns on the empty leg, determine its inductance, calculate its time constant, and use that figure as your pulse period. I wouldn't worry too much about the waveform, as the primary will be almost entirely reactive, so a square wave applied to it will produce a sawtooth current anway.
If it was me, I'd stick with the design ~10 kHz, but I'm sure you could raise this a bit without incurring too many core losses. 10 kHz is good because you can hear what's going on!
That's a very interesying link, PM.
I had suspected that the other windings would play a part here, and had wondered about using capacitors, etc. to 'recover' this energy, but from my initial calculations, I'd need an inductor as well (I think).
My main reservation about using a separate primary on 'the other leg' is the increased leakage inductance that results as a result of lack of 'close coupling' of primary and secondary (I'm uising this to learn about 'flyback topology' for my other projects).
As far as 'waveform' is concerned, yes, a square wave pulse train will give a rising slope, but there are duty cycles, etc. to consider as well. A 625 line LOPT has a primary switch duty cycle of ~20-30%.
Presumably this method of recovering energy from the yokes to increase voltage on the secondary means that more energy is is transferred to the secondary than can be stored in the core/gap alone?
Maybe some variation of the circuit you linked to could be used to increase the overall energy transfer through a flyback transformer in other applications?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
As far as 'waveform' is concerned, yes, a square wave pulse train will give a rising slope, but there are duty cycles, etc. to consider as well. A 625 line LOPT has a primary switch duty cycle of ~20-30%.
Presumably this method of recovering energy from the yokes to increase voltage on the secondary means that more energy is is transferred to the secondary than can be stored in the core/gap alone?
Maybe some variation of the circuit you linked to could be used to increase the overall energy transfer through a flyback transformer in other applications?
Firstly I should remind you that there are many more people here on 4HV who know much more about flyback topology than I ever will. And with this general disclaimer, I'll have a stab at some of the questions.
As I noted in an earlier post above, the EHT overwind is almost an afterthought in the design of these early LOPTs, and remains much the element least complex and least requiring of innovation to this day. So I'll talk about all the other windings that often seem to mystify people on 4HV.
European tellies made after the War avoided the use of heavy and expensive HT transformers by using the less safe, less reliable, 'live chassis' technique. (i.e. the mains Neutral line was connected directly to the chassis)The various HT voltages needed by the sub-circuits were derived from taps on big, roasting, wirewound dropping resistors which usually had a green or black ceramic coat. These often developed blackish hot spots and burnt out - but how much cheaper than a heavy mains HT transformer! As there was now no mains transformer with an LT winding to supply the valve heaters, the heaters were connected in series with a dropping resistor across the mains - hence the odd-looking heater voltages of TV valves, which had to add up to as near to 240V across the whole heater chain as possible. When one valve heater went open circuit, the whole lot went down.
It was soon realised that at least some taps on the smoky dropping resistors could be done away with by using extra windings on the LOPT to meet the different HT needs around the sub-circuits. This was done by extending and tapping the primary to produce auto transformers, and by having completely separate secondaries where electric isolation was required. This trend has continued to this day, so the LOPT has become a sort of Maypole around which all the other circuits must dance.
So the main load on an LOPT was not - and is not now - the EHT winding, but the sub-circuits feeding off the auto-transformers and secondaries (if any other than the EHT overwind)
A few of the valve heaters could not be daisy-chained because of high cathode voltages (that exceeded then possible heater-cathode breakdown voltages), and the EHT heater supply was one of these. This was usually supplied by two or three turns of EHT cable on what is an empty leg on the LOPT I gave you. Sometimes the winding was supplied already fitted for a specific make of telly, and at other times it was left to the TV manufacturer to add it - allowing them to make their own choice of EHT rectifier heater voltage. The EHT heater voltage swung wildly in the first 60 seconds or so, while the telly was warming up, so the heater was designed to operate with a broad safe operating area.
Now with that little bit of background, a stab at your questions: the duty cycle of LOPTs is defined by the line frequency and is as much concerned with its subsidiary and dependent components and circuit elements - the EHT rectifier, the booster diode, the line output valve, and even the scorching dropping resistors - as it is to do with the properties of the transformer itself.
As an example, have a butchers at this data sheet for EHT rectifier EY51, which would have been a popular choice to go with your LOPT. Notice in the footnote to page 2 the caveat "maximum pulse duration ½% of the time between two pulses with a maximum of 5 µsec" This limitation has little to do with the LOPT, but is to stop excessive, destructive heat dissipation in the EHT rectifier, allowing it time to cool down between pulses.
In the case of your LOPT, you will not be loading it with a heater winding, or feeds coming off whatever auto transformer taps it has, so - aside, of course, from all the many sorts of losses - what energy you put into the primary will come out again on the EHT winding. This is now its only load, so you have much less concern about overload than a TV designer, and the device will last a long time if you don't draw arcs from it, or otherwise short the output, or try to make it produce more than about 15 kV, when it will break down irrecoverably in short order.
As to energy stored in the CRT yoke that is dumped back into the primary by the booster "efficiency" diode circuit, I suppose you could use a surrogate inductor of similar value in place of the yoke, but I can't really see the point of this unless you are going to use valves - high impedance devices - in your generator, which I do not recommend.
The original primary was designed to match with valves - high impedance devices using higher voltage with lower current - so you will not get good results with it if you try to drive it with semiconductor drivers typically using high current and low voltage. You will not be able to get enough current to pass through the primary's high AC resistance at 10 kHz. .
My advice for getting started: wind a five turn primary on the empty leg, dump the energy from a 1 uF motor-run or similar capacitor charged to a few hundred volts through it with a 'chicken stick' contact, and estimate the EHT output from spark length if you don't have a suitable meter to hand. Then you can see what effect changing the number of primary turns and the capacitor value has, until you arrive at something workable, when you can automate it with a suitable switching transistor.
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1957 RGD 'Deep 17' model 'A' (405 line b/w TV)
