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29th Nov 2020, 4:22 pm | #1 |
Triode
Join Date: Dec 2011
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How a Flyback works? induction and Ferris
Hi All and thanks for reading my post,
I have a technical query that needs expert knowledge about the construction and workings of a line output transformer. I have over the last few years got all involved with restoring old arcade machines from the late 70s and early 80s. The cabinet restoration is easy for me as I have grown up restoring old classic cars. I send the game PCBs to other fellow collectors for repair and also for the monitor chassis. All of my technical help I have be able to source from the related arcade collectors forums. However this question has drawn a blank so I have tried here where I know there is a wealth of vintage knowledge. I have this monitor chassis from a wells Gardner monitor which I want to send off for repair. During cleaning it I noticed that there was a loose nut and broken long bolt wobbling about on the top of the flyback. After investigating i discovered that its purpose is to hold the two halves of the core together, I decided to make one and after some head scratching and a few goes in the vice i managed to make one that fitted. Suddenly it dawned on me that the original broken clamp is semi ferrous as i tried it with a powerful magnet and the new one i made is just mild steel and is definitely much more ferrous. The magnet sticks to the new one FAST with a click. From past experiences i learnt that i can’t use a ferrous object when adjusting a width coil as the inductance causes the object (screwdriver) to get hot very quickly and potentially damage the components feeding the coil. This replacement bracket i made might upset the flyback in the same way? I mean it passes right through the middle of the flyback. My question is to you clever people, can I use my new clamp made out of an old coat hanger or do I need to obtain the correct metal? I have copper I can use and happy to purchase something if anyone knows what it’s made of. Hope you chaps can help, and thank you. Frazer |
29th Nov 2020, 7:28 pm | #2 |
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Re: How a Flyback works? induction and Ferris
1) How does a flyback circuit work? Well, if you have a classic car background, you should already be used to them. As an engine turns, the points close and battery voltage is applied to the primary of the coil. With a fixed battery voltage, the inductance of the coil increases linearly at a rate set by the inductance of the coil and that voltage - You get V/L amps per second. In a car, the current hits a ceiling set by the wire resistance in the coil (They are crude and inefficient). In a flyback switch mode power supply or a TV line output transformer, the on-time is controlled and the current is stopped before it would have limited.
The coil/transformer now has a significant current in it, making the core strongly magnetised. The points of the car open and all of a sudden the current is not there, so the magnetic field collapses, fast. As it collapses, it creates a voltage in any windings around it. The voltage is proportional to the number of turns and the rate of collapse, so this can make a far larger voltage than the supply used to rev-up the primary field. So the primary winding sees a large spike. Both the car coil and the TV line output transformer have secondary windings with many more turns than the primary. This means the secondary will show an even larger voltage spike. In the car, this gets routed to a spark plug, in the telly, the high voltage spike goes to a rectifier, and then it uses the graphite coatings inside and outside the bell of the CRT as a reservoir capacitor. For colour TVs and monitors, it is usual for the rectifier to not be a simple diode, but to be an array of them with capacitors in a circuit called a Cockroft-Walton voltage multiplier. This rectifier (and likely a multiplier is built into the plastic box that's merged onto your transformer. Also in there appears to be a somewhat lower voltage supply with an adjuster pot which will be for the focus electrode voltage of your tube. This is why you have two high voltage cables coming out of the rectifier box part. Note that the rate of ramping up of the current depends on the inductance of the primary. The transformer needs a good cross section of ferrite to carry all the magnetic flux without saturating, but alone it would give too much inductance for the on-time it will get, and it will make too much flux for the ampere.turns it will be run to. To keep this under control, line output transfomers are gapped with a chunk of non-conductive, non-magnetic material where the two U-shaped cores meet. You can see one of them on the outside. there will be a matching one buried inside. The cores emit a strong 'fringing' flux from these gaps. Now on to that clamp.... It's very interesting that the break is right where it will have been next to the gap and that it looks to be burned or corroded. I'm wondering it it's been arcing away for some time, buried inside the transformer. The photos show the broken leg of the clamp going down inside the wound section itself. There appears to have been arcing. There may even be damage to the inside of the plastic bobbin the windings are wound on, so that there could have been arcing to the clamp. Seeing that far up the hole is going to be difficult, but running something smooth along the side of the hole away from the core might allow you to feel roughness or smoothness. Otherwise there are 'ringing' tests for LOPTs Your replacement clamp looks rather good, but a bit bigger in diameter, which would be a problem with the fringing flux. This is a weird one, and I've not seen anything like it before. David
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29th Nov 2020, 11:31 pm | #3 |
Octode
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Re: How a Flyback works? induction and Ferris
Considering all the clamping arrangements used on LOPT's effectively form a 'shorted turn' around the core of the transformer, how comes this does not have a detrimental effect on the operation?
Would a ferrous clamp have a different effect to a non ferrous one? Interesting failure- I don't think I've ever seen such a thing before! Cheers Nick |
29th Nov 2020, 11:54 pm | #4 |
Dekatron
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Re: How a Flyback works? induction and Ferris
Is it possible in this case that someone has been in previously and omitted insulating washers associated with the steel bracket? Seeing the state of the wire clamp remanded me of school physics classes where darning needles were melted on demountable transformers!
I don't think that the clamp forms a full shorting turn- when a LOPT is secured to a chassis, it has insulating bushes between it and metalwork, in other cases (like this), the clamp threaded ends secure to something like a Paxolin/GRP/other insulating strip across the fourth side of the transformer core. That's my recollection of things, anyway, Colin Last edited by turretslug; 30th Nov 2020 at 12:00 am. |
30th Nov 2020, 12:03 am | #5 |
Nonode
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Re: How a Flyback works? induction and Ferris
As TS points out it doesn't make a shorted turn as it is only three sided and insulated at the bottom end thus preserving the magnetic gap.
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30th Nov 2020, 1:05 am | #6 |
Dekatron
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Re: How a Flyback works? induction and Ferris
What other hardware is there between the steel plate and the top of the core?
I have just had a look at one out of an old VGA screen. It has a "C" shaped spring clip with no rod through the coil. That looks like an earlier transformer. I suspect that the steel plate was added when the board was converted from some other use and that the insulation between the threaded ends got lost. There might be a "bodge" to overcome the change in width that was caused by the steel plate. I am sure those "U" bolts normally go into a plastic bar on the other side of the PCB as opposed to a steel plate on top. It has been turned over. You can see the extension on the coil former to allow for it to be bolted down with the "U" bolt the other way up on your PCB side photo. There is also a resistor that looks like it has been running hot at an odd angle right next to the transformer adding to my thoughts about a possible bodge. The other part of the conversion can be seen on the far side of the frame in the same photo as the resistor. It looks OK. |
30th Nov 2020, 3:01 am | #7 |
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Re: How a Flyback works? induction and Ferris
The circuit round the clamp and the plate runs parallel to the mag flux, not transverse- around it. So there isn't a direct shorted turn effect, but there will be some asymmetry of the flux in the core leading to some current in the clamp's loop.
FRinging flux at the gap will indude eddy currents circulating within the thickness of the clamp rod. Also the metal will likely be lossy in magnetic terms at these frequencies giving an imaginary component to its relative permeability. Straight circulating current would have heated the clamp round its full length. But the failure looks tightly focused at the location of the gap. The gap within the coil will have the rod running hotter that it will by the open-air gap because of the heat from the other windings, and because of the former etc acting as heat insulation. Insulation is good practice. But running a big metal thing right past a core gap is bad practice. David
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30th Nov 2020, 1:09 pm | #8 |
Triode
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Re: How a Flyback works? induction and Ferris
Wow, I knew you were the guys! thanks Wrangler for your detailed explanation, yes I made sense of that and have copied the text for reference, very well explained, thanks.
I spoke to the chap that does the repairs on these boards and he thinks I’m mad wanting to replace this clamp, perhaps he is right? He suggested using a cable tie, he has seen them broken like this on other monitors. I’m worried about any sort of “bodge” in case it causes a problem down the line hence speaking to you chaps (this is a rare and expensive monitor now) it’s a Wells Gardner K4915. I guess I’m happy with the cable tie idea but it might get brittle down the line and snap, also I’m not yet sure if the metal clamp is an intended part of this electrical design. I’m more concerned that if I use my home made one with more ferrous content it might cause damage if it interferes with the transformer in some way. Some extra info, the aluminum plate shown on top is all original and does not have any insulators, nothing is missing. The monitor chap confirmed no insulators and also I believe my monitor is 100% original and there wasn’t anything insulating when I took it apart. I googled this chassis and they all seem to have this aluminum support plate on top and no sign of insulator that I can see. David, it’s a little complicated for me but when you say “direct shorted turn effect” are you saying that the effect won’t be much? So it’s not too important to the life of the flyback? So the effect it has it won’t make it work harder or upset it? Trying to convert it to laymans terms for myself. I have a spare flyback but different type where the metal clamp just fits at the side, see photo. I’m considering using its metal bracket (correct metal type I think) to make a “half bracket” as its not long enough to make a full size complete one. I will use my original broken part to make the other half bracket so in effect I will be able to clamp it but the clamp will be in 2 halves L+R with the gap underneath. from the top it will look normal. What do you think? Half brackets, a cable tie, my steel bracket or buy a new flyback if I can find one? Thanks again everyone for all your input. Frazer |
30th Nov 2020, 1:31 pm | #9 |
Dekatron
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Re: How a Flyback works? induction and Ferris
The clip on that spare transformer is more like the VGA screen ones.
The clip is likely to get pulled about by heat expansion of the steel. That later clip will be a good indicator of how tight it needs to be. |
30th Nov 2020, 2:42 pm | #10 |
Dekatron
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Re: How a Flyback works? induction and Ferris
Hi Frazer, my vote is for a humble cable tie! I can't imagine this component gets very warm, but there are specialist ones if that's a worry and I'm sure someone on this forum will have some...
I just wrote a longer post but it self-deleted when I pressed go to post it, so this is a more rushed version. Not to be pedantic, but it might be clearer to specify 'flyback transformer.' 'Flyback' on its own refers to when the electron beam from the cathode ray tube (CRT) pings from right to left across the screen. (Ok, that's horizontal flyback. There's also vertical flyback, from the picture bottom right to picture top left). It is your flyback transformer that does the work for this, via a coil on a 'yoke' in the neck of the CRT. The pulse that determines this characteristic behaviour is a sawtooth shape, long on the diagonal up and steep on the way down. Flyback transformers are/used to be also known as line-output transformers. The current in the flyback transformer induces a voltage pulse at the top of the diagonal. As a modifier, 'flyback...' can team up with '-diode', or '-converter' - as in 'flyback diode' or flyback converter,' so maybs best to say 'flyback transformer' when that's involved. I'm following your ingenious repair with interest and hope you get it sorted! Cheers
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Al Last edited by Al (astral highway); 30th Nov 2020 at 2:47 pm. |
30th Nov 2020, 3:10 pm | #11 |
Dekatron
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Re: How a Flyback works? induction and Ferris
Could it be due to magnetostriction on an already faulted length of rod?
A common failure mode in TV's etc LOPT, FROPT and SMPS stages was due to magnetostriction cracking the soldered joints. Lawrence. |
30th Nov 2020, 3:29 pm | #12 | |
Octode
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Re: How a Flyback works? induction and Ferris
Not to be even more pedantic, but I find the term "flyback transformer" almost as confused. It is actually a Line-Output Transformer (or in the other case a Frame OPT) , and the "flyback" bit is just a part of its important job. I've no idea why they should ever be called just "Flyback Transformers", or where/when that started,
Mike Quote:
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30th Nov 2020, 3:46 pm | #13 | |
Dekatron
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Re: How a Flyback works? induction and Ferris
Quote:
For most of my life I used the descriptor 'Line output transformer - it's one of the first bits of a TV - and its associated circuitry pulse-forming circuitry that I got to know about and investigate, probably like many, as a teenager. But things seem to have moved on. Manufacturers such as Wurth do indeed call them 'flyback transfomers' and the search term will yield many contemporary components - Here's just one page from Farnell, which clearly illustrates my point. And obbviously, a search on these lines will not yield a classical line-output transformer for TV repair or restoration. But note how the term 'flyback transformer' is also up in the search title for SEO purposes. The generic term they use is on this page is pulse transformer, and they clarify that with a sub-category, 'flyback transformer'. Clearly not all pulse transformers are flyback transformers, whereas all flyback transformers.... My major point was about the origin of the 'flyback' part of the term, which remains valid. Whether we remain loyal to the OG 'Line output transformer' or give a bit of a welcome to 'flyback' transformer is of course a matter of preference. Cheers!
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Al Last edited by Al (astral highway); 30th Nov 2020 at 4:05 pm. |
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30th Nov 2020, 3:55 pm | #14 |
Dekatron
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Re: How a Flyback works? induction and Ferris
It's a line output transformer, when I was in the trade.....LOPT, LOPTX or LOPTY.
Lawrence. |
30th Nov 2020, 4:02 pm | #15 |
Octode
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Re: How a Flyback works? induction and Ferris
Yes, I wasn't particularly criticising the usage - just puzzled about the change from an accurate and well established name to a seemingly less accurate one.
Mike |
30th Nov 2020, 4:02 pm | #16 |
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Re: How a Flyback works? induction and Ferris
Ah-HA!
That ties off one mystery, why haven't others failed like this?...... I'd go for a cable tie, or else a metal clamp that clamps the top of the transformer, but doesn't go down the sides of the limbs past the gaps. More elaborate to make, though. David.
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30th Nov 2020, 4:43 pm | #17 | |
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Re: How a Flyback works? induction and Ferris
Quote:
No such look the other way around, yanks love to abuse anyone who uses british english on a website where they can see it. |
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30th Nov 2020, 7:56 pm | #18 | ||
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Re: How a Flyback works? induction and Ferris
Quote:
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30th Nov 2020, 9:10 pm | #19 |
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Re: How a Flyback works? induction and Ferris
FBT versus LOPT, but both sides of the pond use HOT for the associated transistor, I think?
Also some sets had two FBT's with only one of them used as a HOT while the other supplied the EHT. |
30th Nov 2020, 11:56 pm | #20 |
Dekatron
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Re: How a Flyback works? induction and Ferris
At the end of the forward scan the magnetic field collapses so inducing the flyback pulse. Doesn't the formula -E = L . di/dt come into it?
Of course other considerations have to be considered such as the capacitances in the transformer and other parts of the circuit. DFWB. |