Driving a CRT monitor flyback transformer with squarewave

I think that's your problem. Don't leave the secondary winding open, at least put a spark gap (an auto spark plug will do) on the secondary.

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reduce the duty cycle to 50% max.

Most people building these things, mostly based on 555 timers, use dutycycles of 90%. With 50%, it's impossible to get any kind of spark out of it.

That would be part of the problem. There's still the problem that the back- EMF suppressor also limits output voltage.

The flyback isn't used to drive the yoke in multiple 'H' frequency monitors.

But the flyback is driven by the horizontal output stage, so the flyback itself operates at different frequencies. So, what driving method is used and how is back-EMF handled?

"Wiebe Cazemier" schreef in bericht news:4d961ef2$0$309$ snipped-for-privacy@news.kpn.nl...

In the old CRT TVs/monitors the LOPT is not driven by a sawtouth, it produces the sawtouth current for the horizontol yoke. This is mainly done by tuning the (complex) load so the current in the yoke raises lineair during the ramp. At the end of the ramp, the primary supply is switched off by a short pulse to retrace the line. The collapsing field produces a back-emf which is used to produce the high voltage, roughly 10kV to 25kV. Though the current provided is small, there's a real load. Often some other, less high voltages are also produced by the LOPT causing a real load. Together they prevent the back-emf from rising indefinitely. Nevertheless, when facing a failing CRT, the line output transistor is a primary suspect.

To protect your switching transistor, you can make an artificial load that only comes in when the collector voltage becomes too high. See below using a fixed font.

--->|-----+----+-----+ fast | | | high | | - voltage | .-. / zener diode diode --- | | - p.e. 150V --- | | A | '-' - | | | | | | +----++----+ | | | | | | | | - | .-. / --- | | - --- | | A | '-' - | | | | | | +----++----+ | | | | | | | repeat as often as required.

created by Andy´s ASCII-Circuit v1.24.140803 Beta

petrus bitbyter

It's not clear to me what you're trying to achieve, but if by "decent output" you mean a high voltage, I think you have a fundamental problem. In an ideal transformer, all the flux links both windings, and the ratio of the emfs in the two windings has, at all times, to equal the turn ratio. So you cannot get a high voltage on the secondary without having that same voltage divided by the turns ratio on the primary. Real transformers, being less than ideal, will simply make the situation worse.

With the ballast resistor in place, and the secondary open, the voltage is going to rise initially to a level equal to the current that was passing through the primary immediately before switching multiplied by the ballast resistance. In your case, if you were actually driving 8A, then the voltage would rise to 1200V, and it would be no surprise if that killed your transistor.

Putting in a zener diode to protect the transistor just has the effect of throwing away some of the energy that was stored in the magnetic field in the transformer, which might as well not have been stored there in the first place (i.e. by using a lower current).

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I thought you were trying to protect the mosfet. The output voltage is related to the back emf, ie. if you reduce one then you reduce the other by the same proportion. You also need to worry about spikes caused by the leakage of the "transformer" you need a clipping circuit for that, typicaly a diode connected to a high voltage supply/sink

There's always a load on the flyback (the rectifier), and the switch transistor used is good for circa 1000V when it's turned OFF.

Either a breakdown device on the secondary coil (the HV side) or a clamp on the primary (the drive voltage side) is required. The transformer coupling makes either sufficient for your needs. For HV, it'll just take a simple spark gap.

Saw tooth wave with a damping component? (Diode)?

Jamie

They are separate circuits in modern monitors, so the flyback operates at a fixed frequency. Other than that, it's a traditional TV/monitor HV supply.

Ya think? Not in the dozen or so monitors I dismantled in the last six months, fair bit of MOSFET power switched capacitors around that HO section to tune it to selected rate. Some monitors you can hear relays click when they go high speed. But the ones I had were about a decade old.

I saved some of the triplers, maybe build a Jacob's ladder or so.

Grant.

I'm trying to achieve about 20 kV at as much current I can get out of the flyback. In the device that I'm going to power, there will be a spark gap, but not to protect, but to give me sudden discharge of high voltage capacitors (long story).

Anyway, at 20V, it draws about 5 amps at 90% duty cycle. So with 150R ballast, that gives me 750V.

I hadn't really though about what you're saying about throwing away energy; I've been increasing the voltage on the primary winding to get higher output voltage, but I might as well increase the back-EMF ballast to reduce the amount of energy I throw away.

More testing is indicated :)

I'm looking at one of those CRT's right now, that clicks when you switch some resolutions.

Anyway, could you elaborate on these MOSFET power switched capacitors? Do you mean that modern CRT's do have circuitry to achieve a resonance?

If I'm visualising your circuit correctly, that will push up the voltage that the transistor sees, and 750V is already way above your transistor's rating.

Perhaps you should look at voltage multipliers.

Sylvia.

I can't see the original post so replying here.

All flyback transformers are resonant in some way or another through parasitic capacitance. It is the final current flowing in the transformer when the MOSFET lets go and the LC resonance which determines the ultimate flyback voltage and finally current reversal or ringing. Less voltage means less current and so requires a reduced "on" time of the MOSFET.

I assume by back EMF ballast, do you mean you are trying to limit back EMF by absorbing most of the energy you've just put into the uinductor? A circuit diagram might be useful with switching times and details of the transformer and precisely what you're trying to achieve.

FWIW, in the past, when I would want to generate HV with current, I would use an old (50's - 60's) TV flyback with an added primary winding. (Only open core transformers). Another benefit was that the HV output was AC and you could do more with it. If this is a one-off project, I would go that route. Those transformers are still quite common, even NOS. I always used push-pull driving transistors, with tranzorbs across the CE connections. Never had a transistor failure, but had to pay special attention to the unused leads on the flyback to insure against corona arc over. Tom

On a sunny day (Sun, 3 Apr 2011 10:32:17 -0400) it happened "hifi-tek" wrote in :

In my poor school days I used an old tube radio, fed back the speaker output to the record player input so it ocillated, and then also connected an ignition coil primary to the speaker output, nice HV for no cost.