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16th May 2009 16:57

Alan Yates wrote...

Bert,

Yeah silicone works fairly well. There are commercial products designed for HV potting, most of them smell a whole lot like silicone caulking...

You can try epoxy, but many are fairly conductive and track arcs easily. You can also immerse the entire assembly in transformer oil, but that is messy.

Drive frequency wise, I guess the obvious advice is "at the design frequency of the transformer in question". Depending on where you got it and how it was intended to be used the design frequency may not be known. I'd drive it at the "best" frequency you can find, i.e. that which provides the best output. Resonances may be the worst possible drive frequencies. You have to be careful that you don't overtax its insulation. Modern potted units can not be repaired, if they flash-over internally they are dead.

Being able to vary the drive duty cycle is probably more valuable, as you can control the output voltage at almost constant efficiency with varying loads. Flybacks are basically magnetic storage transformers, if you operate them at too high of a frequency there will be insufficient time for the primary current to grow to a significant level because of the primary inductance. But operate them at too low of a frequency and efficiency will suffer as the core will saturate and DC will be wasted before the switch turns off. The breakdown voltage of the switching device is also important, the voltage at which is Zeners will limit the maximum possible output voltage.

You can hold the duty cycle constant and vary the frequency to control the output voltage. Most people seem to use them that way. Their power handling and efficiency will vary greatly doing that - put often that's fine.

Regards,

Alan

16th May 2009 11:36

Bert wrote...

For your information, I use MOSFET to drive it. The circuit driving the MOSFET has variable frequency and duty cycle. By the way, what is the best frequency to drive a Color TV flyback?

16th May 2009 11:28

Bert wrote...

Hello Alan,

I have some problem with flyback transformers. How can I seal(the terminals and connections) it to prevent or reduce the HV overrun?(leaking to the driving circuit) Using silicone?

This problem prohibited me to drive the flyback to its maximum power.

Thanks

5th March 2009 15:45

Alan Yates wrote...

Hello Mikey,

With the light bulb one of the filament connections is touching the HT output of the flyback and the alligator clip lead is grounded and just wrapped around the bulb to couple to it capacitively.

The flyback transformer I've had for years. I got it out of an small monitor colour TV in High School of all places. It was being thrown out because someone dropped it and smashed the tube. The HT supply was largely OK, but the phenolic insulator that terminated all the low-side windings was also smashed. Not a huge deal for my use of the transformer.

I have several others. Many of the more modern cylindrical units with inbuilt focus dividers, etc, but also an ancient one out of a AWA deep image BW vacuum tube TV that is enormous. The secondary of the AWA unit is lightly waxed to hold it together but otherwise not really potted at all. The wire has several filaments braided together, a Litz wire I guess? It does not have the diodes between each layer of the secondary like modern transformers do, so it outputs HF AC which makes is kinda useful. Similarly that potted transformer in this experiment is also an AC unit, it fed an external tripler.

A mate just gave me another transformer out of a vacuum tube TV. I haven't played with it yet. The secondary is heavily potted in wax, but I assume it is an AC unit too, as it was driving a large HT rectifier tube, typical of vacuum tube TV designs.

Regards,

Alan

4th March 2009 00:15

Mikey broom wrote...

Hi,

im mike from malasia.

I'm doing pretty much the same as you but I managed to get 5 flybacks in series to produce really nice, fat sparks. I'm using a simple 555 timer circuit operating a TIP3055 on a massive heatsink! Loved what you did with the lightbulb! how exactaly did you make it work? did you just wrap the wire round?

Thanks 4 reading.

BTW the transformer you'r using dosn't look alot like a T.V flyback transformer, may I ask you where you got it from.

Thanks alot.