Driving a CRT monitor flyback transformer with squarewave

For more information about my schematic, see my reply in another thread. It was a reply to whit3rd's message, who replied without a reference, so the message didn't appear in this thread.

You're going at it wrong... square wave is wrong.

Turn on, charge to a current limit, dump.

Otherwise you have fry baby, fry ;-) ...Jim Thompson

OK. That's pretty much what I thought.

With that circuit, it seems to me that the maximum output voltage is the transistor's voltage rating multiplied by the turns ratio of the transformer.

To get any higher, you need a way of preventing the transistor from seeing higher voltages from the primary *without sinking current from the primary*, which is something of an ask.

Sylvia.

This maybe true if the input to the coil was a sine wave or some sort of slow ramp down after the coil is magnetized. Using the square wave the signal is being turned off immediately and thus the field is allowed to collapse abruptly. This as you know, is going to generate more voltage than what you see in the turn ratio.

We have plasma generators at work used to treat the surface of wire for printing and the coil in those are being driven via a saw tooth, where output is regulated by monitoring current in the primary side of the coil. We set current limit to what we need for correct output.

So when viewing this on the scope it may look like a trapezoid of sorts at times. But the important part is when the driver drops to zero is when we get our high voltage we need to generate the plasma.

Jamie.

Jamie

What I've argued is that the emfs in the primary and secondary are always in proportion to the turn ratio, on the grounds that the change in flux linkage is in that proportion.

Attempting to stop the current in the primary abruptly will cause higher voltages in both primary and secondary than a slower change would, but it doesn't alter the relationship. So if you want a high voltage in the secondary, you have to handle a correspondingly high voltage in the primary.

The OP's problem is not an inability to create high voltages, it's an inability to cope with the high voltage that appears in the primary, which destroys his transistor, as I would expect it to.

Sylvia.

I've only looked at the PCB, not the circuit, so while I see some MOSFET switched caps, I don't know what they're connected to, but one of the MOSFETs is mounted on a heatsink. No photos and nothing here at the moment.

Got an old monitor in the car, it's 1995 though, may check it later.

Grant.

Jim Thompson expounded in news: snipped-for-privacy@4ax.com:

..

All you have to do is to take a drive through your neighbourhood on garbage day. Any TV set with a picture tube in it, will have a flyback transformer.

With folks still switching to flat screen TVs, they still show up on the curb.

Also any old computer CRT monitor would also have the same. Their natural operating frequencies will be different though.

Be careful disposing of the picture tube-- they implode and leave a 50 foot radius of dangerous hunks of glass if you improperly break one. My grandfather once threw a steel pipe at one in a bushel basket covered with a blanket. It blew the blanket off and glass was strewn all over the place. Cleaning up became a hobby for several weeks.

As a young feller, I eventually learned how to difuse them. But I never got comfortable doing it. It was like difusing a bomb-- filing the glass end and waiting for that sudden pfffffffft!!

Warren

Wiebe Cazemier expounded in news:4d985557$0$577$ snipped-for-privacy@news.kpn.nl:

The best Jacobs ladder I ever had was using an ignition transformer from an old furnace oil burner.

I took that thing to high school once and demonstrated it in an electrical class (motor/generator stuff). I recall holding some magnets near the arc, to influence the arc. Then I must have got some charge from being a bit too close because suddenly my pant legs balooned out from the static charge collected. That classroom had a nicely varnished wood floor.

Warren

I think for a good Jacobs ladder you need to put lots of power into the arc to cause enough heating so that the arc rises on the electrodes.

Just get an old neon transformer. ...Jim Thompson

Spehro Pefhany expounded in news: snipped-for-privacy@4ax.com:

An oil burning ignition transformer does that very well. I don't know how that compares to a neon transformer though.

Warren.

You can also take the base off and crack the evacuation tube in the middle. When I was a kid, I used to shove them in a plastic garbage can and shoot them with my slingshot. Fun.

Cheers

Phil Hobbs

When I was a kid in my dad's TV shop, we had a 55 gallon drum, with a round hole in the side just big enough to accommodate a crowbar.

Insert picture tube, replace lid, jam crowbar :-) ...Jim Thompson

Phil Hobbs expounded in news:7ZGdnToPtfDGiQfQnZ2dnUVZ snipped-for-privacy@supernews.com:

That's exactly where I did the filing (the little glass nib inside the base). To file anywhere else is not safe, AFAIK. Even so, I would take precautions with a wood shield. :)

Warren

Mains frequency neon transformers have a magnetic shunt that gives them a relatively high output impedance (to act as a ballast for the negative resistance tube). You'll get a much more intense arc out of an old ignition transformer, but you might toast the xfmr.

Spehro Pefhany expounded in news: snipped-for-privacy@4ax.com:

It is probably not designed to run continuously but it never seemed too strained when I used it. If your oil burner has trouble it can be active for fairly long periods of time (several repeated startup cycles). Mine never got warm or hot. But then it was the kind of thing that was never casually left on anyway.

Warren

The one I found at the dump as a teenager weighed about 10lb and was potted. It didn't get particularly warm, IIRC.

Then I was in high school I used an oil ignition transformer to drive a Tesla coil. It was 10KV @10mA. 100W should be enough to get the air moving in a Jacobs ladder. ;-)

You're completely missing the point of flyback! It's not about turns ratio as more about charging energy into the inductor and letting the flyback generate the high voltage. Others have pointed out that the OP is on the wrong track driving the thing open loop like that. The web circuit reference is bad in so many aspects, from burning out pots or the 555 on user adjustment, to having no safe control of the power in the inductor.

Yeah, sorta. Close but not quite...

OP does not even have proper snubber network, if he follows that 555 circuit.

If you're not doing the power supply method of applying a voltage until current rises to a turn off point, then wait for field collapse, you would need to do a tuned oscillator topology where you add power at just the right point in the cycle. Sort of saying the same thing in two different ways, I know. But power circuitry like this is not rocket science.

Says me, has some bits in a box, not yet produced a nice sparking voltage source -- though I can draw little sparks from an ioniser I built couple weeks ago to clean the air in here.

Grant.