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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Nov 2017, 8:21 pm   #21
astral highway
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Default Re: Wireless Self-Resonance Experiments

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The spark gap is three spherical copper RF knife-switch contacts on the end of the 50Hz supply (photo) so two gaps in series, and I put a vacuum cleaner nozzle adjacent to suck out the arc and leave the spark. It's very noisy! I have an old tumble-drier motor and had planned to make a rotary spark-gap, but life (and work) gets in the way sometimes.
Nice design indeed, ingenious and fascinating-looking! It's astonishing how much current is measured across these spark-gap switches - on some coils, in the 1,000s of A, and indeed, the most spectacular Tesla coils are those that are mechanically excited, although some semiconductor designs with IGBT bricks in a full bridge are catching up.

It would be interesting and challenging in an enjoyable way, I imagine, to make a rotary spark gap with quenching.

I haven't yet made a mechanically-excited Tesla coil, though, and I'm sticking to my valve design for the time being - less noisy for a smaller environment such as where I live, for starters.

The challenges are entirely different, with impedance matching being at the crux, and preventing instantaneously devastating flashovers between the secondary coil and grid windings, or inside the big power oscillator valve.
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Old 22nd Nov 2017, 1:04 am   #22
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The challenges are entirely different, with impedance matching being at the crux, and preventing instantaneously devastating flashovers between the secondary coil and grid windings, or inside the big power oscillator valve.
Ever considered a thyristor crowbar circuit? We used them on the Marconi B6126 HF transmitters to mitigate the effects of valve flashover. In fact, the valve manufacturer insisted we had such protection to offer valve warranty.

It consists of a current transformer on the 26kV HT reservoir capacitor line feeding an ignitron pulse unit (a thyristor alone would do your app) which, upon receipt of a sudden current pulse caused by a valve flashover, would open the a.c. supply vacuum switch and simultaneously (meant to be simultaneously...) dump the d.c. HT to deck via an ignitron.
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Old 22nd Nov 2017, 5:29 pm   #23
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Originally Posted by russell_w_b View Post

Ever considered a thyristor crowbar circuit? (a thyristor alone would do your app) which, upon receipt of a sudden current pulse caused by a valve flashover, would open the a.c. supply vacuum switch and simultaneously (meant to be simultaneously...) dump the d.c. HT to deck via an ignitron.
Hey Russell, it's a good idea and indeed, I think it is now essential to protect the power valve as well as the power supply downstream. Thank you

I will also put a gas discharge tube (ex Radar modulator) which strikes at 28kV, across the tank circuit. This will hopefully be a lower impedance pathway than the adjacent winding, a few centimetres away. 20KV is the theoretical tank circuit voltage, so this may strike too easily - I'll have to experiment. I could also put a current sense component on the earthy side of the tube, to trigger a crowbar.

I am wondering about the current sense component - you say a current transformer is ideal. I think I should full-wave (bridge rectify) the RF component and then sample it through a (Op amp) comparator that is preset to an appropriate level.
Then I can latch up a 555 and send a pulse to a thyristor as you suggest...

Does that sound fast enough? It all needs to be over in less than 5mS, to protect the rectifier chain in the power supply. They're good for a huge overload for around 10mS, but perhaps not repetitively. I know 5mS is actually quite a long time, but it's important for me to think this through.
-------------


Back to David Knight's transmission line model, I now have a related question on corona inception voltage.

Until I have the transformer that is being wound for me, I am building for test purposes a temporary EHT power supply, which is an SMPS effectively.

The pulse transformer came out of a Panasonic microwave oven inverter.
I was driving it resonantly last night at 28KHz with my MOSFET power modulator and the open circuit secondary of the pulse transformer produced corona at both ends.


This was from a drive voltage of anything above just 18V. The original circuit was had a power IGBT as the switch, and was also powered from a bridge-rectified mains voltage, not a MOSFET at 18-30V! And so I have two questions:

1) Does the corona at both ends of the open circuit secondary relate to David Kinght's experiments? Neither of the ends is connected to the ferrite core. There seems to be no other plausible explanation.

2) How am I getting corona, in any case, at such a low drive voltage? Does it mean that corona inception voltage is lower at high frequencies than at lower frequencies? AND/OR, is it partly explained by the fact that my MOSFET is hard switching on and off at around 25nS, whereas the original IGBT was switching on and off at several hundred nS?

Hence my dI/dT is much faster, way more than an order of magnitude faster... Could that be it?


Thanks everyone
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Old 22nd Nov 2017, 6:28 pm   #24
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Does that sound fast enough? It all needs to be over in less than 5mS, to protect the rectifier chain in the power supply. They're good for a huge overload for around 10mS, but perhaps not repetitively. I know 5mS is actually quite a long time, but it's important for me to think this through.
We used to test ours by hanging a piece of thin wire (the diameter of 5A fuse wire but not actual fuse wire) to the 26kV d.c. rail, through a vac switch, to deck, switching on the transmitter and firing the vac switch remotely. If all was well, the wire would be intact. That's how quick it was. I would say it would carry 10A before the crowbar kicked in and took the supply off. Sometimes the wire discoloured a little.

A 1us trigger pulse was initiated via one of two C/Ts (or the modulator valve grid deck C/Ts) to fire the ignitron, via a thyristor, followed by a 150ms sustain pulse to dump energy and at the same time, throw in a vac switch across the d.c. via a 320 Ohm discharge resistor.

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Does it mean that corona inception voltage is lower at high frequencies than at lower frequencies?
Correct. As the frequency rises, the corona onset voltage falls. Not only that, but at RF, the corona onset voltage and flashover damage point are very close indeed, as opposed to power line networks where there is a distinct gap between corona and flashover. I take it you've no sharp edges?
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Old 22nd Nov 2017, 6:36 pm   #25
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Correct. As the frequency rises, the corona onset voltage falls. Not only that, but at RF, the corona onset voltage and flashover damage point are very close indeed, as opposed to power line networks where there is a distinct gap between corona and flashover. I take it you've no sharp edges?
Thank you, Russell.

The pulse transformer should be a resilient component as it was designed to provide continuous 300mA to the magnetron, after a full-wave doubler. No sharp edges, except in the waveform driving it!! I have put silicon sleeving on the lead-outs but there is full-on plasma there at both sides. It's really odd, from just 18V (at 3.5A RMS on the Drain, goodness knows what it is pulsed) I need to get a move on with constructing my EHT probe (calibrated) too, to see the waveform here on the secondary.

I have ordered a power IGBT which has a much slower rise and fall time, and a long switch-off delay. I think the inconsistent behaviour of a bespoke pulse transformer, driven at its design frequency, must be the blistering fast dI/dT with the MOSFET I've been using. But do you think the open circuit nature of this phenomenon is relevant to David Knight transmission line theory?
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Old 22nd Nov 2017, 10:46 pm   #26
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But do you think the open circuit nature of this phenomenon is relevant to David Knight transmission line theory?
I haven't studied this yet, but if your pulse transformer is designed to work into a specific burden like a current transformer - in your case, 300mA - you really shouldn't run it open-cct as there will be a very high voltage developed across the secondary. Can you hang a resistor across the secondary winding so that it will load it up to 300mA?

What sort of current are you passing through the primary winding of your pulse transformer and how many turns has it?
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Old 28th Nov 2017, 5:02 pm   #27
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I haven't studied this yet, but if your pulse transformer is designed to work into a specific burden like a current transformer - in your case, 300mA - you really shouldn't run it open-cct as there will be a very high voltage developed across the secondary. Can you hang a resistor across the secondary winding so that it will load it up to 300mA?
Hi Russell, this is a good point indeed. I hadn't thought of the current transformer analogy, although I am familiar with the danger of leaving a current transformer o/c. I'll load it up as you suggest and see what happens. In the meantime, I've somewhat reinforced it with additional insulation. I think one side of the secondary was normally returned to the core - I'll have to check.

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What sort of current are you passing through the primary winding of your pulse transformer and how many turns has it?

I can't assess the peak current. RMS is the max that my DC bench power supply is capable of, 4A .However, I've also put a fat electrolytic right across the Source-Drain, so that will up things a bit. Also, the switching device is a MOSFET being hard switched and dI/dT is very high as it's switching in 25nS or so, so large pulses.

The manufacturer's design switch was a much slower IGBT, with a long transition time, I think 225nS.


It appears to have around 8 turns, heavy Litz. That would make sense as the DC bus for the inverter it came from was half-wave rectified mains.

I'd appreciate any comments, thank you!
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Old 29th Nov 2017, 9:58 am   #28
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I'd appreciate any comments, thank you!
I'll make the comment that with that sort of waveform and rate-of-change of flux, your pulse transformer is behaving rather like a car ignition coil.
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