Problem with LOPT

[Andy Kay]

I have a ZVS driver and flyback transformer driving a 5-stage Marx Generator, producing nice fat sparks.

I stripped the LOPT out of an old (but functional) computer monitor and wound a 5+5 turn primary on the core (just as is the case with the flyback supplied with the ZVS driver), then located the HT ground pin by finding which pin gave me a plasma arc. I replaced the flyback transformer with the LOPT, but no sparks from the Marx. The HV secondary terminals happily drive a fluorescent tube in Cold Cathode mode, but not the Marx.

Before I bin the LOPT, is there anything I might be overlooking that could make this work?

[profdc9]

I thought a LOPT was a type of flyback transformer where the flyback works on the principle of storing energy in a flux gap in the ferrite core, but I could be wrong...

You may have too few turns on the primary and the voltage could be too high on the secondary.  It is best to start with more turns and be conservative so you don't overvoltage the transformer.  With the cold cathode fluorescent load, there can be a continuous discharge and this prevents the voltage from getting too high, but basically once the capacitors on the Marx generator are charged they are like an open circuit, and this could cause the LOPT to arc internally because of the high voltage.  The diodes in the LOPT could be damaged and therefore it may be charging and discharging the capacitors, but you would still see an arc from the HV terminals.

Do you have a safety gap for your secondary for your LOPT?  This might prevent the voltage from getting too high on the secondary. 

Dan

[Andy Kay]

Thanks Dan.

I thought a LOPT was a type of flyback transformer where the flyback works on the principle of storing energy in a flux gap in the ferrite core, but I could be wrong...

You have hit on an interesting point for me. I don't have any expertise in this field so I've often wondered why the word 'flyback' is used in so-called "flyback transformers" and "flyback diodes". Regarding the former, I presumed that it was a linguistic hangover from the use of ferrite core transformers for generating not just the HT for CRTs but also the line-scan sawtooth (and also the frame-scan sawtooth, though it remains a mystery to me how both sawtooths can be generated on the same core). Hence my conflation of the terms "flyback transformer" and "Line OutPut Transformer". The term "flyback diode" remains an enigma to me since I've only encountered them in the context of back-EMF protection when driving inductive loads.

You may have too few turns on the primary and the voltage could be too high on the secondary.  It is best to start with more turns and be conservative so you don't overvoltage the transformer.  With the cold cathode fluorescent load, there can be a continuous discharge and this prevents the voltage from getting too high, but basically once the capacitors on the Marx generator are charged they are like an open circuit, and this could cause the LOPT to arc internally because of the high voltage.  The diodes in the LOPT could be damaged and therefore it may be charging and discharging the capacitors, but you would still see an arc from the HV terminals.

I was copying the pattern of the LOPT that works, but in both cases there is a constraint on the number of turns given the gauge of wire being used for the primary winding. The possibility that the reclaimed LOPT has been damaged by internal arcing hadn't occurred to me because I was able to draw 2cm arcs externally, but this no longer appears to be the case so you may be correct that the device has failed in this manner. I did try it with a Cockcroft Walton voltage multiplier (just in case it was generating AC) but this produced nothing either.

Do you have a safety gap for your secondary for your LOPT?  This might prevent the voltage from getting too high on the secondary. 

I haven't been using a safety gap because the working system I'm copying doesn't use a safety gap, but it would seem prudent to employ one from now on. Thanks for your interest and for your advice Dan.

[profdc9]

By the way, if you want a cheap source of flybacks in the US, try Goldmine Electronics

http://www.goldmine-elec-products.com/

http://www.goldmine-elec-products.com/prodinfo.asp?number=G20792
http://www.goldmine-elec-products.com/prodinfo.asp?number=G22434

For this price it's just easier to buy than to scavenge out of a CRT, and you can buy several in case you fry them.  I can confirm that the JCM 6174V-8005 is pretty powerful, I have used it to power a Tesla Coil, though flybacks are fragile and have to be protected pretty well from RF spikes coming back from the coil or they die quickly.

Dan

[Andy Kay]

By the way, if you want a cheap source of flybacks in the US, try Goldmine Electronics

Flybacks start around £9 here in the UK... for the same price I bought a flyback AND a ZVS driver from China, including shipping!

Old CRT TVs and monitors are getting rare now as well.

[profdc9]

I have never built a Marx generator myself but I read that often lenses are placed between the gaps so that some ultraviolet light from one gap is focused onto another gap, so that after the first gap breaks down, the ultraviolet light from the discharge stimulates the next gap to break down, and so on.  If you use a safety gap, place an opaque barrier to block the light between the safety gap and the Marx generator gaps so it doesn't accidentally trigger the Marx generator pulse.

By the way if you want to read about an interesting Marx generator, read about the Z-Pinch Marx Generator for pulse power

https://en.wikipedia.org/wiki/Z_Pulsed_Power_Facility

It's used for nuclear fusion energy research.

Dan

[Andy Kay]

I have never built a Marx generator myself but I read that often lenses are placed between the gaps so that some ultraviolet light from one gap is focused onto another gap, so that after the first gap breaks down, the ultraviolet light from the discharge stimulates the next gap to break down, and so on.  If you use a safety gap, place an opaque barrier to block the light between the safety gap and the Marx generator gaps so it doesn't accidentally trigger the Marx generator pulse.

The thing about Marx generators is that they are so easy to build, and given the absence of semiconductors, very robust (if using appropriate capacitors or MMC arrangements). They work well enough without the lenses unless you need that level of precision. The safety gap has not been an issue, since I just have to make the gap narrow enough to  permit a plasma arc to strike when disconnected from the Marx, but wide enough to permit the first Marx gap to trigger before the plasma arc can be established. It's all grist to the mill when you're "learning by playing".

By the way if you want to read about an interesting Marx generator, read about the Z-Pinch Marx Generator for pulse power

https://en.wikipedia.org/wiki/Z_Pulsed_Power_Facility

It's used for nuclear fusion energy research.

"As of 2012, fusion shot simulations at 60 to 70 million amperes are showing a 100 to 1000 fold return on input energy"...

Fascinating... I had no idea that fusion had come so far.