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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You might want to consider the following configuration. Notice the first capacitor in the Marx is not connected directly to ground, but through a charging element. It is grounded by the operation of a TSG (triggered spark gap). As the TSG has one main electrode grounded, the trigger source can also conveniently be grounded.
Because the Marx is triggered by grounding the first cap, it erects oppositely to the charging power supply. If this is a problem and you still want a +ve output, then the charging power supply can be inverted. As the spark is for photography, this should not be an issue.
Even though the Marx erects in the opposite polarity, the stray capacitances to ground still augment gap breakdown, and the remaining gaps fire promptly.
For examples of three electrode triggered spark gaps, do a google images search for 'triggered spark gap'. You get some very useful stuff like this one for instance.
Registered Member #56316
Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13
Russ Edmonds wrote ...
With the schematic you have It looks like the ignition coil trigger circuit will have to be isolated and be able to withstand 30KV. Once the trigger gap fires the low side of the ignition coil will raise by 30KV relative to the Marx power supply low side. Assuming the bottom of the Marx power supply is low.
Which option for the earth are you talking about? I showed "x" and "y" as options, one to Marx ground and one simply opposite the spark gap.
Dr. Slack wrote ...
You might want to consider the following configuration. Notice the first capacitor in the Marx is not connected directly to ground, but through a charging element. It is grounded by the operation of a TSG (triggered spark gap). As the TSG has one main electrode grounded, the trigger source can also conveniently be grounded.
Because the Marx is triggered by grounding the first cap, it erects oppositely to the charging power supply. If this is a problem and you still want a +ve output, then the charging power supply can be inverted. As the spark is for photography, this should not be an issue.
Even though the Marx erects in the opposite polarity, the stray capacitances to ground still augment gap breakdown, and the remaining gaps fire promptly.
For examples of three electrode triggered spark gaps, do a google images search for 'triggered spark gap'. You get some very useful stuff like this one for instance.
Interestingly I had also considered "adding a gap" before the capacitor array - but wondered if it was actually better for the 30kv trigger spark to be on the other side of one of the ballast resitors to prevent the charging system trying to jump the gap into the coil. But judging by the diagram you have drawn the trigger electrode is not central but rather closer to the 'earth' electrode of the spark gap[?] I suppose this guarantees the spark jumps in the right direction. I will start building a spark gap switch. Thanks for the help, I wil have to 'refine' my last attempt at a spark gap switch with a central trigger :)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I've looked at your trigatron link. When the main voltage is <1000v, you can use whatever tight geometry you like, the electrons aren't going anywhere until the trigger gap fires. That design was OK for 325v, but it is not going to work for a Marx.
When the main voltage is >10kV, you need smooth electrodes. As the trigger electrode will be at the same potential as the ground main electrode, you need to make sure the hot electrode doesn't break down to the trigger spontanteously. This means the hot to trigger gap should be at least as large as the hot to ground gap. Even though the trigger electrode will tend to have a smaller radius than the main ground one, if it is close enough to it, it will tend to be shielded somewhat by its electric field. This all leads to a very offset trigger electrode. Do what I suggested and follow that link I gave you.
Registered Member #56316
Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13
Dr. Slack wrote ...
I've looked at your trigatron link. When the main voltage is <1000v, you can use whatever tight geometry you like, the electrons aren't going anywhere until the trigger gap fires. That design was OK for 325v, but it is not going to work for a Marx.
When the main voltage is >10kV, you need smooth electrodes. As the trigger electrode will be at the same potential as the ground main electrode, you need to make sure the hot electrode doesn't break down to the trigger spontanteously. This means the hot to trigger gap should be at least as large as the hot to ground gap. Even though the trigger electrode will tend to have a smaller radius than the main ground one, if it is close enough to it, it will tend to be shielded somewhat by its electric field. This all leads to a very offset trigger electrode. Do what I suggested and follow that link I gave you.
Does it need to be enclosed like that or will it be ok "out in the open"?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
There is no benefit of enclosing it for this service. The Marx has other open gaps which will be noisy, let off nasty gasses etc so there will be little benefit to enclosing just the first one. In fact there's a benefit with a Marx to mounting all the gaps in proximity in a straight line so the UV from the first assists the other gaps to break down. In this case, I would simply build the Marx as a conventional Marx, with the modification to where the ground goes, and then dangle a trigger wire near to the ground electrode in the first gap. The point of the links was to show the assymetry of the trigger, and the smooth electrodes, not the plastic holder.
Registered Member #56316
Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13
Dr. Slack wrote ...
There is no benefit of enclosing it for this service. The Marx has other open gaps which will be noisy, let off nasty gasses etc so there will be little benefit to enclosing just the first one. In fact there's a benefit with a Marx to mounting all the gaps in proximity in a straight line so the UV from the first assists the other gaps to break down. In this case, I would simply build the Marx as a conventional Marx, with the modification to where the ground goes, and then dangle a trigger wire near to the ground electrode in the first gap. The point of the links was to show the assymetry of the trigger, and the smooth electrodes, not the plastic holder.
Ok thanks - I'm thinking of rearranging the construction now so that the spark gaps are all on top of the device and easier to work with - (maybe even a mirror at the end to reflect the UV light? :) I never knew it made so much difference). I'm always in so much hurry to get something working I need to slow myself down and make it sturdy and adjustable so I can fine tune it without using a glue gun :D
Registered Member #56316
Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13
I went shopping this morning - should I use the small or larger size domes. [i]
I pulled it apart and reassembled it so the spark gaps could be on top. Just as a test I replaced the first spark gap with the bolts with rounded caps so I could find the right size gap I would need before it triggers. No matter how closely I adjusted them it would not fire with the rounded bolts - even if I put a screwdriver in the gap it just jumps from one to the other but does not trigger the main spark. I disconnect it from the bolts and fit the original wire gap again and it fires perfectly? Ignore the trigger screw from the side for now - it will be adjusted correctly when it is in use.
Update: I tried again, smaller gap than all the others - and if I leave it charging long enough waving a screwdriver between the contacts fires it!
Something else I was thinking about - in the diagram you have provided where we switch the earth I imagine it is very dependant on the size of the final spark gap[?] The way I look at it if it is capable of jmuping a 10cm gap and we try to trigger an earth gap then the earth gap will have to be an exact size to complete the main gap - otherwise that voltage will esily find its way across that gap and around resistors if it is suddenly reduced to 5cm or something similar.
I had several successful triggers using the first spark gap. I set my camera up and tried a few more but before I could get a photo of it my old camera flash started smelling bad so I think its circuit got fried.
I had several successful triggers using the first spark gap. I set my camera up and tried a few more but before I could get a photo of it my old camera flash started smelling bad so I think its circuit got fried.
Most likely the charging transformer is fried. Or atleast partially fried.
Registered Member #56316
Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13
I'm wondering if I would be better off with a giant ignition coil. I have an 800 volt 3500 amp SCR wired up to a 3300uf 450 volt capacitor that I charge up with an old flash and dischrage when I take a picture. Perhaps if I were to discharge that through a primary winding I could get my 200 000 volt spark rather. Less hassle with being able to switch it then.
You can get some pretty impressive voltages from an ignition coil by dumping a large capacitor into it. The issue is its not a very high power spark like you would expect from a high voltage capacitor so it may not be bright enough for your application. But it may be worth a shot. Maby even just setup a quick test without the SCR just keep in mid you will need to insulate the high voltage terminal of the coil or it will ark to the primary terminals.
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Triggering a Marx generator with my camera.
