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Triggering a Marx generator with my camera.

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DesmondD

Mon Aug 17 2015, 03:26AM

Registered Member #56316 Joined: Sat Aug 08 2015, 05:24AM
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Posts: 13

brandon3055 wrote ...

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.

I've done it before with a 280 volt charge from my 3300uf capacitor and the SCR - it was a rather weak spark compared to what I expected. I imagine it is also dependent on how fast the voltage is disconnected because a good spark needs a sudden cut-off from the voltge for the field to collapse. I bought a capacitive discharge coil as well and dumped 500 volts through it without any spark happening so I'm not sure what happened there.
I was wondering about the idea of getting a coil with a longer core and simply ignore the primary winding and make my own primary with only a few turns of really thick wire.

Dr. Slack

Mon Aug 17 2015, 07:58AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you are using an ignition coil, and the same goes for all high output voltage transformers, it's safer to have a spark gap across the secondary at all times, than to 'insulate the secondary terminals', whether using or just testing. If too much welly is delivered to the primary, you might deliver too much secondary voltage for even the insulation in the transformer. Insulating your secondary terminals is like replacing your fuse with a 1/4" brass rod, because the fuse blows 'too often'. The output arcing to anywhere prevent excess voltage on the secondary. If you manage to stop it arcing from the already nicely insulated HV output, then it might just break over inside. Keep a spark plug, or some other reasonable sized gap connected at all times. Tracking across the HV insulation is another matter, make sure you keep the surface of the insulator clean, dry and free of fingerprints.

Commercial CDI (capacitor discharge ignition) systems use a very modest primary capacitance, 1uF at 400v is typical. You don't need giant Cs, big enough is sufficient. They do need to be non-ploar, plastic film is good.

The main thing I see your box around the spark gap doing is shielding the UV from the other spark gaps in the Marx, so preventing the very effect you ought to be promoting.

Your marx is already made with bent wire gaps, demonstrating just how smooth the gap electrodes need to be for this voltage regime. Dome nuts are not needed until a rather higher voltage. I would recommend building a neat bent wire trigger gap, like the others, in line with the others.

Your woodscrew trigger electrode has got a nice sharp edge running along its length. Although it is at ground potential during charging, it is in the field of the hot electrode in the gap, which may promote breakdown to it. The main electrode will struggle to shield it with its field. Use a straight bit of wire as your trigger.

DesmondD

Mon Aug 17 2015, 08:50AM

Registered Member #56316 Joined: Sat Aug 08 2015, 05:24AM
Location:
Posts: 13

Dr. Slack wrote ...

The main thing I see your box around the spark gap doing is shielding the UV from the other spark gaps in the Marx, so preventing the very effect you ought to be promoting.

Your marx is already made with bent wire gaps, demonstrating just how smooth the gap electrodes need to be for this voltage regime. Dome nuts are not needed until a rather higher voltage. I would recommend building a neat bent wire trigger gap, like the others, in line with the others.

Your woodscrew trigger electrode has got a nice sharp edge running along its length. Although it is at ground potential during charging, it is in the field of the hot electrode in the gap, which may promote breakdown to it. The main electrode will struggle to shield it with its field. Use a straight bit of wire as your trigger.

I will have to get hold of another old flash to trigger my spark - as mentioned I managed to trigger this gap about a dozen times before the flash suddenly smelled bad and stopped firing.
Once I have that ready I will try with the bent wire method - I got the dome nuts based on your suggestion to follow that link.

Also: Does the polarity of my trigger spark matter? I imagine that as long as the gap gets ionized it will fire[?]

Mads Barnkob

Mon Aug 17 2015, 10:15AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Stop before this double posting madness continues!

DesmondD, read the site rules before posting again:

Part II: Posting

F. Any images you include in your post must be less than 600 pixels wide. If the image is too large, link to it with a URL tag rather than actually embedding it with an IMG tag. If it's something you intend to attach to a post, the board will automatically resize it for you, and create a link to the full-size image. Offending images may be removed at any time -- if you post oversized images, don't be surprised or angry if you find all your images deleted.

G. Avoid double-posting (FYI: double posting is posting twice in a row in a thread, without allowing any replies in between). Double posting just to get a topic marked as new is a no-no. Every topic deserves a chance. If yours slips into obscurity, its chance came and went. If 48 hours have elapsed, and you have some new information to add, you may double post. This is the only exception, there are no other exceptions! Posting an identical or nearly identical question in two individual threads is also considered double posting.

DesmondD

Fri Aug 21 2015, 07:48PM

Registered Member #56316 Joined: Sat Aug 08 2015, 05:24AM
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Posts: 13

I've decided to use a different method of switching my ignition coil rather than having it in series with the flash capacitor since that killed my flash the last time.
I have this SCR sitting in the cupboard waiting to be used and am thinking of using it to dump a capacitor through an ignition coil for my trigger spark.
[As far as I know every image I have posted has been 600 wide so if there is an issue with these images as well please let me know why]

This is a clip of the pdf I was sent when I bought it.

This is the wiring diagram of the previous SCR I used but was warned that it may not switch the same way around with this other one.

This is what I saved more than a year ago when I bought it off someone: "
They are similar but in this model the stud is the anode so it will be live so beware of that. The white lead should be the gate if I remember correctly, the other lead is not required for triggering but I'm sure it is connected to the cathode so it means you don't have to run a lead to the crimp lug. First check the continuity between the cathode (crimp lug) and the red lead (and white) to confirm that. Use a low voltage non-inductive test load Â to start with to check. If your circuit is using an 82.2Ohm resistor on the gate it would probably be better to go with something smaller because it likely won't switch. It will require more than 200mA for hard switching. Check the anode and cathode in reverse polarity too and make sure it's blocking.Â

I would also recommend putting a good fast switching diode across your coil to prolong the life of the capacitor and reduce stress on the SCR, ideally another antiparallel diode across the SCR and inline with the electrolytic capacitor would be better to but across the coil has the biggest impact and allows the current to keep flowing in the same direction and sap the rest of energy stored in the magnetic flux. I have destroyed countless components trying to defeat the forces of inductive voltage spikes! Generating 5 or 10kV and beyond is not difficult with a coil pulse so it's worth looking into at some point.Â "

Before I destroy it I was hoping that someone could advise me on the best switching circuit for it.

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