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Marx generator powered tesla coil

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Benjamin

Wed Nov 23 2016, 04:58AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

I was reading through the TCML archives and came across the idea of using a marx generator to charge the tank cap of a tesla coil. However, in this configuration the spark gap is eliminated from the tank circuit and the tank cap is connected directly across the primary coil, eliminating the spark gap losses! The marx generator is connected across the tank cap through a spark gap. This circuit relies on the impedance of the primary coil to charge the tank cap in one quick pulse. The spark gap would have to quench before the tank circuit could start ringing. Would this work? The power supply wouldn't have to be a marx generator, it could really be any capacitor bank or pulsed hv transformer.

edit: here is the TCML post

hen918

Wed Nov 23 2016, 02:54PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The problems with parallel LC circuits, as you are describing, are given here:

In short, parallel LC tanks are not suitable to be driven with a square-wave. It is, however, possible to use a Mazzilli driver to drive an LC tank with a sine wave, as is done here:

Benjamin

Wed Nov 23 2016, 06:17PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Here is a schematic of how I think it would be wired. Apparently It was done in 1964, that is why I posed the question.
]marx_powered_tesla_coil.pdf[/file]

Uspring

Thu Nov 24 2016, 11:42AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Might well work, but there is an efficiency problem: Only at most 1/4 of the energy stored in the Marx will go into the tank cap. The rest will be dissipated in the spark gaps.

Dr. Slack

Thu Nov 24 2016, 12:22PM

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

Suppose in Bejamin's pdf we did away with the tank capacitor. Now when the Marx fires, it's a number of C's in series with a number of gaps and the primary, basically an MMC with a multi-gap gap, with the MMC charged in parallel rather than series.

That eases the high voltage requirement on the power supply. If you really can't get an NST, then you could use a couple of MOTs and a marx configuration instead. One problem is that marx gaps tend to like higher voltages for stable breakdown, so there's a tension between stage voltage and output voltage, which is eased by reducing the number of stages.

For a mains-powered coil, this could be quite nice. Re-imagine the marx configuration slightly so that all stages *are* charged in parallel, rather than a ladder, so that all stages charge equally. Use charging inductors that are low impedance at mains but withstand the full primary operating voltage at the TC frequency, so no need for iron cores, they'd look like mini secondaries. Build the gaps like a multi-gap gap, setup with shims so they are all the same width so should self-fire reasonably consistently, with a common line of sight for UV so they all fire at the same time.

It's an interesting sledgehammer, it would probably crack the 'no NST' nut.

Benjamin

Thu Nov 24 2016, 06:36PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

So something like this

just with more stages and inductors instead of resistors. Would the air core charging inductors change the resonant frequency of the primary circuit?

Uspring

Fri Nov 25 2016, 02:35PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

As Dr. Slack said the charging inductors would be low impedance only at line frequencies. At the TC frequency their effect on the primary resonance is very small.

Benjamin

Tue Nov 29 2016, 04:29AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

What inductance would be enough? I was thinking 500mH air cored with 24 gauge wire.

johnf

Tue Nov 29 2016, 05:52AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Benjamin please go and build a 500mH aircored inductor out of 24g wire and then see how big it is and how it behaves with line frequency
after doing all the above come back and report

Benjamin

Tue Nov 29 2016, 06:06AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

I can calculate all of those parameters. 1.5'' diameter by 5'' long, 230 turns, reactance at 60 hz is 188 milli ohms, so almost a short circuit. However what I'm interested in is how it will behave at high frequency connected across the tank cap.

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