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Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
I'm going to start building a Marx generator soon, and I was thinking about the design. I made a quick sketch below of the traditional schematic, and one that I'm considering.
The traditional schematic has always bugged me a bit, because it seems that the capacitors will have to charge to different voltages. This could be alleviated to an extent by tuning the spark gaps, but why not fix the problem at the source. It seems to me that it would be better to use the second design I have posted above, so that all of the capacitors will reach the same voltage.
Has anyone used the second design? I've never seen it used and I'm wondering if there is a particular reason, or if it is just convention, or if it may make little difference.
Registered Member #1911
Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165
Is it just me, or is the first schematic the exact same as the second?
Edit - Posting a message at 11:00 PM when you hadn't gotten more than two hours of sleep each for the post three nights may not be the greatest idea... Thanks for clearing that up.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
In the second one, there is the main high voltage bus with individual current limiting resistors for each capacitor. The voltage across each capacitor must be the same at all times. In the first one the high voltage source passes through a series of resistors. For capacitors at the end of the chain the resistors act as a voltage divider so they charge up slower. So that make me wonder if in practical Marx generators not all capacitors are being charged up as high as they could go.
With the classic marx bank design, the last capacitor will definately have a lower voltage than all the others. So for the second-to-last etc. The voltage difference between the capacitors will be not significant if the charging resistor (the first one) is somehow bigger than all the others, for example 10 times as big, and i guess also if the charging cycle is a bit bigger than the RC characteristic of the first stage.
In the Edwards and Scoles model, besides the charging resistor, the "damping" resistors are also included in each stage, which act as current limiters and wavefront shapers.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Thanks for the feedback. I think the most important fact as mentioned here is that with my second schematic, during discharge, the resistors will see a much greater voltage, so a lot of extra care would need to be taken to ensure they don't arc over. So since I can get the capacitors closer to equal voltage by using a large resistor in series with the entire bank, there is not a whole lot of motivation to change to the second design.
Harry, I found a paper which references that Edwards and Scoles design (this), so if my reading of it is correct, the damping resistors are there to slow down and lengthen the pulse of current leaving the marx generator. This is no doubt important for using marx generators in practical test setups, but I mainly want big, loud, impressive arcs, so I can't tell immediately if this circuit helps.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Mattski wrote ...
Thanks for the feedback. I think the most important fact as mentioned here is that with my second schematic, during discharge, the resistors will see a much greater voltage, so a lot of extra care would need to be taken to ensure they don't arc over. So since I can get the capacitors closer to equal voltage by using a large resistor in series with the entire bank, there is not a whole lot of motivation to change to the second design.
Harry, I found a paper which references that Edwards and Scoles design (this), so if my reading of it is correct, the damping resistors are there to slow down and lengthen the pulse of current leaving the marx generator. This is no doubt important for using marx generators in practical test setups, but I mainly want big, loud, impressive arcs, so I can't tell immediately if this circuit helps.
Yes, the wonderful Haefely works- home of Marx generator innovation!
The lesson to be learned here is that it is a mistake to have all resistors of equal value, and that the resistor values should be calculated on the need for the keep alive current of the arcs - say 5A - 10A - rather in the rather arbitary way in which values seem to be assigned in a lot of DIY Marx circuits I've seen, without concern for the time constant, or the output impedance of the erected circuit.
One should always design backwards from the needs of the application, but if sparks themselves are the application, then such attention to detail may be unnecessary, as you seem to suggest.
I like the Edwards & Scoles iteration, because it makes you think about Marx topology in a fresh way, which is always the best and most exciting way forward with engineering.
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Different Marx Generator schematics
