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Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133
Im making 20-stage marx generator using 1nF/15kV ceramic capacitors. I have finished first 6 stages and its working very good. Maybe 12cm sparks. Powersupply is simple NE555+mosfet+modern flyback transformer giving 20kV (more than enough). Resistors are two 680k/2W in series. Problem is that i have no more left those 680k, I only have some 390k, 560k, 820k similar resistors. So my question is that: How important it is to have same value for all resistors? Lets say I have now 2x680k = 1.36M, is it good idea using 2x560k or even more, 2x820k resistors for final stages? Actually I dont see any problems with that but is that so?
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253
The most important thing is to use resistors that have good surge capability - film types (even HV ones) don't last long. Solid carbon composition ones are ideal if you can find them. Value isn't too critical, but if you use a mix, put the lower value ones nearer the charging end as you are charging more caps through these than the ones further up.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The choice of resistors should be based on the desired time-constant and output impedance of the Marx. Ultra fast Marxes usually employ wire-wound components of 50R or so.
Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133
Ok, i have now 2x560k resistors at lower part of marx and 2x390k at top end. How ever, it works....for a while. It fires few times and then stops working. Reguires some time to work again, why is that happening? Too much corona losses ionizing surrounding air that prevents all caps to fully charge?
However, there is now 16 stages, 30-35cm sparks. Spark gaps: first and second gap is set to 10mm, rest ones around 6mm.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Have you thought of measuring the resistance of all your resistors after the Marx has failed?
My guess - and it is only a guess - is that the voltage developed across such high resistances exceeds their maximum working voltage, and that one or more of them has been destroyed, and gone open circuit, or grossly changed its value.
What is the maximum working voltage of the resistors? Are they special high voltage types?
Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133
Harry wrote ...
Have you thought of measuring the resistance of all your resistors after the Marx has failed?
My guess - and it is only a guess - is that the voltage developed across such high resistances exceeds their maximum working voltage, and that one or more of them has been destroyed, and gone open circuit, or grossly changed its value.
What is the maximum working voltage of the resistors? Are they special high voltage types?
It cant be resistor failure because it is still working. It just fires few times maybe 10-15seconds and then stops. After that I just switch powersupply off for few seconds and then back on and it works again. I dont know working voltage ratings for those resistors..
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Tonskulus wrote ...
Harry wrote ...
Have you thought of measuring the resistance of all your resistors after the Marx has failed?
My guess - and it is only a guess - is that the voltage developed across such high resistances exceeds their maximum working voltage, and that one or more of them has been destroyed, and gone open circuit, or grossly changed its value.
What is the maximum working voltage of the resistors? Are they special high voltage types?
It cant be resistor failure because it is still working. It just fires few times maybe 10-15seconds and then stops. After that I just switch powersupply off for few seconds and then back on and it works again. I dont know working voltage ratings for those resistors..
Then the value of the resistors is too high, and after a few firing sequences a state of equilibrium is produced.
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Marx generator resistors
