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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Tonskulus wrote ...
One Mohm / stage...is it really too high? I have seen many similar marx generators that has 2.2Mohm resistors without any problems.
That would depend on the size of the capacitors, and the desired time constant and output impedance of the Marx. Professional laboratory machines usually have resistors in the range 50R - 500R
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
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..
Not necessarily - a failed film resisitor will often arc over internally when HV is applied, and so not always be immediately obvious. Check them with a meter, ensuring all the caps are _completely_ discharged.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The problem in answering your question is that you haven't specified your application: if you want a steep wavefront, and a short pulse with a short tail - the nearest a Marx can get to a square wave - you make the resistors as small as possible that will still fire.
If you want a long tail, which gives the subjective impression of a bigger, fatter spark, as the tail side of the pulse sags, then a larger resistor would make you happier.
Compromises between pulse speed, pulse energy, and pulse shape are inevitable.
There are lots of papers on the theory of the Marx generator available online. Look out for author names Algilbers, and Mayes, as well as free scientific document downloads from the divineHaefely Test AG
Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133
Ok sorry I forgot some info about its purpose. I need as long sparks as possible, so highest possible output voltage is the main goal. But im pretty satisfied as it gives 40cm sparks now from 14 stages. Input voltage is around 20kV.
Im going to use this to make miniature "thunderstorms". There will be about 40x40cm earth(GND) filled with model trees, buildings etc..:)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
My first suggestion was that one or more of your resistors had failed, and this still seems like the most likely answer to me, as others too have suggested.
What is your inter-stage voltage? If you remember that 'standard' resistors are usually rated at 350V, or 500V, then you will quickly see if your two in series are likely to have been stressed.
But why not just measure the resistances and see? And if it's not the resistors, why not test the leakage of the capacitors at 1kV or more? Much quicker and more certain than going backwards and forwards here... surely measurement is at the very heart of all we do?
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Marx generator resistors
