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Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I just bought one of those Chinese Marx generators that you see listed on E-Bay. It came fully assembled but I haven't tried it yet. My question concerns the capacitors. This is a 20-stage model with twenty 2 nF capacitors. I measured the capacitance of each capacitor and found one that only measured 0.6 nF. Will this affect the operation of the Marx generator? Do I need to replace the capacitor?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You don't mention the value of the other 19 capacitors, presumably they are somewhat closer to the nominal 2nF?
The obvious problem is that of lower output energy than you were expecting.
The more serious problem is that the voltage on this runt will change faster than the other 19 on discharge, and it could end up dangerously reverse charged.
I presume it's not electrolytic, so on the face of it there's no reason it can't support a reverse voltage equal to its rating. However, for pulse discharge applications, manufacturers generally advise not allowing reverse charge to 100% of rating, as dielectric memory (aka relaxation or charge storage) leaves charge in the film that adds to the voltage stress the film actually sees. Often a 50% rating is advised. And that's for professional capacitors that are rated for pulse discharge applications, not cheap knock-offs that happen to turn up in unexpectedly low values.
Let's do the sums for a pure resistive load, which is the best case, inductance could make things worse. With 19 other capacitors, they will dominate the voltage across the load. If the output arc continues until the load voltage is nearly zero, the 19 caps will change voltage by almost their initial charge voltage. The voltage on the runt will change 3 times faster then the others, due to the capacitance ratio, so will change voltage by nearly 3x the initial charge, to nearly -2(initial).
What that means is that yes, capacitors in a Marx should be matched to a better ratio. If there are unmatched capacitors, an odd high value one doesn't matter, as its extra voltage gets distributed amongst the others. A single low value one will get 'ganged up' on by the others. This means the critical ratio is the smallest to the average capacitance.
For a resistive load with 50% reverse derating, for a marx with infinite number of stages (20 is quite close to infinity here), and initial charging to the capacitors' voltage limit, it looks to be that the smallest should be at least 67% of average (unless I've made an off by one error, I've not written anything down, just shooting from the hip here). Any stray inductance will require tighter matching.
Two things to your rescue. Capacitors don't necessarily fail at their rated voltage, there's usually something in hand. And secondly, you don't need to charge your bank all the way to their voltage limit, that derating will give more margin for the runt's survival.
You have several options. a) swap out the low value capacitor for a new one b) simply shorten the marx by a stage c) run it as it is, you're doing this for impressive bangs, right? I assume any bang will be at the level of entertaining rather than life-threatening. And deal with replacement or marx-shortening if and when you need to.
Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I forgot to state that the nominal capacitor value should be 2 nF. I ordered a replacement capacitor and I will install it before I operate it. Does anyone know the best way to remove hot glue? That is what they used at the capacitor terminals to minimize corona I assume. I sort of expect this type of situation with Chinese stuff. The prices are good but the quality is a crap shoot.
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Marx generator capacitor question
