Re: Exploding Maxwell caps (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 16 Aug 2007 11:27:31 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Exploding Maxwell caps (fwd)

Time for a quick review for the benefit of inexperienced coilers using 
commercial pulse caps...

There really is no problem using Maxwell (or other major pulse capacitor 
brands) as long as the caps are constructed using a polypropylene (PP) 
dielectric system and as long as they are properly derated for TC use. 
Commercial pulse caps need to be properly derated in order to handle the 
actual voltages and degree of ringing seen within a TC primary. It is 
true that most of Maxwell's pulse caps are rated for only 15-20% voltage 
reversal (VR%), and a few may go up to 80% VR. As long as your pulse 
caps are rated for higher duty (typically 500 PPS - meaning a PP 
dielectric system), they should be usable in Tesla Coils when suitably 
derated for voltage reversal. Newer self-healing capacitors (which use 
metalized film instead of foil-film construction) cannot handle as much 
tank current as film-foil caps due to the higher resistance of the 
capacitor plates. However, this is not _usually_ a problem for well 
designed Tesla Coils with higher inductance primaries.

The key to using commercial pulse caps is to remember that any rapid 
voltage reversal creates a transient voltage stress across the 
dielectric system equal to the total peak-to-peak transition. In other 
words, a pulse cap that is initially charged to 20 kV and then rapidly 
discharged into a high Q ringing circuit will "see" a voltage stress 
equal to about 40 kV across the dielectric system. If the tank cap's 
faceplate voltage reversal rating (i.e., Vdc * (1+VR%)) is significantly 
less than 40 kV, the capacitor's life may be significantly reduced, or 
it may immediately fail.

A spark gap type Tesla Coil primary circuit has relatively high Q 
(typically in the range of 10-15). When the main gap fires, the tank cap 
will see voltage reversal of 85-90%, so the peak-peak voltage change 
during the first RF cycle will be about 180-190% of the cap's initial 
voltage.

To maximize capacitor life, the voltage swing that's applied to the cap 
should always be less than the capacitor's voltage reversal rating of 
Vdc * (1 plus VR%). Although coilers often exceed this limit, they do so 
at their own peril, since excessive voltage reversals cause partial 
discharges to occur on surface of the PP film (along the edges of the 
foil or metalized plates). These can progressively damage/track the 
dielectric surface and degrade nearby dielectric fluid, and leading to 
premature/catastrophic failure.

Let's plug in some numbers for a typical coil. Let's assume that we want 
to use 15 kVRMS NST as our HV source. In theory, the peak voltage on the 
cap will approach 1.414*15 kV, or about 21.2 kV. This assumes there are 
no additional mains frequency resonant rise effects between the tank cap 
and NST. If the gap fires near this peak, then the peak-to-peak RF 
voltage swing across the tank cap will be about 21.2*1.9 or 40.2 kV. A 
pulse cap rated for 35 kV and 15% voltage reversal is rated for a 
peak-to-peak voltage swing of 35*1.15 or 40.2 kV. Initially, it looks 
like a single cap would be OK to use (with no safety margin).

However, in practice the actual NST peak voltage is often significantly 
higher, especially when the tank cap is sized to be mains resonant or 
Smaller Than Resonant (STR), and if the main spark gap and safety gaps 
are set too wide. Under these conditions, actual tank cap voltages can 
be significantly higher than 21.2 kV - perhaps more like 24 - 30 kV, 
with voltage reversals of perhaps 47.5 to 57 kV! Under these conditions, 
a single Maxwell #37667 tank cap will fail quite quickly (if the NST 
doesn't fail first!). Coilers who use groups of two of these caps in 
series should have no problems running in 15 kVRMS systems.

In order to provide adequate margin, the tank cap's DC voltage rating 
should be a _minimum_ of three times the RMS voltage of the HV 
transformer. This provides a bit of a safety margin to allow for 
somewhat higher peak tank circuit voltages, and provides fairly reliable 
operation with most commercial pulse caps. However, with mains 
resonant/STR NST's and ballasted distribution transformers, I'd 
recommend using a factor of 4 times the RMS voltage for bulletproof 
operation.

Bert
-- 
***************************************************
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, Lichtenberg Figures (our "Captured
Lightning") and out of print technical Books. Visit
Stoneridge Engineering at http://www.teslamania.com
***************************************************

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Wed, 15 Aug 2007 22:46:23 -0500
> From: resonance <resonance@xxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Exploding Maxwell caps
> 
> 
> 
> I don't use them, but a few people on the list, in years past, have reported 
> problems with them.  As Scott pointed out, the voltage reversal will kill 
> them.  You need a Maxwell rated for 100% reversal.
> 
> We make our own now, conservatively rated at 80 kV DC with a Q factor 
> greater than 20,000.  60 nF and 100 nF sizes available.
> 
> 
> 
> Dr. Resonance
> 
> Resonance Research Corp.
> www.resonanceresearch.com
>