Re: Good & Bad Maxwell Caps, MMC questions?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mark Broker by way of Terry Fritz <twftesla-at-qwest-dot-net>" <broker-at-uwplatt.edu>

>Are MMC's really any better?  To what extent are they "self healing"? 

Best left to the "experts" to explain the details, but basically, an MMC is
the same construction as a Maxwell pulse cap.  The added benefits of an MMC
are that failures can be easily repaired/replaced with a single 
$2.25 cap, as opposed to finding a new $225 Maxwell.  The other benefit is
"tweaking" the capacity to your transformer/charging circuit.

The C-D 942Cs are pretty self healing.  We've mildly abused our MMC without
any problems....  

>To build a really sturdy MMC with the geek group caps (942c), for use
>with 15/180 NST (three 15/60's), what is the minimum string length and
>number of strings to handle the voltage and current they'll see?  I can
>adjust upwards to get my desired capacitance, which is between 0.06uF
>and 0.045uF --- 0.06 because that's what I was using and it worked,
>0.045 because that's the "right" LTR value for 15/180 with a static gap,
>but I don't know (I haven't calculated yet) if my primary has enough
>extra turns that I'd still be able to tap it in tune with 0.045 (I
>currently tap around 7.5 with my biggest topload, and have 10 turns).  I
>understand that the strings need to be of adequate length to handle the
>voltage (though I'm not sure what that length is to get a long MMC life
>with 15kV AC in tesla coil use).  Am I right that there is also some
>minimum number of strings needed for adequate current handling (180ma,
>in my case)? 

The biggest concern is the voltage rating.  Experiments performed by many
individuals have shown that the DC voltage rating should be approximately
the same as the peak voltage of the transformer.  This will utilize 
the approximately 2x to 2.25x overrating by the manufacturer (so The Geek
Group's C-D 942Cs can  withstand 4 to 4.5kV instantaneously), and will also
untilize the fact that we don't need our MMCs to last for one 
million hours.  If you run your coil 30 minutes per day, 365 days per year
for 5 years, you'll still have a runtime of about 1000 hours.  Of course in
reality, you'll probably get 30 minutes per week runtime for only a couple 
years before "upgrading".....

The current rating for The Geek Group's bigger cap is sufficiently large to
accommodate most systems without worry.

Using our (The Geek Group's) big cap (2kV, .15uF), I get an excellent
combination for your system.  4 strings of 10 caps will give you .06uF at
20kV.  3strings will give .045uF, and 2 strings will give a resonant cap 
(.030uF).  The largest LTR value for 60Hz 15/180 with an SRSG is .083uF
(according to MMC Calc2).  Adding a string (5 strings of 10 caps) is
.075uF, the closest value using 10 caps per string.

50 caps at your price of $2.25 is $112.50 plus shipping.  Not a bad deal....

Mark Broker
The Geek Group's chief engineer
G-5 # 10