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RE: MMC Question.
Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
Hi Glen:
There are two similar polypropylene-dielectric capacitor product lines
made by Cornel Dubilier - the 940 and the 942 series. The 942 series is
widely used in Tesla coils and has proven to be reliable in that
application. The 940 series is not rated for as much current and has
been seen to fail in Tesla coil duty. It's the inferior 940 series that
has the suspiciously identical specs to your STK caps.
In a spark gap (A.K.A. disruptive) coil, the cap is charged slowly at a
relatively low current, limited by the power supply rating - 60mA (or
thereabouts) in your case. But when the spark gap fires, the discharge
current is very brief, but can be in the hundreds of Amps. This places
some very stringent demands on the capacitor, and very few caps are up
to this challenge.
BTW, I don't recall what voltage your NST is, but if it's a 15/60,
you'll want a cap .015-.02uF to be optimal, so your .0094uF cap is
slightly light if you're shooting for a LTR cap. It will definitely
work, just not as well as it might. LTR means that your cap is
significantly larger than the value that would be mains-resonant (Larger
Than Resonant) with the NST secondary. The mains-resonant value was
once thought to be optimal, but current wisdom is to use a value 1.5-2
times this value, for both performance and durability reasons.
Regards, Gary Lau
MA, USA
> Original poster: "Glen McGowan" <glen.mcgowan@xxxxxxxxx>
>
> Thanks for the extra effort regarding the caps.
>
> Regarding the peak current, both Charts show the same values for peak
> current. I must be missing something(?). I'm not building a beast by
> any means. But,this is my first coil and plan to power it with a
> 15/60 NST. My MMC will use 16 of the STK's in series.
>
> Wouldn't the current remain at 60mA throughout the tank?
>
> I'm all ears, still learning so everything is educational at this
> point. As long as I don't stop my heart I consider it a learning
> experience. I would like to avoid the "power up and run for the
> hills" avenue as much as possible.
>
> On 8/7/06, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
wrote:
> Original poster: Vardan
>
<<mailto:vardan01@xxxxxxxxxxxxxxxxxxxxxxx>vardan01@xxxxxxxxxxxxxxxxxxxxx
om
> >
>
> Hi,
>
> I am a bit "concerned" about the STK caps. One will note the specs
here:
>
>
<http://www.stkelectronics.com/pdf/MP89.pdf>http://www.stkelectronics.co
m/pdf/M
> P89.pdf
>
> And the specs for CD caps here as part number 940C20P15K-F:
>
> <http://cde.com/catalogs/940C.pdf>http://cde.com/catalogs/940C.pdf
>
> They are "exact duplicates"... Now this stuff is not easy to measure
> and "same setups" do not repeat even the next day!!! So I worry
> about "copying specs"...
>
> But the raw materials are "commodities" and maybe they just "roll
> them" just the same...
>
> But the peak current is 1/3 and so they are not as "tough" as our
> usual favorite capacitor...
>
> Cheers,
>
> Terry
>
>
>
>
> At 09:51 PM 8/7/2006, you wrote:
> >Glen -
> >
> >Your questions about series vs. series-parallel connections have
> >been answered, and you have been cautioned about the use of aluminum
> >buss-bars as connection points. However, no one questioned your
> >selection of capacitors. Your STK type MP89 utilize metallized-film
> >construction. This type of capacitor uses an extremely thin layer of
> >aluminum evaporated on to a polypropylene film as the capacitor
> >"plates". This design has been shown to be marginal for Tesla coil
> >use because of localized overheating and burning away of the end
> >connections to the capacitor roll.. The type of capacitor
> >construction recommended for use in Tesla coil tank circuits is
> >film/foil, where the plate material is thin aluminum foil
> >interleaved between the polypropylene dielectric film. This type of
> >capacitor is well proven in Tesla coil use, while many people have
> >experienced failures with the "metallized film" construction. Now if
> >you design your MMC very conservatively, and calculate the peak
> >current in your tank circuit, and make sure that your MMC has an
> >appropriate current rating, you should be OK. However, for the best
> >reliability and safety factor you should use film/foil capacitors.
> >
> >For instance, the Cornell-Dubilier Type 42C film/foil caps are very
> >widely used in Tesla coils, and have gained an excellent reputation
> >for reliability. The C-D type 942C cap of the same voltage and
> >capacitance ratings as your STK caps (.15uF at 2,000 volts DC) has a
> >peak current rating of 432 amps, and an RMS current rating of 13.5
> >amps. Your STK caps have a peak current rating of 144 amps, and an
> >RMS current rating of 8.9 amps. The CD cap has a peak current rating
> >3X greater than the STK caps.
> >
> >Now if your STK caps were free, than just carefully design your coil
> >so the peak primary current stays within the MMC's rating. Bear in
> >mind that the C-D film/foil caps are available for under $3.00 each,
> >and have a peak current rating 3X greater than the STK caps. Based
> >on peak current rating, you should not have paid more than $1.00 per
> >STK cap, or you would have been better off to go with the C-D type
942C caps.
> >
> >Post all the design specifications for your coil, and the list
> >members will help perform a "design review" to verify that your MMC
> >is appropriately matched to the rest of your system.
> >
> >Regards,
> >Scott Hanson
> >
> >
> >----- Original Message ----- From: "Tesla list"
> <<mailto:tesla@xxxxxxxxxx> tesla@xxxxxxxxxx>
> >To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> >Sent: Monday, August 07, 2006 12:24 AM
> >Subject: MMC Question.
> >
> >
> >>Original poster: "Glen McGowan" <
> <mailto:glen.mcgowan@xxxxxxxxx>glen.mcgowan@xxxxxxxxx>
> >>
> >>Just received my new shiny bag of caps that Dr. Resonance kindly
> >>provided. I got my fancy polycarbonate MMC perfboard fabricated.
> >>But now that I got to thinking; what would be the best way to wire
> >>them up? Option #1, would it matter if I terminated *both* strings
> >>to the same two pieces of metal at each end of the MMC? Or should I
> >>go with Option #2 and keep them all in a strict series? Wouldn't it
> >>still perform the same function in either scenario? Forgive the
> >>newbie question I'm sure this is basic electronics.
> >>
> >>
>
>
>><<http://www.cyplexia.com/IM000525.JPG>http://www.cyplexia.com/IM00052
5.J
> PG>http://www.cyplexia.com/IM000525.JPG
> >>
> >>
> >>Option #1
> >>
> >> | |
> >> |-----||--||--||--||--||--||--||--||----|
> >> | |
> >>--IN----| |---------- OUT --->
> >> | |
> >> |-----||--||--||--||--||--||--||--||----|
> >> | |
> >> | |
> >> ^ ^
> >> Aluminium Strip Aluminium Strip
> >>
> >>
> >>
> >>
> >>Option #2
> >>
> >> |
> >> |-----||--||--||--||--||--||--||--||-------------- OUT --->
> >> |
> >> |
> >> |
> >> |-----||--||--||--||--||--||--||--||---------------IN-----
> >> |
> >> |
> >> ^
> >> Aluminium Strip
> >>
> >>
> >>
> >>
> >>
> >>-Glen
> >>
> >>
> >>
> >>
> >>
> >>
> >>
>
>
>
>