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Re: Current peak from MMC (fwd)
---------- Forwarded message ----------
Date: Fri, 16 Apr 1999 10:54:50 +0100
From: Alex Crow <user-at-alexcrow.clara-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Current peak from MMC (fwd)
Hello All -
I've just tested my MMC for the first time. It uses Evox Rifa 0.047u MKP, 2500V, 7
in series, 10 strings in parallel. It works beautifully and makes bangs just as big
as my PE cap. I was worried about the low dU/dt rating (1800V/uS) but I checked the
data sheet and they have been tested at five times this rating with only twice the
normal heat dissipation. Indeed, they ran stone cold in my coil. At the standard
rating, at my calculation (allowing for peak V less than the max of 2500) they
should take about 750 Amps in total. Looking at some of the Microsim analyses and
real (lower bandwidth) measurements (Terry Fritz's site) this would appear
adequate, as his 10" testbed coil seems to be giving bang currents of around
400-500 amps. The high bandwidth analyses show a peak of about 4500 A, but Terry
has stated that this may be too high and due to saturation of his sensors. Even if
there was an initial pulse of about 4000A, this would be not much more than the 5x
test value and is of very low duty (a few nanoseconds per mains half-cycle).
Obviously this is only my first test, but my suspicions are that MMC users are
saved by the low duty cycle inherent in a Tesla primary, and the high safety
margins on these units - most tests the manufacturers conduccct on these are *well*
over the stated maxima. PE cap users don't need to worry about duty cycle so much
as their internal heating would be low anyway (PE cap = low loss, large volume).
I haven't yet heard anyone on the list smoking one of their MMCs!
Alex Crow
Tesla List wrote:
> ---------- Forwarded message ----------
> Date: Thu, 15 Apr 1999 08:54:49 +0300
> From: Marco Denicolai <Marco.Denicolai-at-tellabs.fi>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Current peak from MMC
>
> Tesla List <tesla-at-pupman-dot-com> on 14.04.99 01:47:01
>
> To: tesla-at-pupman-dot-com
> cc: (bcc: Marco Denicolai/MARTIS)
> Subject: Re: Current peak from MMC
>
> ------------------------------------------------------------------------
>
> >Original Poster: Finn Hammer <f-hammer-at-post5.tele.dk>
>
> >Tesla List skrev:
> >>
> >> Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> >>
> >> Hi Finn.
> >>
> >> If you look at the MKP 375 characteristics, don't forget to have a look
> >to the
> >> CAPACITANCE:
> >>
> >> a MKP 375 rated for 2 kV has got 30000 V/us dV/dT but only in models up to
> >> 2700
> >> pF (yes, pF!). Models up to 0.01 uF have got dV/dT = 10000 V/us and models
> >> up to
> >> 0.022 uF drop down to 6700 V/us.
>
> >Marco, I don`t know where you get your numbers from,
>
> >From the Philips original datasheet, of course.
>
> >but if you read my
> >post carefully, you will see, that the dU/dT values change, according in
> >fact with the envelope size of the cap.
>
> Certainly, but the size affects the CAPACITY they can put inside. As a matter of
> fact, a higher capacity needs a bigger size that, in turn, reduces the dU/dT
> available. This is not a rule, it is simply how the MKP 375 is specified by Mr.
> Philips!
> You got to look to the right component, exactly to it, to find its ratings.
>
> >>In the specksheets that I downloaded from Philips, the 375 is rated at
> >>3300 V/µS for the big envelope with 27 mm Pitch, and 5000V/µS for the
> >>envelope with 22,5 mm pitch.
>
> Again, this is true for the 1000V devices (see datasheet 1998 Mar 04 p.12). If
> you look at p.20 you find the 2000V devices with 10000 V/us for the 22.5 mm
> pitch and 6700 V/us for the 27.5 mm pitch.
>
> >>
> >> The MMKP 376 (which is MY CHOICE) rated for 2kV is available with all
> >> values up
> >> to 0.033 uF and for ALL values Philips ensures a dV/dT > 10000 V/us.
>
> >Oh! the lovely 376, I was only put off by the pricing in the Farnell
> >catalogue. But yesterday I called Philips, and asked them to suggest a
> >wholesaler, so that I could get a better price, and BINGO: They sell
> >376ès at the same price as the 375 : 0,022µF -at-1600 V for 1/2 dollar, so
> >I ordered 600 of them <SNIP>
>
> Get a good tin bobbin and Happy Welding! :)