Re: MMC from Microwave Caps

["Tesla list" <tesla-at-xxxxxxxxxx>]

Original poster: "Jason Johnson" <hvjjohnson13-at-hotmail-dot-com> 


----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, November 30, 2000 9:59 AM
Subject: Re: MMC from Microwave Caps


> Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net>
>
> Alan,
>
> All of the problems you mention are real. In addition, the dielectric in
> these caps is usually Mylar, which tends to be quite lossy when stressed
> with RF. This leads to reduced coil performance and internal capacitor
> heating. Since these caps are hermetically sealed units, dielectric and
> plate heating in Tesla Coil use can create an overpressure situation.
> Metal-cased [or even worse, glass-enclosed (Glassmike)]

I tried using glassmikes once. I set up a simple spark gap test with two
MOTs and a .1 ufd 5kvdc glassmike. I plugged the cord in and the gap fired
once, as it did the glassmike detonated sending the glass casing all over. I
never did find one of the metal endpieces which took off like a rocket
across the garage. Also the non-viscous silicon oil in them is a pain to
clean up. I'll bet the failure was due to overvolting the cap, but I had
tried the same thing with a polyester cap of similar ratings (.1ufd 6kdvc)
with no problems. I did this while looking for a cheap cap and decided that
even though glassmikes are easy to get with similar ratings much cheaper
than MMCs they're not suited to tesla coil duty at all. "I" would try the
microwave caps, but put them in a box made of some plywood and plexiglass,
and maybe some oil for cooling (make sure to seal the box good first!). I
think that they should stand up to the current, but the frequency could be a
problem for heating. If you try them and they don't explode or something let
me know cause I'd like to get a hold of some more microwaves.

Jason Johnson

Mylar caps have
> been known to forcefully explode with no warning(!), showering the
> unsuspecting coiler and the surroundings with hot oil and capacitor
> innards.. :^(
>
> As long as you put the caps in some type of container for operator
> protection, and if your string of caps uses approximately the same
> capacitance/cap, using a string of them should work, but not nearly as
> well as a true polypropylene MMC. Keep your runtimes reasonably short,
> with cool down times in between. This will allow to heat that builds up
> deep within the capacitor rolls to safely transfer to the outer surface
> of the capacitor package. And put parts for an MMC cap on your Christmas
> list... :^)
>
> Safe coilin' to you!
>
> -- Bert --
>
> Tesla list wrote:
> >
> > Original poster: "Kelly & Phillipa Williams" <kellyw-at-ihug.co.nz>
> >
> > Hi All,
> >
> > I had an idea for a cheap 'MMC' using microwave capacitors.
> >
> > The capacitors are 1 uF, (1 E -6 Farads)
> > If I put 20 caps in series, (one string)
> > the voltage rating would be a whopping 40 kV *RMS* or 56 kV DC!!
> > (Assuming the faceplate voltage of 2000 VAC)
> >
> > The capacitance would be 50 nF (0.05 uF)
> >
> > At first glance, this seems like a great MMC, but there are a couple of
> > problems I have
> > thought of.
> > 1) I remember someone telling the list that these caps have small
internal
> > leads and thin
> >       plates, perhaps limiting their current discharging capability
> > 2) Their dv/dt (Volts per microsecond) may not be high enough for TC use
> > 3) Due to tolerances, the voltage stress on each cap may be unequal,
leading
> > to early failure.
> >     (although the voltage rating of the whole cap is very high)
> > 4) One string of caps may not be able to handle the current stresses
> >
> > I have 23 microwave capacitors.
> > I'd really like to be able to do this, (as it's free,) and I'm desperate
for
> > a cap for first light
> > so I can spend some time thinking about what I want to buy for a more
> > traditional MMC,
> > as it will cost me quite a lot of money (for a student).
> >
> > *Does anyone know of any problems with this idea, or can confirm my
> > thoughts?*
> > I don't care if the caps die in a few hours, I got them free.
> >
> > Thank you very much,
> >
> > Alan Williams
>
>
>
>
>