Re: MMC from Microwave Caps

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

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)] 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