Re: MMC potting: (Was Re: UK MMC bulk order)


A few comments:

Tesla List wrote:

> Original Poster: "David  Dean" <deano-at-corridor-dot-net>
> -----Original Message-----
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> Date: Thursday, July 08, 1999 4:49 AM
> Subject: MMC potting: (Was Re: UK MMC bulk order)
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
> Hi Reinhard
> > Original Poster: "David  Dean" <deano-at-corridor-dot-net>
> Snip
> > BTW my mmc consists of two strings of 11 for a total of 10nf. I put
> >the strings side by side inside a 1 1/2" O.D. polypropylene tube
> >12" long. perfect fit. Potted with epoxy resin. A 1/4" X 1 1/2" brass
> >bolt serves as a stud on each end. Much smaller than the equivalent
> >rolled poly in oil.
> I have a few comments / questions about your MMC construction:
> 1.) You say, you are using two strings. This means each string
> will have to take 50% of the primary amperage. Depending on
> design (of the coil) this might lead to severe overstressing of
> the caps. Do you know how high your peak and rms primary
> current is? What power are you running?
> Please correct me if I am wrong, but it seems to me that the amperage
> through one string would be exactly the same if there was only one string as
> it would be if there were a hundred in parralel. The amperage through a
> string would be dependent on the capacitance of the string. the voltage it
> was charged to just before it was to be discharged, and the resistance (make
> that impedance) of the primary circuit.
> Now if I had 22 caps in series for each string, the capacity would be half,
> the current would be half. and I would need twice as many strings. Or four
> times as many caps to reach the same value.

No, the strings are there to share out the current. The peak discharge current
from a cap of this type is only limited by the ESR and load Z. Imagine the
difference between using a single string of big caps adding up to 0.02u vs.
ten strings of smaller ones adding to the same value. The same discharge
current will be observed, but it the 1st case one string will take the whole
current, in the second case each string will take 1/10 of that current. I am
sure you are probably exceeding the dV/dt rating of your caps. They may not
last long! (Max current=dV/dt*C) Basically, the more caps you use the
longer your MMC will last.

> 2.) You write, that you potted them in epoxy resin. Why? I
> can see NO advantage in doing this. However, there are a
> number of disadvantages doing so:
> Because I have epoxy resin sitting one the shelf. I do not have any perf
> board or any pc board material lying around, and would have to drive 60
> miles or more during the week when I am supposed to be working to get any,
> or else order some and have to wait for UPS to take their sweet time to get
> the stuff to me. Or I could go to Radio Shack and buy some of their sucky
> stuff that would be to small to use for such a thing anyway. Or I could
> build some kind of fancy apparatus like Terry did but I am just to lazy to
> do that. Or I could copy your design, which I like, BTW, but it would not
> incorperate well into my design. I need the cap mounted vertically.

You do not really need a board either. Just solder the strings together and
make a wooden support to grip them. Then if a string fails you can 'pull' it
within seconds for repair.

> a.) Internal heating of the caps canīt be removed. The epoxy
> will act as an insulator.
> I don't buy that. Epoxy has a fairly high thermal mass. It absorbs a lot of
> heat. It conducts it rather slowly. It acts not as an insulator so much as
> like a thermal flywheel. Let me give you an analogy. If you build a house
> out of concret and cover it with six feet of dirt, the heat of summer will
> be reaching the structure at about the same time as the cold of winter is
> outside. The cold of winter will reach the structure at about the same time
> as the heat of summer is outside. Silicone rubber acts the same way.
> If I were planning to run this thing for hours I might worry about it. But
> just a few short bursts from time to time, I think it will work O.K.
> Most of the reports I have read on this list describe the heating as not
> noticeable to bairly noticable.
> If I am wrong, time will tell, and I will let you know.

I would say that having the caps exposed enables you to touch each one
individually to check for excessive heating. Potted, you can't tell if one
single cap is weak and is about to fail.

> b.) This setup is unrepairable. One of the MMCīs advantages
> is easy repair (in case something blows). By potting them, you
> defeat this advantage.
> It is really easier to unpot something from epoxy than the tar in NSTs. All
> you need is an oxyacetalyne torch with a small tip, a stiff brush, alittle
> skill and some patience. You play the flame across the epoxy and it turns
> all crumbly kind of like dried out jello.9 If you heat it too much it
> changes color, gets darker.) Then brush the crumbs away, and flash with the
> flame again. I have used this method to unpot timer modules and solid stat
> relays so I could repair them in the field in emergancies when replacement
> parts were not available or would take too long to get. I have done it many
> times just to see what is inside the little magic black boxes.

Well, as I'd say, if you're prepared to spend time doing this then it's OK.
However it is nice (if you are doing a demo away from home) to be able to
make a rapid repair with just a soldering iron.

> c.) The epoxy will do nothing for corona suppression, simply
> because you wonīt experience any ;o) (outside) corona. As
> for internal corona (partial discharges), the epoxy does
> nothing to prevent this.
> I don't know what you mean by internal corona. If you mean inside the caps,
> then there is nothing I could do about that anyway. If you mean corona at
> the points where the leads are soldered together, I would not expect too
> much there anyway as i twisted the leads of the caps together and then
> wrapped the resistor leads around them, soldered and snipped the exess off
> so that makes some ball shaped blobs which
> would not emit much anyway.  As far as the epoxy causing a problem with
> corona, I have seen too many high voltage parts that were potted in epoxy
> work fine and last a long time to worry about that.

The epoxy should supress corona a little more, but since the lead spacing is
wide anyway nothing but air is necessary as an insulator.

> e.) Voltage versatility. Your setup also defeats the voltage
> versatility of the MMC design (as you canīt change the
> number of caps in an expoxied string).

I'd be inclined to agree here - if you change your transformer you may want
to alter the cap to match it.

> As I understand it, you are not running any resistors across your caps. I
> would be very interested to know if you have found the same thing I did when
> I got shocked by the middle of the string when it did not have resistors on
> it.  As I said, I found some pretty high voltages on some of the caps, and I
> found that the polarity of the voltages was sort of random. Some one way,
> some the other. I wonder if this might be happening while the caps are
> running in a coil. If so, then the caps might be seeing higher voltage
> stress than they would if the resistor were there. Just a thought. Anyway, I
> will always use resistors on any caps that I build in the future no mater
> what design I use if the caps have more than two plates.

I have been bitten my a large part of my MMC. It hurt, bad! Resistors are a
good idea - reducing any excessive voltage stress is a useful aim. Also there
will be a better 'equalisation' of charges during runs, and maybe the cap may
discharge more fully.

> Hope this answers your questions.
> later
> deano

Good luck with getting sparking again!

Alex Crow