Re: MMC question

At 04:13 PM 6/7/99 +1000, you wrote:
>    Hi all.
>My name is michael and I am new(ish) tp this list.
>I would like to know if standard metallised polyester greencaps will
>work at all in a coil. I was recently given a few hundred (like 500) of
>these each rated at 100Vdc and 0.49uF.

I don't know what cap value you want but these caps look like they would be
very difficult to use.  You need a lot in a string to get to a reasonable
voltage but then the capacitance is pretty high.  Polyester will heat
quickly and you will only be able to use one or two strings which means
they will see very high current.  For example:

250 caps in two string gives a 25kV 4nF cap.  The current will really be
too high for just two strings.  I fear you would wire up 500 caps and it
would fail quickly.  I don't know all the variables but I would really be

>I understand that these are probably undesirable, however it's all i've
>got at the moment (other to a 2nF rolled cap that's too small for my
>I would also like to know if  "Plastic film Boxed casing MKT 250V" caps
>will work. I have made a few bricks (5" X .5") of 360 each, but am
>worried about blowing them when they have other uses. They are each
>rated 250V and 0.022uF.

The key is if they are polypropylene or not.  If so they have a good chance
of working.  Just don't let them heat up.

>My other question is this.
>I am running a 4" coil with 16 (30 degrees incline) turns of 3.5mm Iron?
>fencing wire for the primary.
>My spark gap consists of up to 10 X 0.5mm gaps.
>I'm using 3 15Kv 30mA NSTs and am only getting 12" discharges.
>My cap is a 20nF (about) rolled PE type.
>I've raised the secondary to reduce coupling but to no avail.
>Does anyone have any suggestions??

It definitely sounds like something is out of tune.  The primary and
secondary my be working at different frequencies.  I would guess you
primary circuit is running around the 100kHz region but your secondary
probably wants to run at a much higher frequency.  Try tapping in the
primary.  Maybe to as little as five turns.  

The fence wire is not the best but that is not the main problem here.  With
more info I could tell your much more about were the tuning was at.

>Thank you very much for your time.