Re: Question regarding first (small) coil

["sundog" <sundog-at-timeship-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi B. Marshall, All!,

  I'd avoid the ceramic caps, as they tend to heat up and their
capacitance will drift.  This makes the coil start out in tune and
then drift out of tune.  That'd be enough to steer *me* away,
unless they were free, and all I had.  If SW caps aren't the way
you want to go, I'd recommend an MMC over rolled poly.  Less mess,
less bulk, seem to be much more robust.  I've tried to kill mine,
and it hasn't died yet.  I *did* kill 2 1/2 trannies, but that's
for another post ;)

 Now, the bleeder resistors, they don't seem to do a whole lot for
the voltage equalizing, but they *do* drain the cap down in a
fairly short time, and keep it shorted when not in use.  The
panasonic caps can hold their charge over a week.  Just trust me on
that.  I use 10 megaohm resistors from Digikey, they run ~$13 for
1,000 of them.  So far, it's a good investment, as I've used them
as bleeders, charge-limiters, current-limiters, voltage dividers,
etc.  The things are more useful than I ever thought they'd be.  10
megaohm's will work fine, and are cheap.  get the half-watt ones.
Check the list, I believe there's an MMC bulk buy going on, and
that'd be great to get in on.
                                                             Caio!

Shad
-----Original Message-----
From: Tesla list <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Sunday, August 13, 2000 4:32 PM
Subject: Question regarding first (small) coil


>Original poster: "B Marshall" <bm-at-bens-stuff.fsnet.co.uk>
>
>I've finally got round to begin construction of my first coil,
using a car
>ignition coil as a
>PSU. I have no idea on the output, but I imagine it is about
12-15kv -at-
>50-60W. The primary is 10
>turns of 1mm ECW in a flat pancake formation, and the secondary is
880
>(approx) turns of 0.125mm
>ECW wound around a 1 1/2 inch pvc pipe. I am planning on building
a
>polythene sheet/Al foil stack
>type capacitor, if I can find a suitable thickness of polythene
sheet (the
>output from the
>ignition coil blasts straight through any plastic bag thickness
material).
>
>My question concerns the capacitor. I understand that it is
important to
>put a bleeder resistor
>in parallel with the cap for safety when the coil is not in use.
What value
>and power rating
>would be desirable in this case?
>
>I have also been wondering about building an MMC. Would HV Ceramic
Caps be
>suitable? The ones I
>have in mind are from RS components (UK), rated at 1000vdc 0.01uF,
and cost
>£0.18 each.
>Unfortunately, the catalogue gives no information for high
frequency
>(>1khz) operation, the
>dissipation factor is 1.5% at 15v rms 1khz.
>
>Regards,
>Ben
>
>
>
>