Science Day TC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Michael Rhodes by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rhodes-at-fnrf.science.cmu.ac.th>

Glad to see the list is back - thanks Chip and Terry, been
trying to post this for several days...

Science Day Tesla Coil Project - First Light!
Chiangmai University, Thailand
Sponsored, designed and supervised by Michael Rhodes
Work group - 10 physics students, two other physics professors and two 
machinists
Design, construction and first light time frame - 3.5 weeks
Home page:  http://www.fnrf.science.cmu.ac.th/tesla/

Specifications can be seen at the above home page.

I was chosen to sponser a 2nd year Physics students science fair
project and my project choice was, yes - Tesla Coil.

I've been operating in panic mode for a week
now getting the TC ready for Science Day here at the university.  I was
initially told it started on Sunday(18th) but on Tuesday was told it started
on Thursday (15th) and lasted three days.  The coil was not wound yet, the 
spark
gap not assembled yet, etc.  Wednesday I received the parts for the spark
gap and the coil was wound but the last coat of varnish was not put on
until about 4:00p.m. so it had to dry overnight.  I assembled and adjusted
the spark gaps and handed it over to the students.  Thurday morning (first
day of the show)  we had to rush assembling the complete system.  This
included making last minute modifications of the assembly, running the
ground wire and driving a 2 meter ground stake in, making the toroid
standoff, the secondary base holder, and wiring it up.  The varnish was
still not dry and some came off when we laid it down on a cloth to connect
the secondary base and toroid wires.  We put another thin coat on after
it was completely assembled.

About 3:00p.m Thursday we were ready to fire it up.  You can imagine
how nervous I was, so many things could go wrong when rushing a big
job like this plus I knew the students would been very disappointed if
they didn't have a presentation for the next two days of the fair.

I had them set up one of the extra 2 Meter ground stakes at an angle to the
toroid and about 20cm from it for initial power up tests.  The tuning point
on the primary coil was based on my QuickTC program which now supplies
not only the turn number but the number of degrees on the turn to place
the cable.  So I put it at this spot.

The control box consists of a key switch, mains power switch, and a big
press/hold button (has to be physically kept pressed for the coil to run - 
safety
feature).  Pushed the button in and started slowly turning the variac up,
shortly the spark gap began firing.  Continued turning it up and 'bang' we
had 'first light', a white streamer from the toroid to the ground rod.  Big
hurrah and aplause sounded out by the students.

That was 20cm without tuning and I showed the students how to tune
the primary and keep a log book of position and streamer length (good
practice for physics students to learn how to keep records).  I left them
to do that.

Next morning (first day for them to show the TC) I came over to see how
it was going.  I could hear the coil running and smell the ozone thirty
feet down the hall.  The windows were sealed with black plastic to darken
the room.  The windows and door were opened between shows to
clear the ozone out.  This room has no air conditioning either, just a
couple of fans

The Science Day fair is held at different universities each year and this was
the first time here in about 4-5 years.  Primary and high school students
from a hundred miles around or more come to see the exhibits.

When I approached the door I was surprised by the large queue of students
waiting to get in for the next show.  When the doors were opened, the
room was filled to capacity (about 60-70 students) and this is with 2/3
of the room roped off for safety (not a big room either).

When they started the coil up I was pleasantly surprised to see over 70cm
blue streamers striking the ground rod and lots of growing streamers
all around the toroid.  Since we had really no time to really tune the
unit and it is monsoon season here now (hot and very humid) I felt this
was excellent results considering absolute max possible is about 100cm.
(15kV-at-30mA NST)
I looked at the tuning point they had set it to - it was only about 2 cm
from where my program indicated.

I had given them some ideas to enhance their show.  One was to walk
around with a fluorescent tube in their hand, the other was to place
a clear light bult on the top of the toroid.   They had both a white and
a yellow fluorescent tube plus the bulb with toilet paper tube cut
to hold it on top of the toroid.  But probably the best was their own
idea.  The took a piece of the #2 AWG stranded wire I used for
hookup, stipped the insulation off and fanned the strands out so
it looked like a widely space bristle brush with the bristles pointing
upward.   Beautiful effect, they were getting free growing streamers
coming off the points upward to 50cm plus all the other points were
glowing with smaller streamers.

The one idea I got off the Internet was to microwave a CD and place
it on top.  They told me they tried it without much success.  I showed
them how to place the ground rod directly over the top  of the CD to
draw the streamers up and through it.  Got the rod to close and when
we started it, the streamers were hot (white).  When we turned the
TC off and the lights back on I looked at the CD.  All we had was a
completely transparent disk and a lots of ash/particles lying on top
of the toroid.  Too much current...

I instructed them to run the TC for about 1 minute then wait for 5
minutes for the next run.  This worked out well in that my students
would explain the basic concepts of the TC to the visitors then
switch to the next group.  I would say our display was the most
popular one since the room was overflowing for every show.

I did run into a problem with the spark gap.  The total gap spacing was
set for slightly less then .28" and I was using a resonant size cap 
(actually slightly lower
6.25nF vs 6.3662 calculated for a 15kV-at-30mA NST.  Further in the day
the spark gap would occasionally arc over (like with a safety gap)
between the two connectors.  These arcs were more then one inch
in length!  I can understand this if the spark gaps missed firing
due to resonant rise but when I put the NST on the spark gaps
only (no primary or capacitor attached) the sparks would start
an run continuously at about 110VAC on the variac.  This NST
is 230VAC input.  As the day progressed the frequency of these
large arcs increased.  Any ideas? I had no time to make any
measurements on the variac output but it was wired correctly.

Also I have a 220VAC 50Hz fan at the end of the spark gaps.  If I look
through the blades they are not stable but jerky like the spark gap is
miss firing.  Perhaps I need to close the gaps a little or at least
increase the capacitance of the primary cap.  Just too late to do it
now but an opportunity for the next group of students to enchance
the coil.

I have some nice pictures of the TC in operation and of the construction.
They can be seen at http://www.fnrf.science.cmu.ac.th/tesla/
Some interesting construction points that may be of use will be included.

Terry, I apologize for the size of this e-mail, just excited about the outcome.
This is the first coil I built after 3 I built 30 years ago.  The conceptual
advances that have been made are remarkable.

--Michael