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Re: Atomic Laboratories 71869 Tesla Coil
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi,
At 08:02 PM 10/20/2006, you wrote:
Volunteered to work on a Tesla coil from the local high school. The physics
teacher complained the spark was weak and he also got shocks when trying to
flip the switch on the metal case.
The spark gap might be dirty and could use a cleaning. It may also
be out of tune. Verify that the metal case and parts people touch
has good and intact grounding.
I was able to draw ~1/4 inch sparks off
the case.
Try to find the case resistance to a true ground. The resistance
should be zero. If you still get shocks, let us know.
The first thing I checked was the secondary. I expected to find
the bottom end grounded. Instead it leads to a potted lump of plastic that
is then connectd to the metal case.
Oh... The bottom of the secondary should probably be connected to an
independent RF ground path rather than the case. If it is only
grounded through the AC line ground, The poor RF conductivity of the
AC line ground might allow the voltage on the case to get
"high"... The AC line ground is for 60Hz line faults and safety but
it is not good for high frequency RF grounding.
Inside the plastic I can see a large
disk capacitor labelled .01M 3KV. Across the capacitor are three resistors
in series red-red-green. When I put an ohm meter between the bottom of the
secondary and ground it measures 25 X 10**5. I was expecting to see more
like 66 X 10**5. This suggests some problem with the disk capacitor. I
just can't figure out the purpose of this arrangement. I'm planning to
attach a piece of ground strap to the case so it can be grounded to a water
pipe. I'm not sure what to do with the bottom of the secondary.
The capacitor might mess up a multimeter reading of the
resistance. Or when the coil was arced to ground or something they
blew the cap out.
I cannot think if any use for this plastic thing. I would remove
it. Somebody might have thought it helped tuning or something. I
think these are vintage coils so someone might have had some wild
idea about something. I could not find any info on this coil.
It might be one of these Morris and Lee (now Science First) coils
which are sold under a giant range of add your own logo sticker names.
http://scientificsonline.com/product.asp?pn=3070301&cmss=tesla
http://www.thebakken.org/artifacts/database/artifact.asp?type=category&category=C6.2&id=1856
Here is the manual:
http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=10205#
Cheers,
Terry
Pete from Virginia