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Re: [TCML] Repairing a Coil - Please Help
If a 60 ma transformer replaced a 120 ma transformer, then the capacitor
will normally be too big for the 60 ma transformer. The .019 in use
there is rather close to the resonant value of the 15/120, and it may
have been over-volted if the gap has been set too wide. The .019 is only
a smidge above the LTR value capacitor for the 15/60, so it should still
be in a usable range. If you can find a 12/60 neon to put in its place,
it will be a good match for that capacitor. I keep my gap at 7/32" and
get good results with my coils. That represents about 17,000 volts
according to the old spark gap charts, if you can believe them.
If the coil was known to work before, check both sides of the
transformer and see if the output on both ends (individually to ground)
are robust. Then, check the capacitor bank for damage. After that, you
will have to use your judgment if neither of those two components give
you a fix.
Comments are added to your text...
Brian Kelly wrote:
Hail to the great TCML geniuses, I am in need of help.
The National Center for Atmospheric Research is hosting a Super
Science Saturday this weekend and has previously done a lightning
demonstration with a Tesla coil. However, it is not working well
today and I have been asked to help because of my limited coil
experience from high school. Any suggestions to get it working again
for the kids would be most appreciated.
My understanding is that the transformer went bad and was replaced.
I was told the coil needs a 60KV, 120mA transformer but that it
currently had "only" a 60KV, 60mA installed. We later discovered
that the installed transformer was actually 15KV and 60mA. I suspect
the original transformer was 15KV and 120mA but I am not certain.
A 60KV 120mA transformer would be rather largish ( avg size about the
same as one of those Milk crates that hold 4 gallons of milk) a 15KV
120 or 60mA transformer would be about the size of a shoe box if not a
tad bit smaller.
The coil itself is modest. The secondary is about six inches in
diameter and three feet tall. The primary is about seven turns of
.25 inch tubing. The capacitor is an array of 16 high voltage DC
caps with a total capacitance of 19.2 nF. Two nice inductors (about
500 turns around 2 inch plastic pipe) protect both poles of the
transformer. The spark gap is enclosed in square metal plates
sandwiched together in a way that made me originally mistake the
whole unit for another capacitor. 4 long nylon screws hold the whole
thing together so I wonder if the gap is adjustable.
sounds like it is adjustable, take a look to see if there is some
dust bunnies or foriegn objects between the plates, could be
artificially reducing the gap width and lowering the firing voltage
which will produce smaller fuzzy output.
Fuzz ... normally know as corona. The coil is putting out a voltage
off of the top load but not nearly as much as it should be.
Right now, when the coil is turned on, the spark gap fires and the
secondary coil gets a bit of charge, I would describe it as fuzz when
you get a ground wire close. It is nothing like the 6 foot sparks I
understand it is normally able to produce. I suspect the coil is
well tuned as the clip location for the primary is very well marked,
although again, I could be mistaken.
Changing the transformer from one amperage capacity to one of a lower
value ( in your case by 50%) it would increase the cap charge time
considerably ( 2-3X factor).
My main question - is it possible that switching out the transformer
with one with half the amperage could make the difference between
fuzz and 6 foot sparks? Would the voltage have to change or would
there need to be another problem?
Without seeing/hearing the coil running, it would be somewhat
difficult to tell you how to do that. Things I would check on (
process of elimination) would be
And my followup - does anyone have any suggestions that might make
this coil function for the kids, even if at a lower capacity?
1. check spark gap for dirt etc between plates. ( avg total gap width
should be around 5/8" to 1/2")
2. consider the multi cap system having one or more caps not
functioning properly ( dried out, shorted internally)
3. see if the transformer is putting out the proper voltage on both
4. look carefully at the primary for previous marks as to where the
lead "was" attached.
5. see if the safety gap is firing too often when running ( more than
once per several seconds is a minimum)
6. MAKE SURE that the secondary coil base wire is attached to a solid
earth grounded item ( preferably its own ground rod)
7. Stand facing N.E. on one foot while holding your left hand palm up
to the East while chanting some monosylabic verse 5 times when you
turn it on. ;)
My thanks in advance,
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