Sparks in scotland
Finally got sparks (sort of). After finishing my 6 inch secondary the
desire to see somthing happen blew all my best laid plans out of the window.
In order to get somthing up and running a very badly shaped primary was
fashioned out of a length of 1/4 x 3/32 flat copper wire taken from an old
transformer secondary.After fighting for about four hours with it, I ended
up with a ten turn conical primary i/d 8 inch o/d 17 inch. The angle of the
winding is around 50 deg. , Way too steep but i had other things to worry
about after four hours.
A trip to the beach, soaked to the waist, three gallons of sea water was
collected. This was mixed with a further gallon of tap water and half a
pound of table salt and added to ten one pint beer bottles stood in a
plastic storage bin. The final measured capacitance was almost bang on 0.015
The spark gap was made by seven 3.5 inch long pieces of 15mm copper water
pipe clamped between two pieces of polythene bar giving six gaps -at- 30mil
each. (layout similar to the gaps in the vacume gap shown in Richard Quicks
The assembly was then laid flat inside a piece of 6 inch pipe and a large
axial fan, used for force air cooling of an old transmiter valve, sat on top
sucking through. The fan isn,t as powerful as a vacume but has about the
same effect as sticking your head out of a car window at 35 mph.
Power supply was a 30 and 60 ma 10KV neon in parralel with safety gaps and
two 3.5mH chokes in each leg. No bypass caps were available.
The safety gap was set to fire at seventy five percent full voltage (7.5KV)
and the main gap set by dropping copper shorting bars between each gap until
it would fire at fifty percent (three gaps). A homemade 18" x 3" torroid was
set on top of the coil.
The power was wound up and the first thing i noticed was that the gaps fired
at about twenty five percent. The tuning point was then found by using an
earth wire at the end of a wooden pole and the primary tap adjusted until
the largest spark could be drawn. The best tune was found anywhere between 9
1/2 and 10 turns.
Slowly, the power was increased and shorts removed the coil was running at
75 percent but no spark breakout was evident. The safty gap was extreamly active
and at times put the main gap out altogether. About 4.5" of spark could be
drawn with the earth stick but the spark was really spindly and blue. I
opened up the safety to try and get more power out but there was a large
flash and a lot of black soot floating around. A big hole right trough the
middle of one of the windings of the unpotted 30mA transformer. (:-( Six
months of looking has not unearthed any more neon transformers so this was a
bit of a blow.
The very next day on my way to work i spotted two old oil fired heaters just
removed from an adjacent factory unit. A quick looksee revealed two 10KV
25mA ignition transformers which were duly liberated and when tested found
to be ok.
These were then tried in parallel with the remaining neon and worked.
The coil was retuned with a hastily built smaller 12" toroid and safety
gaps reset to my origional settings and eventually, with a sharp point sat
on the torroid, about 5" of discharge was seen. A lot of corrona from the
top of the coil was also seen, not surprising given the shape of the
primary, so the secondary was lifted out untill it disapeared. I dont know
if the discharge could be considered as a real spark, it wasn't the faint
blue haze that the corrona from the top of the coil had, but niether a hot
white spark. Certainly not very visible under a 100watt bulb.
I know there are a lot of areas of vast improvement to be had on this system
i.e. copper tubing primary with lower angle, better capacitor(this four
gallons of water and couple of pounds of glass actually gets quite warm
aftera few mins. run time), but now i have a spark the fun is now going to
be improving on it.
These tentative first steps have led me to a couple of questions though,
1) Would anybody have expected a better perfomance from this initial lash up?
2) Is it normal to see the main gap break down at a much lower voltage than
when set without the tank cct connected?
3) What would be a ball park figure for a safty gap at this voltage/power level
(my gap is almost fully energised at 75 percent power but im affraid to
open it up again for fear of losing another transformer?
As an additional note the system earth probably leaves a lot to be desired,
a six foot copper pipe about five feet away from the coil base connected via
the outer braid of URM67 (RG58) coax and an area of ground plane (counterpoise)
covering an area below the coil at least as wide as the secondary is high.
I look forward to hearing your comments.
Ian Hopley ----> i_hopley-at-wintermute.co.uk