Neon Sign transformer / Tesla Coil question

J

J Shrum

Guest
Greetings.
I have been working on a small tesla coil based around a 10KV 30ma neon
xformer.

I've built the spark gap, tested, works.
I've built a 12 pack of bottle capacitors, measured 8nF.
All seemed well until I tried to hook up the spark gap and the capacitor. W/
the spark gap alone, it throws sparks just fine... but as soon as I
introduce the capacitor in either series, or parallel... it stops the spark
gap from firing until I turn off the supply and turn it back on. But if the
capacitor is hooked to it... nada.

Could this be a protection in the neon transformer that shuts off w/
overload? Shunt? I'm not real certain what to do at this point, as I've
spent weeks searching google for everything I could find prior to building
this thing, and now I'm beyond frustrated.

Any help is much appreciated. Thanks

Jim
KC9FFX

(cross posted to sci.electronics)
 
In article <-7-dnTOBTrnXyK_fRVn-tQ@madisontelco.com>, J Shrum wrote:
Greetings.
I have been working on a small tesla coil based around a 10KV 30ma neon
xformer.

I've built the spark gap, tested, works.
I've built a 12 pack of bottle capacitors, measured 8nF.
All seemed well until I tried to hook up the spark gap and the capacitor. W/
the spark gap alone, it throws sparks just fine... but as soon as I
introduce the capacitor in either series, or parallel... it stops the spark
gap from firing until I turn off the supply and turn it back on. But if the
capacitor is hooked to it... nada.

Could this be a protection in the neon transformer that shuts off w/
overload? Shunt? I'm not real certain what to do at this point, as I've
spent weeks searching google for everything I could find prior to building
this thing, and now I'm beyond frustrated.
Click to expand...
I suspect 8 nF is rather high for many to maybe most neon sign
transformers and will load down their secondary voltage.

I suggest:

1. Reducing the capacitance. I think 8 nF is quite high to charge within
half a cycle of AC line frequency from a neon sign transformer. 8 nF at
30 mA only gains 3.75 KV per millisecond, and at 60 Hz zero-to-peak time
is about 4.17 milliseconds, and also consider whatever may not be ideal...
I have experience of only a couple nF doing well with a neon sign
transformer for this!

2. Blowing forced air through the spark gap if the sparking through the
spark gap with the lower capacitance results in the "spark" conducting in
an excessively continuous (quiter) manner. The sparking should be a very
bright bluish white bright enough to put spots in your eyes and noisy
enough to interfere with normal conversation at close range unless noise
abatement measures are taken.
Also make the spark gap electrodes bigger, bulkier, beefier, and better
able to stay cool!


CAUTION - such sparks generate nitrogen dioxide - a gas that is to some
extent corrosive to lung tissue and maybe some rubbers, has an
orangish-brownish color when accumulated in a confined area (I saw this
happen in a jar enclosing a Tesla coil primary spark gap), and has an odor
somewhat like somewhere between chlorine and ozone.

3. Increasing the number of turns of the primary, in a proportion that is
inverse square root to capacitance as capacitance is changed. Reduce
accordingly (as in proportional to square root of capacitance) the
secondary voltage.

4. ALTERNATIVE - fullwave rectify the output of the neon sign transformer
before feeding this to the capacitor. Expect the sparking frequency to be
on the low side, less than once per half cycle of the AC line frequency if
the problem turned out to be requiring tghis fix or the above fix due to 8
nF being too much capacitance to charge to an adequate voltage within a
half-cycle.

5. Make sure the spark gap is not too wide. Depending on spark gap
geometry, the voltage required to spark through can somewhat easily be as
much as about 3 KV per millimeter plus 300V! More extreme - 3.3 KV per
millimeter plus 300V!

- Don Klipstein (don@misty.com)
 
One additional comment:

Whats very strange about this, is that I don't even have the primary
connected. Its just the NST, cap, and gap in parallel.

I only connected one side of the capacitor to the gap, so the current isn't
even going through the cap, and still the sparks are very light and easy to
blow out. W/ just spark gap, bright sparks that can't blow out. But when I
hook even one side of the capacitor, it goes out. Does it appear that there
is something wrong w/ the NST?

If it helps, I can draw a diagram...

Thanks again for all the help.

"J Shrum" <shrumj@spam.is.evil.madisontelco.com> wrote in message
news:WKadnVmRUL5Pya_fRVn-iA@madisontelco.com...
Greetings.
I have been working on a small tesla coil based around a 10KV 30ma neon
xformer.

I've built the spark gap, tested, works.
I've built a 12 pack of bottle capacitors, measured 8nF.
All seemed well until I tried to hook up the spark gap and the capacitor.
W/ the spark gap alone, it throws sparks just fine... but as soon as I
introduce the capacitor in either series, or parallel... it stops the
spark gap from firing until I turn off the supply and turn it back on. But
if the capacitor is hooked to it... nada.

Could this be a protection in the neon transformer that shuts off w/
overload? Shunt? I'm not real certain what to do at this point, as I've
spent weeks searching google for everything I could find prior to building
this thing, and now I'm beyond frustrated.

Any help is much appreciated. Thanks

Jim
KC9FFX
Click to expand...
 
In article <FbSdnfqRsO9sO67fRVn-ug@madisontelco.com>, J Shrum wrote:
One additional comment:

Whats very strange about this, is that I don't even have the primary
connected. Its just the NST, cap, and gap in parallel.

I only connected one side of the capacitor to the gap, so the current isn't
even going through the cap, and still the sparks are very light and easy to
blow out. W/ just spark gap, bright sparks that can't blow out. But when I
hook even one side of the capacitor, it goes out. Does it appear that there
is something wrong w/ the NST?

If it helps, I can draw a diagram...

Thanks again for all the help.
Click to expand...
I suspect the several picofarads of stray capacitance are loading down
the voltage of the neon sign transformer - which I would not expect unless
the transformer is one of those high frequency electronic ones - in which
case putting just one nanofarad across the secondary will probably load
down the voltage to less than 1 kilovolt.

If the transformer is a conventional one, I suspect the spark gap is
marginal (too wide) and only intermittently sparking through, although I
thought that if it sparks through without the capacitor the spark would
become a somewhat flame-like (but not really bright) arc. Also check for
bad connections.

CAUTION with the more conventional transformers and capacitors - do not
connect the capacitor without a spark gap. If the capacitance is that
which results in resonance with the leakage inductance (around a few
nnofarads) and no spark gap is present to limit the voltage, the voltage
may become excessive for the transformer's insulation.

- Don Klipstein (don@misty.com)
 
Thank you for your reply. I was thinking the same thing about the stray
capacitance... it seemed to bog down jus w/ an extra few feet of wire
attached to it.

Your right about the arc, its a firey flaming one... not a white one. I'm
pretty sure at this point that it is a high freq xformer, and thats likely
my story...

Thanks for the help, at least it makes a pretty cool jacobs ladder.

"Don Klipstein" <don@manx.misty.com> wrote in message
news:slrnd37gas.d9d.don@manx.misty.com...
In article <FbSdnfqRsO9sO67fRVn-ug@madisontelco.com>, J Shrum wrote:
One additional comment:

Whats very strange about this, is that I don't even have the primary
connected. Its just the NST, cap, and gap in parallel.

I only connected one side of the capacitor to the gap, so the current
isn't
even going through the cap, and still the sparks are very light and easy
to
blow out. W/ just spark gap, bright sparks that can't blow out. But when I
hook even one side of the capacitor, it goes out. Does it appear that
there
is something wrong w/ the NST?

If it helps, I can draw a diagram...

Thanks again for all the help.

I suspect the several picofarads of stray capacitance are loading down
the voltage of the neon sign transformer - which I would not expect unless
the transformer is one of those high frequency electronic ones - in which
case putting just one nanofarad across the secondary will probably load
down the voltage to less than 1 kilovolt.

If the transformer is a conventional one, I suspect the spark gap is
marginal (too wide) and only intermittently sparking through, although I
thought that if it sparks through without the capacitor the spark would
become a somewhat flame-like (but not really bright) arc. Also check for
bad connections.

CAUTION with the more conventional transformers and capacitors - do not
connect the capacitor without a spark gap. If the capacitance is that
which results in resonance with the leakage inductance (around a few
nnofarads) and no spark gap is present to limit the voltage, the voltage
may become excessive for the transformer's insulation.

- Don Klipstein (don@misty.com)
Click to expand...
 
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