RE: troubleshooting tesla coil (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 15 Jun 2007 09:32:24 -0400
From: "Lau, Gary" <Gary.Lau@xxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: troubleshooting tesla coil (fwd)

Hi Marko:

I see several problems.

The main problem is your NST.  You have a solid-state high frequency
unit, which is fine for powering signs but simply cannot be used for a
Tesla Coil.  For use in a Tesla Coil, the NST needs supply power at 50
or 60Hz, not 30 KHz.  What you need is called a core and coil
transformer.  They're much larger and heavier.

A second problem is the construction of your capacitor.  Mylar is a good
insulator, but when used at the high frequencies in a Tesla Coil, is
becomes lossy.  This means it absorbs some of the power, and gets warm
as a result.  If it goes from warm to hot, it will fail.  If you can
charge the cap and fire the gap when using DC, I think your capacitor is
OK, beyond it being lossy.

The circuit you describe - the NST, gap, and capacitor all in parallel,
does nothing now because your NST is supplying too high a frequency and
the capacitor can't charge fast enough to fire the gap.  But if you had
a proper NST, this circuit is asking for trouble.  The capacitor WOULD
charge to the gap firing voltage, the gap would fire, and would short
out the capacitor.  This would be LOUD, as all of the energy would be
dissipated in that spark.  Some capacitors might be damaged by this, as
the current of a direct discharge through the gap is higher than the cap
is designed to sustain.  (Normally in a proper coil circuit, the primary
inductance serves to limit the discharge current to a safe value)  You
can probably do this for a couple seconds, but I wouldn't push it.

Regards, Gary Lau
MA, USA



> Date: Thu, 14 Jun 2007 19:32:17 -0400
> From: Marko Ruban <Marko@xxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: troubleshooting tesla coil
> 
> Hello all
> 
> I was looking for some help on tesla coil construction, and joined
this
> list as a result.  Looks like I came to the right place :)
> 
> I'm building my first tesla coil, and could use some help "debugging"
> it.  All of the coil parts have been assembled, according to various
> recipes out on the web, but when put together, the spark gap doesn't
fire.
> 
> I've got the circuit down to a bare minimum:  5.5KV, 30Khz NST
provides
> the power, spark gap connected across the transformer output leads,
and
> a capacitor in parallel with spark gap.  Without the capacitor, spark
> gap fires just fine, with it, I just hear humming sound (I think
coming
> from the vibrating capacitor plates), but no spark.
> 
> Capacitor was home built, consists of 8 copper sheets separated by
10mil
> Mylar insulator, roughly 8"x6" area.  Measured C is 7nF.  When DC
power
> is supplied (through a rectifier circuit), makes the gap fire at
> intervals, indicating that cap is storing charge.  I thought this
could
> be my problem component, so I built a different type of capacitor
(beer
> bottle salt water, 800pF), but that didn't change a thing.  Neither
> capacitor is shorted out, according to my multimeter.
> 
> Is there any definitive way to test the capacitor for faults?  Am I
> missing something else?  What could be going wrong?
> 
> Thanks, for any thoughts you can provide on the subject.
> 
> Marko
> 
>