RE: [TCML] Building my first tesla coil

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Jeremy,

Correct - current best practice is to size the tank cap at roughly 1.5 times the mains-resonant value.  There's nothing magical or precise about the number 1.5; I've found that 2.0 works slightly better for me, but the final number will likely be dictated by how your particular cap goes together.  BTW, don't forget that the mains resonant cap size depends on the mains frequency.  As you have 50 Hz power, you will be using a larger cap than someone in 60 Hz land.

The reason for using an LTR value is not to protect against spikes.  It's not immediately obvious, but the voltage to which the cap will charge to is not as simple as the peak voltage that the NST is capable of putting out (15kV x 1.414 = 21.2kV in your case).  If you were using a cap that was "near" the mains-resonant value (the value that would resonate with the NST secondary inductance at 50 Hz), AND if the spark gap was set so far apart that it would not fire, the voltage would get higher and higher with each mains cycle (resonant rise), and within a few cycles, would reach in excess of 50kV or more, until something - a spark gap, safety gap, capacitor dielectric, or NST insulation, breaks down and sparks.  This is why it's so critically important to set the spark gap width correctly - so that it fires as soon as the voltage reaches the maximum value that the NST can safely endure.  By using a cap value that's significantly larger than the mains-resonant value, the degree to which the voltage will rise above the nominal peak voltage (should the gap be accidentally set too wide) is reduced (but not completely).  

Additionally, I think it's been shown that the maximum power throughput through the NST is achieved by using an LTR cap value.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Jeremy
> Sent: Wednesday, January 21, 2009 3:20 AM
> To: TCML
> Subject: RE: [TCML] Building my first tesla coil
> 
> 
> ok, i've kind of given up on precisely measuring the NST specs, i'll just assume it's
> 15kV 30mA since those are the closest reg specs. Now i have questions on the
> MMC. I have heard from sources that a larger than resonant bank of about 150% is
> recommended can anyone here confirm this as well as the rationale behind it?
> They claim that it protects the components from HV spikes though i'd tend to think
> the contrary since a larger bank accumulates higher voltage and current. The last
> thing I want to do is fry my NST.
> 
> thanks and regards,
> jeremy
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