[TowerTalk] WWII Caps

[Jim Lux]

> 
>They have little in common with the Tesla application, which requires
>1000-amp-plus pulses to be delivered at high rep rates, and loss factor
>(ESR and DF) optimized in the 100-200 KHz region.
> 

Kind of a digression here..

Tesla coils have fairly reasonable RMS currents and not too extreme primary currents.  A really, really big (tens of kVA) coil might get to kiloamps, but it's pretty unusual, a few hundred amps for a pole pig powered coil might be more typical.
Taking as an example, a 5kVA coil powered by a pole pig, the primary voltage might be 14.4kV (RMS) or 21kV peak. The primary capacitor would be about 40nF.  The primary inductance might be  100 uH. (fres around 80kHz)
The cap stores about 9 Joules, and the peak current, assuming NO losses is about 420A.  In reality it will be less. A typical loaded Q is about 10, so after a few hundred microseconds, the current is down to zero.

The RMS current is a lot less (if you're drawing 5kW, with no loss, at Erms=14.4kV, then the rms current has to be about 350mA)

A coil powered by a 15kV, 30mA neon sign transformer might have a 5-10nF primary C, 80 uH primary L, 200kHz, and have 200 amp peaks, with an rms current around 15mA.  Even DC filter type doorknobs work, although they do get warm.  A stack of 10-15 0.10uF 1.5kV or 2kV PP caps (called a MMC in the tesla coiling world)  would last forever in this application. (So would the beer bottle capacitor with salt water).

Higher powered coils don't have hugely larger currents, because they're physically larger, so have a lower resonant frequency. Both factors lead to more primary inductance, which keeps the primary current reasonable.  (that is, the C/L ratio doesn't change a whole lot, and everyone works at 15kVrms kinds of voltages, because it's convenient)


Vacuum Tube TCs (VTTCs if you google) at a basic level are power oscillators, and not much different from a high power 160m transmitter in construction and operation. Think of it as a oscillator where the tank is the antenna system, and you have a very short radiator with a capacity hat and a big base loading coil.  There are folks building what's called in the laser business a MOPA: Master Oscillator/Power Amplifier where you have a master oscillator to set the carrier, and you key the amplifier. 

Solid State TCs, which have much lower primary voltages and lower primary inductances, tend to have much higher RMS capacitor currents (tens of amps), but they also have lower peak currents. THey're almost always a big switcher run by a master oscillator.

For folks looking to build a really robust 160m antenna or matching network that can handle a lot of power, looking to the VTTC and SSTC folks isn't a bad thing to do.  They've done a lot of work on tracking down low loss capacitors, inductor building techniques.  Especially if you need an adjustable high power capacitor or inductor (Sure, you really want that 1kW compact loop, right?) and you want to homebuild it, there's a lot of useful resources there.  (heh.. several TC builders I know did things like design RF sources and the tuning networks for plasma etchers for a living)