Re: Solving the DC coil mystery

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Kevin, Steve, Antonio, and all,

Kevin's circuit has some additional complexities, and its behavior is
considerably different than the simple DC source, switch, and series
resistor case I previously described. The major differences include:

1. The tank capacitor is not being charged from a DC source, but instead
from a low impedance full-wave rectified source with no filter circuit.
Unlike most other DC/resonat charging systems, there's no interim energy
storage/filter cap in this design - fully unfiltered full-wave DC is being
applied to the resistor. Since the effective voltage source is changing
while the capacitor is charging, the situation is changed considerably -
See Antonio's recent post about the effects of changing the applied voltage
WHILE charging the capacitor.

2. The RC time constant of the circuit is about 310 uSec, so that the cap
is "mostly" (95% in 3*RC seconds) charged to the instantaneous mains
voltage every millisecond or so after the gap quenches from the previous
firing. Since gap presentations occur at approx. 1.67 milliseconds (600
BPS) intervals and the source impedance (PT and Variac) is comparatively
low, we can assume that the capacitor voltage will lag a bit behind the
instantaneous mains voltage from the PT (if it's rising) or meet it (if
falling) before the gap fires again at the next presentation, but the
capacitor voltage should almost track to the instantaneous PT output
voltage after every 1.67 millisecond recharge interval.

3. Per Kevin's ftp site, his rotary gap will fire at a given presentation
whenever the tank capacitor voltage is greater than 7 kV. The circuit
behaves sort of like a sampling circuit where the tank cap voltage (when
the gap fires) is a "sample" of the incoming mains voltage taken every 1.6
milliseconds (for Vcap > 7 kV). 

A SPICE simulation would be required to properly estimate the power
dissipated in the resistor versus that reaching the tank circuit since the
system is considerably more complex than the simple DC RC charging case I
presented. 

Nothing is ever simple with Tesla Coils.  :^)

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com


Tesla list wrote:
> 
> Original poster: "Kevin Ottalini by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <ottalini-at-mindspring-dot-com>
> 
> Steve:
>         Sorry for the minor typo there ... the current is ~0.292amps and I
> left an
> earlier number (.250 amps) in the calculation ... only a small difference
> though,
> it raised the loss to 426watts (from 425) and lowered the efficiency to
> 87.8%.
> 
> The scenario here is that the power supply is charging a purely capacitive
> load
> (the tank cap) so there is no resistive divider present to decrease gross
> efficiency or limit the max voltage going into the capacitor.
> 
> Certainly, if I was only running 50% efficient my resistors would go out in
> flames, but this is not the case (sorry Bert!).
> 
> Given long enough, the capacitor would charge to the full output voltage as
> if
> the resistor wasn't there at all.  The only losses are dynamic ones caused
> by
> the current passing through the resistor, and mostly only during the peak
> charge current phase when the capacitor is starting up from ground.
> 
> In reality, the capacitor is never fully discharged while running so the
> resistor
> never usually sees the absolute max possible current, although I see
> something a little worse during the discharge itself and during ringing.
> 
> You can see the schematic for my system here:
> ftp.mindspring-dot-com/users/ottalini/highvoltage/DCCOIL/DCSCH2A.PDF
> 
> Since this is a working system and the resistors don't get hot, the caps
> and diodes are cool,  the caps charge up to around 20KV if I don't spin
> the ASRG and I do run at power ranges up to 4Kw, I can only say that
> conventional wisdom (and reality) holds true.
> 
> Best,
>         Kevin
> 
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Wednesday, July 25, 2001 6:19 AM
> Subject: Re: Solving the DC coil mystery
> 
> > Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <youngs-at-konnections-dot-net>
> >
> > Kevin,
> >
> > Very interesting.  But how do you get much better than 50% efficiency?
> That
> > does not agree with Bert Hickman's proof that half the power is dissipated
> > in the series resistor.  Why do you use 0.250 amp in your resistor power
> > loss equation?  If you meant 0.292 amps, that is the same mistake I made -
> > one can't use the average current to calculate the resistor loss.  If Bert
> > is correct, and I believe he most certainly is, then your resistors are
> > dissipating more like 1,750 watts if you are charging your tank cap
> through
> > them.  Maybe your circuit setup is different than I am imagining.  Can you
> > do an ASCII rendition of it or let us know where it is posted?
> >
> > --Steve