[Prev][Next][Index][Thread]

Re: Dynamic Q



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Dynamic Q
> Date: Thursday, August 27, 1998 9:45 PM
> 
> 
> From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
> 
> > Date forwarded: Thu, 27 Aug 1998 06:05:33 -0600
> > Date sent:      Thu, 27 Aug 1998 05:39:35 -0600
> > To:             tesla-at-pupman-dot-com
> > Subject:        Dynamic Q
> > Forwarded by:   tesla-at-pupman-dot-com
> > From:           Tesla List <tesla-at-pupman-dot-com>
Malcolm Wrote:
>" How are you calculating unloaded Q?  I have measured an extensive 
> range of coils with unloaded Qs ranging from somewhere around 60 
> (terrible former) to around 300 in the practical Tesla coil range.
> Other coils built for measurement purposes went even higher. These 
> were measured without breakout. Loaded Qs vary considerably depending 
> on the degree of loading. Corona only is not such a huge dip but 
> attached sparks cause a massive drop as one would expect.
> 
> Malcolm"
> 
 Malcolm, Bert, James et all:

I guess the calculation is the problem. As you pointed out in my
AC resistance post, it may be off by a factor of 2. For some reason
I have an adversion to unloaded Q tests.  There are so many subtle
things that can introduce errors, stray capacitance, scope probes,
ground plane, height, orientation, etc. Considering all that, I feel
the results may be tainted.

One method used was a test with the base grounded to a mesh
plane, which in turn was earthed.  In all cases the bottom turn was 
elevated at least one diameter above the plane.

Excitation was provided by VERY loose capacitive coupling from a
signal generator,consisting of #20 insulated wire wraped around the 
loose coil wire (one or two "stretch turns").  This was dressed away
from the coil at a 90 degree angle to minimize other coupling.
If memeory serves, a 1khz square wave worked well.

Pickup was accomplished by clipping the probe around the magnet
wire (but not electrically connected) near the excitation point.

The coil would ring at it's natural frequency, and the periods could
be counted and the Q calculated.


The system dynamic Q was a more stable type measurement,
owing to the top load and direct measurement.

Here I put a series resistor between the generator ( 100 to 1000 ohms)
and the coil base lead. No, the resistor does not have any effect on the Q!
In a few cases I tried both values for the series resistor and the
reflected
load calculation came out the same!
It's just a matter of measuring the drop across the resistor, calculating
the current, then the refllected coil series resistance,Rs=E/I , where
E is the voltage measured at the base.

>From this, the top relected resistance can be calclated by using
a series to parallel conversion.  Take this RL (top) and divide by 
the charactoristic (surge) impedance and voila!, the dynamic Q.


Yeah I get around 100 for dynamic Q, but what bugs me is. this is so much
lower than the coil's unloaded Q.  I'm running high frequency for Tesla
stuff (900 khz +) so my Qu is high say around 800.

So what's the deal?


Jim McVey