How to rise the secondary?
From: terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent: Thursday, July 02, 1998 11:26 AM
To: Tesla List
Subject: Re: How to rise the secondary?
you asked many good questions! I added my thoughts as to the answers.
At 11:13 PM 7/1/98 -0500, you wrote:
>From: John H. Couture <couturejh-at-worldnet.att-dot-net>
>To: Tesla List <tesla-at-pupman-dot-com>
>Date: Wednesday, July 01, 1998 2:58 PM
>Subject: How to rise the secondary?
> To Ed, Antonio, Bert, and Malcolm -
> Many thanks for all the information regarding the raising of the
>secondary. It is gratifying to know that with the Internet and Tesla List it
>is possible to ask technical questions of this nature and be able to receive
>immediate replies from around the world. Coilers of only a few years ago
>were at a tremendous disadvantage without this type of resource.
> As is typical with TC design, answers to a technical question sometimes
>brings up more questions.
> 1. Doesn't changing the coupling (K) only change the time of energy
>transfer for a tuned TC system regardless of frequency? For example with K =
>.20 the energy will transfer for all coils and operating frequency:
> Transfer time = 1/K = 1/.20 = 5 half cycles.
In my coupling coefficient paper, I scoped the charge time for various K
values and found the following:
K 1/K Multigap peaks Simple gap Peaks
0.0226 44 19 24
0.0256 39 17 27
0.0286 35 17 23
0.0324 31 15 23
0.0369 27 15 21
0.0421 24 13 17
0.0489 20 13 15
0.0557 18 12 14
0.0647 15 11 13
0.0760 13 10 12
0.0888 11 9 10
0.1046 10 8 9
0.1242 8 7 8
0.1475 7 6 7
0.1753 6 6 6
0.2069 5 5 5
The 1/K formula tracks fairly well for values above 0.08 but below that
there is growing error introduced.
It can be seen in the paper that that very low K values produce lower
secondary currents (and thus lower output voltages). Since we work with the
higher values the energy transfer does seem to be somewhat consistent but
there are issues as shown in the paper.
BTW the web page is www.peakpeak-dot-com/~terryf/tesla/experiments/experiments.html
The paper is "A Comparison Study of Multi-Gap and Simple Tesla Coil Spark
Gaps with Various Primary to Secondary Coupling Coefficients."
There are many high resolution scope pictures which make the page about 4
megabytes long.... be patient!
> 2. The total amount of energy transferred will occur when the TC system is
> That is when LpCp = LsCs
> What are the equations relating the amount of energy transferred to
>special coupling coefficients?
The equation LpCp = LsCs is a definite winner! :-)
Barring any miss-tuning and massive design flaws, a good working coil should
transfer almost all of the primary energy into the secondary. The only real
loss in that system is in the spark gap. If the coupling is high and the
quench is proper the transfer will be very efficient. If the coupling is
low, other losses start to steal energy from the system before the energy is
transferred and the efficiency drops.
My resent testing shows that the secondary corona losses can be very high.
It is necessary to make the transfer time very fast so that the losses do
not have too much time to steal energy from the system. With high coupling
the transfer time is very short and the corona losses are not too bad. Good
secondary design is very important to insure that the losses are minimal at
> 3. If the primary is designed with enough clearance to prevent sparkovers
>between the pri and sec coils, raising the secondary would be unnecessary?
>In other words the coupling is determined only by the sparkover clearance
>limitation. Using insulation instead of air to separate the pri/sec coils
>could be used to reduce the clearance and increase the coupling. But this
>would not change the amount of energy transfer or the length of sec term
If the coupling is too high, the gap will not quench well and energy and
output are lost. As my paper shows, over coupling can waste a great deal of
energy and generally mess the system up. Even though the peak voltages are
high the system looses output. I can actually see this in my system when
the coupling is just at the point were it flips between proper and bad
quenching. When the quench is good the output is good. When the quench is
bad the output drops way down (and the spark gap is obviously dissipating
more power). Even though the peak voltage is still high, the voltage drops
back down and the follow-up peaks are not available for output. the energy
is transferred back into the primary system where is gets chewed up. Good
quench is VERY important! Today's gaps are not able to quench well enough
to use maximum coupling. Thus, it is necessary to lower the coupling until
the quench is proper. Perhaps some of the recent gap information will give
us better gaps. What I learned from my latest gap and mentioned in
yesterday's post is very significant!
> 4. What are the equations relating the quenching characteristics to
>coupling and output spark lengths?
There are no equations at this time. Coupling depends on the current
magnitude, parasitics, frequency, gap type, gap width(s), etc. My paper
gives some information but there is little in the way of hard equations to
predict quench behavior. Perhaps some type of "empirical" (God, forgive
me!) technique could be used. This is one area that needs much research!!!
As far as output goes the following rule is key to good output.
"The coupling should be as high as possible while allowing quenching at the
first energy transfer."
> 5. In #1 above the number of half cycles required to transfer all of the
>energy of a tuned TC from pri to sec coils can be found. However, how many
>half cycles are required to properly charge a suitable sec terminal so there
>will be a sec output spark and quenching time is not important?
I think you mean is there a safe K value that is low enough so that the coil
will always work. No! The performance change as you approach the critical
quenching point is dramatic. It is well worth carefully adjusting and
pushing the coil to the edge of good quenching.
> 6. The point of over/under coupling (critical coupling) is determined when
> Rp = Rs How are these two parameters calculated at the time of
> The equation R = Xl/Q cannot be solved because both R and Q at
>high voltage operation are unknowns at time of design.
Throw this equation away! Over/under coupling, Rp=Rs, and critical coupling
assume the Telsa coil is using standing waves. This has been pretty well
disproved on spark-gap type coils. The jury is still out on CW coils (the
losses are probably too high in these to support standing waves). The
coupling needs to be much higher than these equations would suggest.
Rp and Rs are insignificant compared to corona losses. My models still use
these values to predict low-power non-breakout behavior but when the sparks
fly these losses are insignificant.
However, before I reached this conclusion I did study them in detail:
Rp is the series combination of the primary coil's resistance with skin
effect, the spark gap's effective resistance (non-linear), the wiring
resistance, and the internal resistance of the primary capacitor. These can
all be calculated before the coil is built (the spark gap is by testing).
However, they should be insignificant in the overall operation of the coil
unless something is wrong.
Rs is the resistance of the secondary (with skin effect), the resistance of
the ground path (the loop from the top terminal to the bottom of the
secondary), and the resistance of the top terminal. In my coil this
resistance is ~270 ohms. There should be more study of the ground
resistance. It does drain the secondary some and the ground loop is very
poorly understood. The resistance of the top terminal has recently gained
new importance considering the high frequency nature of output arcs. There
is evidence that a highly conductive, RF friendly top terminal will support
longer arcs much better than a sloppy resistive terminal.
I now have the ability to measure the resistance of corona losses. however
this will change drastically from system to system. If the coil is well
designed, it should hold the maximum terminal voltage with low losses. This
is often overlooked.
> John Couture
Often, with the internet, when you ask a question you end up with more
answers than you want. The number of original questions just can't keep up
with all the answers!
All the best