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Re: TC Critical Coupling (was Overcoupling


  Malcolm, All -

  The Tesla coil overall efficiency issue is a conundrum like the power vs
energy discussions of a couple years ago on this List. The equation
    efficiency = output/input   looks simple enough  but for a researcher
this equation represents many difficulties when applying it to electrical
devices especially the Tesla coil. 

  In the past many coilers have used the capacitor energy equation
    joules = .5 C V^2     to find the TC efficiency. But is this correct?
The losses found in this way do not represent all of the losses in the Tesla
coil system. Left out are the losses for the power transformer, DC power
supply, wiring, etc. Also there is the problem of the unknown amount of
charge left on the capacitors before they are recharged by the power supply.
This affects the efficiency. Note that the output of a capacitor is reactive
and must be used with caution. The scope waveforms can tell you a lot about
what is happening inside the TC system but have limited use for determining
the overall efficiency.

  These problems can be eliminated and all of the losses can be easily
included by using the proper tests. Tests using known loads are the answer.
For example to find the efficiency of an electric motor a Prony brake, a
known output load, is used. For a Tesla coil a controlled spark or
incandescent lamp which are known output loads can be used. There are other
possibilities.

  The "output" in the above efficiency equation means useful output, or
useful work, that is, work done outside the TC system which can be easily
measured. This output can be of a type selected to give the necessary data
for finding the efficiency. The controlled spark can give you an "input
watts per foot of spark" and the incandescent lamp can give you an "input
watts per output watts". Efficiency graphs can then be developed using this
data. I show in my books a preliminary start for this type of test. I also
show an efficiency graph based on the limited data presently available.
However, much more work needs to be done. 

  I will be interested in what advanced coilers like you, Marco, Terry, etc,
will produce regarding this thread in the near future.

  John Couture

----------------------------------------

At 10:13 PM 5/12/99 -0600, you wrote:
>Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> 
>
>Hi John,
>
>> Original Poster: "John H. Couture" <COUTUREJH-at-worldnet.att-dot-net> 
>> 
>>   Malcolm -
>> 
>>   Thank you for the reply. To determine the amplitude of the TC energy don't
>> you need to measure the amplitude of two parameters plus the time period?
>> 
>>   Energy = Volts x Amps x dt
>
>Not at all. Knowing capacitance and instantaneous voltage, E pops 
>straight out. 
>
>>   To determine the overall efficiency you need the input energy and the
>> output energy.
>> 
>>   % Efficiency = 100 x Output energy/Input energy  
>> 
>>   If I understand your reply correctly you measured only one parameter and,
>> therefore, did not have the coil energy.  
>
>See above. E = 0.5CV^2.  In order to determine the energy retained by 
>the system between secondary maximums as a percentage of initial 
>energy, you go thus:
>
---------------------------   big snip