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Re: [TCML] K-value? - was: New pic of 60ma coil with suggestions applied

Hi Steve,
Coupling is no big deal as a few good programs will calculate coupling as good as your meter could measure (or better depending on meter accuracy). Just throw in the coil dimensions to any of these programs and they will calc the coupling for you. But keep in mind, output is only as accurate as the input (so precise dimensions help give precise output in both dimensions and inductance). 

JAVATC at   http://www.classictesla.com/java/javatc.html
MANDK at http://drsstc.com/~sisg/files/mandk/ <http://drsstc.com/%7Esisg/files/mandk/> 
ACMI at   http://abelian.org/acmi/
INCA at http://www.coe.ufrj.br/~acmq/programs/ <http://www.coe.ufrj.br/%7Eacmq/programs/> 

You can also measure coupling:
I've pasted a procedure from Terry Fritz back in September of 1998 describing the method I personally use. I've used this method since and it is easy and accurate.
I do recommend either a hair dryer or a heating element in water. You want the current and volts to "not" fluctuate (you want the resistance to stabilize). The water method works, but water adds to the danger. I like the hair dryer method. To put the hair dryer in series, one of the plugs terminals goes to hot, the other terminal to the primary. Of course to measure current, a meter goes in series with the hair dryer and primary also. The hair dryer is easy and accurate. Let the hair dryer stay on for about 30 seconds then take volt and amp readings. The blown air across the heating element naturally balances out the current through the heating element and it's an easy and handy tool that most have on hand. I've tried "many" ballasts for this measurement (including a 3000W resistor) and nothing was as stable as a simple hair dryer.
Anyway, if you do measure, be careful as when current is flowing, the primary is live with 120Vac. Several coilers have made the measurements many times. Even though I "know" the program is correct, I still like to do the measurement on a new coil as a cross check. 

Here's the procedure from Terry Fritz back in 1998
BTW, the resistor and cap across the secondary is unnecessary.

Take care,
Bart
---------------------------------------------------------

All,
       I have tried all the suggestion I have received (Thanks Malcolm, Fr.
Tom, John C., Mark Rzeszotarski).  The best method I have found that does
not require expensive equipment or great theoretical challenges consists of
the following.

       Apply a heavy 60 Hz AC current to the primary coil.  This is best
done by placing a space heater, hair dryer, etc. in series with the primary
to limit the current to about 10 amps.  Measure this current with a
multimeter.  Note that the space heater gives a fairly stable resistance.
Light bulbs have a non-linear resistance through the AC cycle and distort
the measurement (they must cool down substantially at the nodes of the AC
cycle).  Of course, use great caution with the live AC on the primary so as
not to kill yourself. Only the isolated primary need be connected to the AC.
The capacitors, transformers, and other wiring should be disconnected from
the primary for this test.  Be cautious of the AC finding its way on to the
secondary! 
Place a 10k ohm resistor and a 1uF capacitor across the secondary and
measure the AC voltage.  It will be on the order of say 100 mV AC.  The
resistor and capacitor will eliminate stray noise picked up by the secondary
and swamp any resonance which is significant at these low levels.

The mutual inductance is found by:

       M = V / (w * I)

Where:

       M = Mutual inductance in Heneries.
       w = the line frequency in radians per second (377 for 60Hz or 314
for 50 Hz).
       I = The measured current in the primary in amps AC.
       V = The measured secondary voltage in volts AC.

As an example:
       If the current in the primary is 10 amps and the frequency is 60Hz
and you measure 0.100 volts AC, you would get:

0.100 / (377 * 10 ) = 26.52 uH for the mutual inductance.

k can then be found by using the formula:
k = M / sqrt(L1 * L2) 
Where L1 and L2 are the inductances of the primary and secondary coils.

       This method is rock solid in theory and easy to do.  The accuracy is
excellent.  There is little that can go wrong compared to other methods and
you don't need anything special other than a multimeter to do the test.  The
accuracy is dependant on the accuracy of your multimeter.  My tests could
easily get within 1%. 
       Thanks again for all the great suggestions and do be careful with
the AC if you try this.

       Terry

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


Stephen J. Hobley wrote:

I don't know how to determine/calculate a K value of a coil - is there a reference somewhere?

Thanks,
Steve

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