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Re: Burned Secondary...




From: 	Mark S. Rzeszotarski, Ph.D.[SMTP:msr7-at-po.cwru.edu]
Sent: 	Thursday, September 11, 1997 2:59 AM
To: 	Tesla List
Subject: 	Re: Burned Secondary...

Hello Dan,
        You said in part:
>I made a couple of changes to my system the other day and promptly had
>several turns weld together on the bottom 1/3 of the secondary. I cut that
>part off, retuned, and had the rest of the secondary burn the same way. I
>don't mind this though...Last time I jumped up a level in coiling, it was
>because I was burning secondaries (arcing through the inside of the form),
>but it's not that way this time. Here's the system before the changes:
>
>I never saw any inter-turn arcing, but pin-point hot-spots shorted the
>secondary turns in several places.
>
>So I'll wind an identical secondary soon, but before I do, I'd like to know
>what burned me up. :)  I can guess over-coupling, so I've purchased 1/4"
>copper tubing to make a flat-pancake primary, spacing 1/4" if it's over
>coupling.

        Although you did not provide complete info regarding your primary, I
ran a couple of computer simulations based on what you probably have:
Current configuration is approximately:
 Saucer-Shaped Spiral Primary Coil Geometry
 Primary coil inside diameter (inches)=      10.00
 Primary coil outside diameter (inches)=      18.25
 Number of primary coil turns =    11
 Last turn elevation in inches =       1.00
 Wire diameter (inches)=      .1600
 Solenoidal secondary diameter (inches)=      6.25
 Secondary height (inches)=      22.00
 Number of secondary coil turns =   800
 Secondary coil wire diameter (inches)=      .0250
 Primary coil inductance in microhenries:           66.16
 Secondary coil inductance in microhenries:          24108.
 Secondary coil capacitance in picofarads:     10.67

 Position is secondary coil bottom turn position in inches
 above bottom turn of primary coil
 M = mutual inductance in microhenries
 K = coupling coefficient
           Position           M                K
            -3.00           369.5            .293
            -2.00           340.9            .270
            -1.00           305.7            .242
              .00           265.1            .210 <============  probably okay
             1.00           223.1            .177
             2.00           184.2            .146
             3.00           150.8            .119
             4.00           123.3            .098


If you build a flat pancake primary, you can expect similar performance:
 Flat Spiral Primary Coil Geometry
 Primary coil inside diameter (inches)=      10.00
 Primary coil outside diameter (inches)=      18.00
 Number of primary coil turns =     8
 Last turn elevation in inches =        .00
 Wire diameter (inches)=      .2500
 Solenoidal secondary diameter (inches)=      6.25
 Secondary height (inches)=      22.00
 Number of secondary coil turns =   800
 Secondary coil wire diameter (inches)=      .0250
 Primary coil inductance in microhenries:           31.58
 Secondary coil inductance in microhenries:          24119.
 Secondary coil capacitance in picofarads:     10.67

 Position is secondary coil bottom turn position in inches
 above bottom turn of primary coil
 M = mutual inductance in microhenries
 K = coupling coefficient
           Position           M                K
            -3.00           259.6            .297
            -2.00           237.2            .272
            -1.00           210.2            .241
              .00           180.0            .206 <===========
             1.00           149.5            .171
             2.00           122.2            .140
             3.00            99.4            .114
             4.00            80.9            .093
             5.00            66.2            .076
             6.00            54.6            .063

        Assuming the bottom turn of your secondary is even with or above the
bottom turn of the primary, you are not over-coupled.  For a coil of this
size, I would shoot for a K of 0.18-0.22 range.  I am curious, however,
about your operating frequency.  Your primary is tuned to around 125 kHz if
I made the right assumptions about your geometry.  Your secondary is
self-resonant around 300 kHz, and is probably operating with a resonant
frequency around 180 kHz, if I am figuring the capacitance of your toroid
right.  If this is the case, you will get some rather nasty standing waves
on your coil, which could explain your results.  You may want to go back and
check your tuning.
Regards,
Mark S. Rzeszotarski, Ph.D.