Re: [TCML] TAR

[Ed Phillips <evp@xxxxxxxxxxx>]

Hi Ed,

What was your technique when you tried the Paraffin? I have used 
Paraffin on 10 kV strobe trigger coils.  I submerged the coils in the 
hot Paraffin for a long time.
At first there was a steady stream of bubbles, but eventually they 
stopped.  But I left the coils in much longer after that,
like 1/2 hour.  The Paraffin is really thin at high temp and it wicked 
into the multilayer coil really well.  It is important that the entire 
coil get up to temperature so that the Paraffin does not solidify and 
trap air anywhere.
These coils I had originally came from the manufacturer with Paraffin  
insulation, but it looked like they had only been dipped while the coil 
was cold so there was only a outer coating of  Paraffin.  There was 
sparking under the Paraffin on the first one I tried.  After the 
thorough soaking there was no problems.  If they ever got too hot 
though, it would be a problem as the Paraffin would run out.

Jeff"

   I did something different and probably not as good but it was 
easier.  I wasn't trying to impregnate the coils but just to insulate 
them to increase the external breakdown voltage.  I put the coil 
assembly in the oven and heated it up to about 220° F.  At the same time 
I melted the paraffin in a can and got it about as hot I thought I 
dared.  Then put the hot coil assembly in the mold and poured the 
paraffin over it.  Clearly the coils were above the melting point of the 
paraffin.  I filled the mold with the hot was and in the beginning it 
was pretty well bubble free but as it cooled and the stuff shrunk on top 
it began to turn milky white and then eventually there were some pretty 
big bubbles.  I can't see the coils of course and don't know what the 
paraffin looks like around them but when I run the coil with sparks of 
4" or more [that's from two coils in series] I see light through the 
paraffin from the sparking so it's pretty obvious there are voids or 
more probably big bubbles at the surface of the coils.  I have 
considered putting the whole assembly back in the oven and letting it 
sit for a few hours but haven't done that since the coil assembly 
immersed in oil works just fine and the "potting" was just an 
experiment.  I do want to repeat it with the "tar" if I can get some.

   Since the GMHEI coils are potted to begin with the sparking has to 
be from the HV terminal of the coil to the core so no permanent damage 
occurs.

Thanks for the note,

Ed

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