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Re: [TCML] Black printing can be *very* conductive!
Good test Phil! Certainly sounds conductive. Now, if we do the normal
prep coating on the secondary, the ink becomes insulated from the
winding (or even if we don't, the wire insulation will insulate). I
think what we end up with is a "hot spot" for lack of a better term. An
area that absorbs energy. I really don't think it's that much, but I
guess that depends on the print coverage and voltage across the area.
This area might even help cause a carbon track. But, there's also the
fact that many including myself have not had problems with the ink, so
that has to be in the mix also.
Given what you have measured, I think DC's advise to sand away the ink
is good advice.
Well, I was sitting in my office today, and I recalled our discussion
about the conductivity of black printing on Sonotube cardboard forms.
So I went to one corner, and picked up a corrugated cardboard box with
black printing on it and in the other corner I got my megger.
My megohmmeter is a slightly older AEMC Model 1015, but it's been just
fine for my purposes. I used the 1000V setting.
The corrugated cardboard box had black printing on the side. The part I
tested had a black rectangle, 1" by 3", with an "unprinted" 5-digit number in
the middle. Call it 60-70% black coverage in that rectangle. And not the
greatest printing job, as it wasn't as solid a color as it could've been.
As I had posted earlier, I knew the black pigment in the ink was
basically soot, but I was astounded at how much a difference in conductivity it made!
From one end of the rectangle to the other, I read 1 Meg of resistance.
Enough that the 1mA of current sparked form the probe to the surface of the
box, and I could instantly, and strongly, smell burnt paper! If I touched the
probe down just a coupla millimeters to the side of the black rectangle, the
resistance read infinite - or at least over 1000 Megs, according to my
megger. Interestingly, the smallest reading I could attain was 300K, with the
probes 1/4" apart on the black rectangle. So it doesn't seem linear with distance.
I was surprised that the basic cardboard box itself was "infinite"
resistance - the corrugated layers are held together with water-based starch
adhesive, and the paper layers themselves have some moisture content.
So there you have it! Black printing on cardboard can be *dangerously*
conductive - enough to pass a few mA at a few thousand volts, or enough to
ignite the underlying cardboard! And this was a rather weak print job.
Center for the Advanced Study of Ballistic Improbabilities
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