Re: [TCML] Black printing can be *very* conductive!

[bartb <bartb@xxxxxxxxxxxxxxxx>]

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.

Take care,
Bart

FIFTYGUY@xxxxxxx wrote:
> Folks-
>  
>     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.
>  
> -Phil LaBudde
> Center for the Advanced Study of Ballistic  Improbabilities
>
>
>
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