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Overhead transparencies are conductive? (RESOLVED)

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Crunchy Frog

Mon Apr 19 2010, 02:49AM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

I'm building an HV cap with aluminum foil and PET (i thought) overhead transparencies, similar to

except flat. So I get the thing built, and try to measure the capacitance, and the meter won't give me a number. So I measured the resistance, ant it comes out a few hundred ohms. So I put the DMM probes on either side of one sheet and pressed it together, and it game me about 50k!
What's going on?

They're 3M write-on (AF4300), if that helps.

Patrick

Mon Apr 19 2010, 04:05AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

My overheads seem to be polyester, and they work fine
is there printing on them?

I am suspicious of your procedure, did you use that hotplate? and that variance in resistance makes me think something is loose or not in full contact.

Proud Mary

Mon Apr 19 2010, 11:54AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

3M OHP film is made from BOPET - the dielectric used in metal foil capacitors.

Bjørn

Mon Apr 19 2010, 12:06PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Overhead transparencies can have different coatings, depending on how they are meant to be used.

Some of the AF4300 packages says "Acid free on the smooth side" whatever that means.

hboy007

Mon Apr 19 2010, 12:38PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

these are write-on transparencies, maybe they made them antistatic so you don't have that hair-raising sensation when working with them. Those for laser printers work fine, they only exhibit surface leakage effects. At 25-30kV you can create glowing branched discharges that reach out almost 2cm around the edges of the electrodes. The dissipated currents are low but they create an awesome smell of ozone.
I'm sorry to tell you that you bought the wrong type of foil. How about adding some conductive silver glue to the game and build a resistive touchscreen instead? *runs for cover*

Crunchy Frog

Mon Apr 19 2010, 01:18PM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
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Posts: 85

I don't think they have anti static coating, they seem to stick to each other pretty well. And what do you mean the wrong kind of foil?
I bought the write-on ones instead of laser printer ones because they were cheaper

hboy007

Mon Apr 19 2010, 02:23PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

I know the write-on transparencies, the coating comes off as a slimy mess (gelatin) when you make them wet. Whatever it is, it's certainly no good for homebrew capacitors. Mine are for monochrome laser printers and have a slightly milky and yellowish appearance when piled up together. The package says "for dry toner printers" and wiping across a sheet with the back of my hand tells me how well every charged particle would like to stick to it. According to the following source, they consist of polyester with some modifications.

"The imaging layer of most commercially available transparencies consist of either acrylic of fully esterified epoxy resins, often mixed with quaternary ammonium ionically conductive polymers. Such systems generally have a glass transition point of from 55 to 75 degrees C. A back side coating is almost invariably an acrylic resin, which contains polymeric quarternized ionic conductors and spacer particles formed by large 5 to 10 microns polymeric beads, made from urea-formaldehyde or acrylic resins. "

Crunchy Frog

Mon Apr 19 2010, 05:28PM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

So I tested it today and it worked fine. Huh. Sorry to bother everyone lol.

hak8or

Mon Apr 19 2010, 10:00PM

Registered Member #2025 Joined: Fri Mar 13 2009, 05:39PM
Location:
Posts: 16

I don't think it holds a charge for too long due to the resistance, if I am not mistaken?

Patrick

Mon Apr 19 2010, 11:34PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i think you have a slight or intermittent short, that would explain why it works now, but not before. maybe that hot-plate is not a very good method.

@Hboy007, it is ink jet and bubble jet transparencies that exhibit that gelatin crap when wet. i am almost certain that the
vis-a-vis pen writing ones have no coatings.

i prefer the laser printer ones myself anyway.

@ProudMary where is your source for this BOPET claim, everywhere i look i see them quoted as polyester!

OOps! Proud mary I recant , O great one.... http://en.wikipedia.org/wiki/PET_film_(biaxially_oriented)#Graphic_arts
wiki "pet film or bopet film" if this link doesnt work.

-Patrick

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