Author Topic: Resistance wheelsets  (Read 2020 times)

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Ed Kapuscinski

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Re: Resistance wheelsets
« Reply #60 on: November 01, 2021, 09:35:11 AM »
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It's possible some of the conductive paint got below the resistor and shorted it out .. I always let them dry overnight and then check conductivity with a meter ..

Yeah, that's likely what happened.

After that burned out though it seems perfectly fine. I guess I'll put the meter to it (since I don't want to torch the place now that it has a semi-working layout).

C855B

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Re: Resistance wheelsets
« Reply #61 on: November 01, 2021, 10:00:09 AM »
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It's possible some of the conductive paint got below the resistor and shorted it out .. I always let them dry overnight and then check conductivity with a meter ..

In the case of the conductive paint Ed is using, this is very important advice. I bought a tube and have been testing its characteristics:

1) It has a considerable resistance value when wet. I squeezed out a 1" line of paint roughly the diameter of the nozzle, the line measuring 5KΩ when wet.

2) It takes a while to fully cure. The test line was still pliable after 24 hours, although resistance had reduced to ~1KΩ.

3) It does not have zero resistance when dry, but in our application it may as well be 0Ω. The test line was 200Ω.

4) The cross-section of the test line played an important role in determining the final resistance value. Thinner = more resistance.

5) A test wheelset where I simply painted a line across the insulator went from ~500Ω to ~5Ω overnight. Prophetic. What I found especially interesting was while it scraped off easily, there was enough residue from the initial scraping-off to still be a virtual short. I had to hit it with a brass brush before it cleared. Probably should follow-up with alcohol and a toothbrush.

My edumaguess is the stuff is graphite powder in an acrylic medium. Resistors are made of some form of graphite mix. So what you have are grains of graphite touching each other in a suspension, the more grains touching, the lower the total resistance. A thin cross-section means fewer grains touching.

Bottom line for Ed's needing to summon the volunteer fire department was because there was enough resistance in the uncured dabs of paint to not anger the short-circuit gods. Like John said, the fun started apparently when one of the globs made its way under a resistor and dried to the point where it drew enough current for the light show.

peteski

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Re: Resistance wheelsets
« Reply #62 on: November 01, 2021, 01:07:10 PM »
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Most commercially available resistors are either carbon composition, or carbon film.  Graphite is a form of carbon.
I wonder if Ed is using silver-bearing conductive paint, which would have even lower resistance than the graphite-based conductive paint.
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C855B

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Re: Resistance wheelsets
« Reply #63 on: November 01, 2021, 01:09:59 PM »
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...I wonder if Ed is using silver-bearing conductive paint, which would have even lower resistance than the graphite-based conductive paint.

No, I'm using the same stuff. It's powdered graphite. The silver concoctions are über-expensive.

Bill H

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Re: Resistance wheelsets
« Reply #64 on: November 01, 2021, 06:28:52 PM »
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No, I'm using the same stuff. It's powdered graphite. The silver concoctions are über-expensive.
I guess one man's uber is another's lyft. The silver conductive paint that Daryl used in his video is about $25-30 and he did over 300 cars with it and remarked he had plenty left over.

Kind regards,
Bill

C855B

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Re: Resistance wheelsets
« Reply #65 on: November 01, 2021, 06:57:57 PM »
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I guess one man's uber is another's lyft. The silver conductive paint that Daryl used in his video is about $25-30 and he did over 300 cars with it and remarked he had plenty left over.

Curious where he bought it, then. Digikey wants $54, Mouser $62, Adafruit $60.

peteski

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Re: Resistance wheelsets
« Reply #66 on: November 01, 2021, 08:19:58 PM »
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Curious where he bought it, then. Digikey wants $54, Mouser $62, Adafruit $60.



When I bought this stuff I'm pretty sure it was under $20, but that was years ago. I needed it for a project back then. The paint is probably dry by now.  :facepalm:
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Sumner

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Re: Resistance wheelsets
« Reply #67 on: November 01, 2021, 08:26:22 PM »
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Curious where he bought it, then. Digikey wants $54, Mouser $62, Adafruit $60.

Bought some "Bare" on Amazon  ( bare conductive electric paint pen 10ml ) for about $12 but haven't used it so don't know how it compares ....


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trainzluvr

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Re: Resistance wheelsets
« Reply #68 on: November 01, 2021, 08:40:11 PM »
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That CW2200 pen is professional grade item for electronics use, and I posted about it because you get what you pay for.

If you got Amazon you can't really expect much for $12, and having questionable ingredients that will burn your layout (and/or house) down. ;)