Colloidal graphite resistors

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George Schmermund
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Colloidal graphite resistors

Post by George Schmermund » Sun Jun 22, 2008 12:38 am

This post is a spin-off from a thread started by Steven S. I was not going to pursue this further, but it might leave someone out if I don't expand on this idea a little bit. Generally, I don't report on ideas or experiments that I haven't done myself. I also subscribe to the idea that, in general, numbers are perfect; they're not opinions.

Now, as for using 'dag' in the roll of both a resistor and a conductor, I've had many instances where it has solved interesting problems. My main use for this material is in electron microscopy sample preparation. I have bottles of both the water based and alcohol based product. Here's a link so that you know what I'm using for this demo: http://www.tedpella.com/SEMpaint.htm#anchor1349940. There is nothing special about their product.

I already knew what the outcome would be (I've done it before), but I yielded to some unknown force to reproduce the results for any of my skeptical brethren. The test is simplicity itself and proceeds thusly:

Take a basic microscope slide and clean it thoroughly. Next, use a couple of strips of Scotch tape and place them onto the slide to make a gap of some desired width between the strips ( I chose a gap of 1 mm). Now paint a stripe of dag down the gap. The product from Ted Pella comes with a brush attached to the lid the same as nail polish. My dag stripe was made with one stroke of the brush and was 2" long. I used the alcohol based product.

When the stripe is dry, remove the tape and you'll have a uniform resistive deposit. Next, place a dab of dag at each end of the stripe so as to form a pad to make your resistance measurement. My "resistor" measured somewhat over 2M ohms.

As to my original premise regarding a helical resistor, one turn of a 2" diameter glass tube will produce a dag track over 6" long. I'll let the skeptics do the math.
Anything obvious in high vacuum is probably wrong.

John Futter
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Re: Colloidal graphite resistors

Post by John Futter » Sun Jun 22, 2008 1:55 am

George

Excellent idea
Steven S could placeeither a continous resistor or several resistors in series in whatever scheme he wishes and can clean them off and try another scheme.

inside of the glass of course

might have two pi's for lunch

john

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Chris Bradley
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Re: Colloidal graphite resistors

Post by Chris Bradley » Sun Jun 22, 2008 8:43 am

Thanks for the post. I will try to obtain some myself and experiment... any known suppliers in UK?

One potential benefit I see (but tell me if you think this may not be so for some reason) is that it can only fail open circuit because a long strip like that is unlikely to spontaneously 'fuse up' into a conductive region (I can't see how that would happen) - so this seems to me to be a safe substitiute for high voltage resistors.

The other two benefits for Steven's idea are that a helical configuration will keep within the principle of only being able to align in one axis at any one time, plus as it will be bonded to the tube I presume it will have suitable 'thermal management' which is otherwise a risk for components in the vaccum.

best regards,

Chris MB.

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Steven Sesselmann
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Re: Colloidal graphite resistors

Post by Steven Sesselmann » Sun Jun 22, 2008 9:48 am

Thanks George for posting this thread, especially since Richard is planning to purge the other thread.

If the spiral resistor stretched from the cathode to ground, one would want a resistance in the 100's of Meg, which would work out about 50 to 100 turns, possible..

In the mean time I have run my experiment and will report shortly..

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Richard Hull
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Re: Colloidal graphite resistors

Post by Richard Hull » Mon Jun 23, 2008 7:37 pm

I'm glad this particular piece of info got transfered to a non-purgable thread. I also have and use the tedpella product. Mostly for GM tube contacts and conductive light shields on 5886 electrometer tubes. I'll have to do the experiment. I usually use about 10 layers of the material after successive dryings to form a light shield as well as a conductive layer. This may account for my lower readings.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

JohnCuthbert
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Re: Colloidal graphite resistors

Post by JohnCuthbert » Tue Jun 24, 2008 5:21 pm

Someone already mentioned the problem of keeping these cool in a vacuum. Does anyone know what the temperature coefficient is?

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Richard Hull
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Re: Colloidal graphite resistors

Post by Richard Hull » Tue Jun 24, 2008 7:07 pm

Disappation is always an issue with ultrahigh ohm divider resistors and especially in a vacuum. Such extremely high ohm painted resisitors with very thin traces can't be expected to be working dividers from which any decent current could be drawn but if they are high enough in value with no tap points and used only as electrostatic gradient establishers, dissapation should not be an issue.

The painted helical resistor in my Tektronix CRT's has a trace that is about 2.5mm wide, but many feet long. (~50 turns in a belling open tube from 2.5"to a full 5" diameter. resistance is 100megohms

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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