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DIY Conductive Paint for Cavity Shielding

Discussion in 'Pickups & Electronics [BG]' started by Pulplogic, Oct 11, 2009.


  1. Pulplogic

    Pulplogic

    Feb 2, 2007
    San Diego
    I recently stumbled onto an “Instructable” on how to make conductive paint from easily obtainable supplies (http://www.instructables.com/id/SEOH3GFFABRWUOD). I used a variation of this technique to fabricate a stretchable strain gauge for a non-bass related project. The results were promising, so I decided to try using a similar concoction for cavity shielding. While the electrical conductivity achieved using this method is not as high as foil tape it should provide respectable electromagnetic shielding.

    The basic idea is to create a paintable mixture containing graphite powder. Since graphite is highly conductive the surface painted with the graphite will have a low resistance once dry. The formula I used was one part 3M Scotchkote Liquid Tape, 1.5 parts graphite powder, and 1 part Acetone. I created the mixture in small batches, just enough for one coat since the solution starts to setup in about 10 minutes. The solution was painted on in three coats using disposable acid brushes allowing 20 minutes between applications.

    The materials can be purchased locally at most hardware stores in small quantities. I bought my supplies from McMaster-Carr mainly because they sell graphite powder by the pound (around $10).

    Supplies.

    Home Depot or Walmart will have something more like this:

    HD_Supplies.

    The test subject is a recently purchased '76 P-bass that had been modded for EMG's before I bought her. My plan is to clean her up and outfit with custom active pickups / electronics and an aluminum control plate. Here is the body with the control cavity cleaned-up and threaded inserts installed.

    76P-Cavity-UnShielded.

    Copper tape strips were laid down over the lip of the control cavity at the inserts to provide a conductive path to the control cover.

    Copper-Strips-Applied.

    The liquid tape was mixed with the graphite using acetone as a thinner.

    Solution.

    The control cavity and pickup routs after three coats of conductive paint.

    76P-Cavity-Shielded.

    After drying for one hour the resistance was measured across the length of the cavity. A resistance of less the 100 ohms was measured between any two points in the control cavity.

    Resistance-after-one-hour.

    The resistance should continue to drop over the next 24 hours as the solvent evaporates. I will post an update with the final results.

    --John
     
    Boom762 likes this.
  2. Fantastic.
     
  3. xaxxat

    xaxxat Supporting Member

    Oct 31, 2008
    Cool...
     
  4. :eek::eek:
    It's definitely going to go way down when it dries, right?
    Not to criticize, but 92.2 ohms is absolutely unacceptable.
    Personally, I wouldn't be satisfied with any shielding job over an ohm.
     
  5. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Nice work.
    Years ago I tested a number of materials and conductive paints in a shielded enclosure over a very wide frequency range looking at RF attenutaion.
    As you pointed out, copper tape (especialy the embossed type) has a much higher RF attenuation potential than a coating.
    While I don't recall the actual test results, I can tell you that any paint that has a low resistance / high RF attenuation is VERY expensive and usualy contains silver.

    Thye down side of tape is that it is very difficult to line internal cavities with no or tight radiuses as it will tear easily. If you really want to go crazy, form a shield from copper shim stock and solder the corners.
     
  6. it's a shield...not a current-carrying conductor...

    On what criteria do you base your decision that 92 ohms is too much?

    personally, I wouldn't go through the hastle of making a graphite paint when you can just use a zinc undercoat instead (available off the shelf)...
    and Zinc also is a magnetic shield as well as electrical...

    but, there's a huge misconception out there about what shielding actually does...
     
  7. Pulplogic

    Pulplogic

    Feb 2, 2007
    San Diego
    You are right, it will certainly not yield the dc resistance of copper foil. As far as EMI attenuation, I am hopeful for acceptable results.
     
  8. gino69

    gino69

    May 9, 2012

    Man you have a misconception on shielding, the 92.2 ohms registered on the digital tester is just to show that there's a continuity on the process of his "DIY project", and we should wait till his next post about the update on that shielding with these process was a success or not.

    Correct me if im wrong but my thoughts is that its not part of the circuit, its a process to cover the circuit to eliminate noise.
     
  9. I know all about the physics of shielding, but even when the voltage drop across 92 Ohms is insignificant, the goal is always to have the most direct path to the ground potential possible.

    In hindsight, I should have waited for an update to see what the final reading was.
     
  10. Here is what my Jazz bass with copper tape reads out at, from the two points shown. 0.27 Ohms total, with 0.1 Ohms of that being the resistance of my test leads.
    7168772514_55a9f047d1_b.
     
    Boom762 likes this.
  11. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

    Well, you can sure see the difference between paint and copper. My copper jobs have nearly nmeasureable resistance from one point to another. 92 Ohms to me is not shielding.

    For those unsure of the process - shielding does not block external noise (this is nearly impossible), it conducts it to ground.

    Your use of the copper tabs under the paint to connect to the pickguard is very clever - I do the same - but you should have only ONE tab. It is like star grounding - only ONE path to ground if you want to do it correctly. Your several tabs could create a tuned antenna and increase the noise - especially with the high resistance of the shield paint.

    This is all so critical because the voltages produced in a guitar are so incredibly small, and the impedance is so high - it is really easy to create audible noise.
     
  12. when I was fixing up my bass I wanted to put some shielding in it, I think DiMarzio makes some, anyway I wanted to do it but you know how it is, I just wanted to get the build over with and play, was it foolish to not put shielding tape in? and maybe should I go back on a rainy day and install some? also, when I was thinking about shielding I was curious about a paint on version, so it's interesting seeing someone try it out!
     
  13. There is tremendous debate over the effectiveness of shielding. At the end of the day, you have to remember that in most cases, you are dealing with a very incomplete Faraday cage, so shielding can only be so effective. Many people feel it is useless, however, others have found applications in which there is a noticeable improvement with shielding. FWIW, I have personally heard a noticeable reduction in high frequency electrical noise with shielding on one of my basses, with a reasonable degree of confidence that I have ruled out other factors, so I remain open minded. I've never seen any reason not to shield all instruments regardless.
     
  14. madmachinist

    madmachinist

    Dec 28, 2008
    whether or not the graphite concoction works , the cost of
    the materials involved would likely be prohibitive (to those
    who didn't already have the stuff )

    imho , copper tape just costs too much . it's a
    specialty item and has limited availability - so the seller can
    charge whatever they want/can get for it........


    i use aluminum-foil tape ........the kind used to seal A/C
    ductwork ....it's available at any hardware store @ $3 to $4
    for a 50' roll - enough to shield 20+ instrument cavities .
    i've used it in every instrument i own which
    didn't come w/ factory shielding - i can tell a night and
    day difference in some instruments ....others never
    had any problems to begin with - but i put the stuff in there
    just in case .

    http://www.google.com/products/cata...1&ie=UTF-8&tbm=shop&cid=15446723062419918669#

    (i have a peavey bass that used to play a
    local AM radio station at certain times of day ....
    the foil tape fixed the interference - the only downside
    is that i can't keep up with the local news while playing
    it anymore :D )
     
  15. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    When it is done correctly shielding does in fact block noise. A good shield will be highly effective even when there is no "ground" to conduct to. A good shield does not have one connection between its various pieces. A good shield does not have several or many connections between its various pieces. A good shield has an infinite number of connections between its various pieces: it is physically and electrically continuous.

    If you connect the "lid" (pickguard shield) of a shielding structure to the "box" (control cavity) with only one tab or only a few tabs then unconnected gap between tabs (or the entire circumference of the box in the case of only one tab) acts as a slot antenna and the frequencies at which it resonates pass through the shield relatively easily. Only a continuous connection between the box and lid can prevent this. Obviously if you have several tabs that are not evenly spaced you have multiple antennas with differing resonant frequencies. This will generally be an advantage over one tab since all the resonant frequencies are higher than that of a single tab because the gaps are shorter and higher frequencies are less likely to cause an issue with a guitar. Of course if your electronics are well designed to reject RF interference a single tab will suffice since everything that gets through will be too high to bother them. It is a little hard to say exactly what frequencies are involved in causing the problems that we use shields to combat so it is hard to say if one tab is enough or if more tabs are better. More tabs are never worse unless you are faced with a single interfering frequency that can be blocked by avoiding a gap length that corresponds to it. In that case one tab could be better than several tabs but more tabs yet would also be good. Infinitely many tabs (no gaps) is always best.

    The only time you intentionally use one connection in shielding work is when you are connecting double shields together. Guitars are never double shielded but a double shield would be one copper box supported inside an outer copper box with an insulating layer between them. Both boxes should be completely continuous and they should connect at one and only one point.

    Ken
     
  16. mrbell321

    mrbell321

    Mar 26, 2012
    N. Colorado
    So... why electrical coating? It is an insulator. What you really want is something to hold the graphite in suspension without adding resistance. Not that I can think of anything that would fill the bill, but latex paint would be cheaper and you wouldn't need the acetone...
     
  17. DrVenkman

    DrVenkman

    Jan 22, 2010
    Pacific NW
    In response to the idea that copper tape is too expensive and hard to find: go to a garden supply store and buy some slug tape. It's a lot cheaper than going to Stewmac. The only drawback I see is it doesn't have conductive adhesive. I solder the joints anyway because I don't trust the adhesive long term, so it's a no-brainer for me.
     
  18. Stealth

    Stealth

    Feb 5, 2008
    Zagreb, Croatia
    Hobby shops that sell glassmaking equipment (stained glass, glass beads etc.) sell it too, it's not expensive - and it has conductive adhesive. But DrVenkman's right, soldering the overlaps is better.
     
  19. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
  20. bassbenj

    bassbenj

    Aug 11, 2009
    Actually, making conductive paint is a very tricky thing to do even not so hot conductive paint. The reason is that while you can add conductive particles like graphite but virtually all "paints" you'd mix them in are insulators. And it's obvious to get a conductive layer you have to have the particles touch each other making contact.

    Even latex paint although water based still forms a rubbery insulator material that can isolate particles from each other. The long time standard conductive "paint" is called Aquadag, and consists of colloidal graphite (much smaller particles than graphite lube) and water. As the water evaporates the graphite particles come together forming a conductive coating that was widely used on the outside of picture tubes.

    To get better conductivity you have to go to metal particles like copper or silver which gets expensive. Aluminum doesn't work well because the aluminum particles oxidize and aluminum oxide is an insulator. I've used the Radio Shack conductive paint pen to ground the poles on G&L MFD pickups and it worked VERY nicely. But it apparently uses silver particles and is way too expensive to do an entire cavity.

    In fact aluminum oxide is the problem with using aluminum tape for shielding. Aluminum can't easily be soldered, so all connections between pieces have to be made mechanically (usually with a wood screw with a lock washer under the head that "bites" into the foil). But except for the contact problem it is widely available, cheap and works as well as copper foil. In fact works better because the copper darkens with age but the aluminum due to the oxide layer stays nice looking. Aluminum foil is pretty standard on the back of pickguards and cavity covers where the mounting screws provide the mechanical connection.

    But my choice is always copper foil (I have used aluminum on covers and pickguards) and my current copper foil source is a stained glass supply although I've also used the art supply copper as well but then you need a can of spray-on glue to hold it in the bass so it doesn't rattle.
     

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