Question about shielding with magnetic spray

[SuperGeo]

Hello

After this article http://www.muzique.com/lab/magpaint.htm at AMZ, I decided to try it so I bought an spray and sprayed all inside my plastic enclosure.

magnets sticks as expected so I think it's 'working'.

My question is: How do I connect the shielding to the ground of my effect?

..forgot to say, I couldn't measure any resistance (condutivity?) with my DMM

[petemoore]

  My guess is...clean and lightly sand a [~1/2''] of the de-insulated stranded wire, and saturate the flat-splayed strands on the substrate which will recieve the magnetic spray material...like a wall of the plastic enclosure, so that a good amount of surface area to bond the material to the strands is available.
  Somebody else has surely used this method and can give more tangible advice...or try it on a scrap sheet and test for continuity.

[Seljer]

My question is: How do I connect the shielding to the ground of my effect?

through ground on the jack touching the enclosure?

[SuperGeo]

what happens is that the paint doesn't conducts when touching it with the DMM probes, sorry if I wasn't clear enough

[zachary vex]

i don't get it.  why does magnetic paint provide electrostatic shielding?

the biggest contributor to noise is electrostatic (capacitive) interference.  magnetic interference is really only an issue in wah pedals where there's an inductor involved which can pick up hum from nearby transformers.

i'd suggest sticking to lining plastic boxes with foil (glue it down.)

[SuperGeo]

well, Magnetic paint  has this name because it make the surface able to stick magnets on it. It isn't really magnetic, it has some iron particles which is what allows magenets to stick.

[SuperGeo]

  My guess is...clean and lightly sand a [~1/2''] of the de-insulated stranded wire, and saturate the flat-splayed strands on the substrate which will recieve the magnetic spray material...like a wall of the plastic enclosure, so that a good amount of surface area to bond the material to the strands is available.
  Somebody else has surely used this method and can give more tangible advice...or try it on a scrap sheet and test for continuity.

sorry, I didn't get it, there's some words that are difficult to understand    What do I do with the stranded wire?

[John Lyons]

If you can't measure continuity from one place on the enclosure to the next through your meter then there must not be enough coated on the box. If there seems like enough maybe the iron particles are suspended in goo and don't conduct? In which case the mag spray is unfortunetly pointless. You can buy conductive paint but it's screamingly expensive.

Foil is a pain in the neck and very ugly/messy but it does work.

John

[rockhorst]

As far as shielding goes, I think the following analogy holds up: Does an umbrella pierced with tiny holes in it get you less wet than no umbrella? Probably, but I'd rather buy a new one and stay dry. Off course this does depend a bit on iron particle size and concentration compared to typical interfering wavelengths...but I'd go for the save option.
In addition, note that the paint itself isn't magnetic, although at a first glance you might have that impression. It's the magnets you stick to the painted surface that are magnetic.

[brett]

Hi.
I'm no expert about shielding, but I would have thought that a "patchy" shield was enough for most stompbox applications.  A demonstration of how wide the holes can be in an effective shield is given in the door of microwave ovens.  Despite a short wavelength and very intense radiation, the shield on the door has quite large holes, and only covers about 50% of the glass.  Same with radio waves not being able to penetrate steel bridges - in that case the "holes" are many feet wide, but the radio waves still don't get in very well.

By the way, has anybody used the Hammond ES boxes (eg 1590ES)?  They are plastic with electrical shielding built in.  The prices seem reasonable.

cheers

[petemoore]

  Anything that makes the connection...I didn't even think about digging a lockwasher or even a regular washer into the stuff with the jack nut keeping hard contact. Seems like as good a method as any, field testing results on this subject...I haven't read any, the main objective is to have a shield that's grounded.
 Test for continuity, or even super low resistance...I have no idea what the resistance of the material in the spray is, or what thicknesses are necessary to improve it's conductivity.

[Paul Perry (Frostwave)]

There are TWO different kinds of interference pickup: magnetic and electrostatic.

Magnetic is pretty hard to stop, and all you can do is use magnetic material. Whether there is enough in that magnetic paint, I don't know (the only magnetic paint appication I am familiar with, is painting blackboards so kids can stick magnets on).

That paint, as has been observed, isn't conductive so it won't do any electrostatic shielding at all. So you will have to put adhesive metal foil or else carbon loaded conductive paint over it.
With electrostatic shileding, if holes & gaps are 'small' relative to the wavelength, there is no problem. (so you can use metal mesh).

[Alex C]

I recently noticed several hardware stores (Lowe's, Home Depot, Ace, etc.) selling Aluminum Tape, which is basically the same width and thickness as duct tape and costs roughly the same.  I used this to shield the control cavity of a guitar, and it works quite well.  There are a few brands, I think 3M makes one.  The adhesive on the stuff I used was very good, and it was easy to cut to size.  I am planning on trying this out on some ABS plastic boxes.  Very simple, no mess, it conducts well, and you can make any size you want.

[petemoore]

  Never tried it. I heard each layer or separate piece of tape must be separately connected to be grounded...

[rockhorst]

Same with radio waves not being able to penetrate steel bridges

Good point...The difference is that a bridge is constructed of solid bulks o' steel connected to each other and to ground, making a big Faraday cage.

[John Lyons]

Since the aluminum foil tape has a stcky side, it does not conduct piece to piece. If you cut a piece to the rough length and fold a 1/4 inch over onto it self you will be able to overlap and get metal to metal contact. The tape I use is backed with paper so it's easy to fold a nice crease and them peel the backing off and fold over to a nice straight line. General use solder will not stick to aluminum so you can either use clear packing tape to cover this overlap or another piece of foil folded over. The clear tape is nice for a insulator at any rate. Just leave the pot and jack holes uninsulated.

[petemoore]

  I found a good way to make ground between two substrates..er..havne't done the long term testing but..
  Anyway I like to dig the threads of a screw right through both substrates, with a 'messy' exit hole on the first one, to be clamped by the final tightening of the screw, nose pliers pressing brads through might make good contact for a guitar cavity, but hardwood would be easiest to drive into if a pilot hole was made first.

[Dave_B]

I just ran across this stuff.  If I read it correctly, the surface resistance of approx. 0.7 Ohms should be what we're looking for, right?  It's about $25US a can.  It would seem to be a good choice for shielding the Cornish-style boxes people are building.   

[John Lyons]

Hmmm

That seems like the stuff to get. Now to find a distributor! The paint on version would be the way to get I think. Paint it where you need it and in a thick coat or two.

John

[Dave_B]

This stuff ain't exactly cheap, but here it is: http://www.lessemf.com/paint.html

Super Shield is the third one down.