RF Shielding

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RF Shielding of Welding, Plasma, and Computer Equipment

Here is my story and what worked for me. Please click on any of the photos to see a larger image.

When I first built my plasma gantry machine, I connected my computer to the drive electronics and then to the motors of the gantry. In my case, the gantry sits outside and the computer and drive electronics are inside my air conditioned lab. I watch the plasma gantry in action through a camera. This little situation occurred when I discovered that plasma dust gets into everything….and I mean EVERYTHING. I soon  moved the plasma gantry out of the garage.

So, the cables from my drive electronics to my motors on the gantry are about 15 to 20 feet long. The cable from my computer to my drive electronics are 5 to 7 feet long. My lab also houses my amateur radio stuff and other sensitive test gear. Below are some pics.

 

These show the computer, drive electronics, outside box connection, and the gantry

When I first started the plasma gantry up and tried to cut something, my x and y motors took off and cut all over the sheet of steel I had clamped up to cut and basically ruined the sheet. This was caused by the rf energy produced by the plasma arc making the drive electronics generate many extra pulses to the motors. I knew then that the plasma arc was feeding back into my electronics and computer.

Having been a amateur radio operator (ham) for over 40 years, I'd run into this sort of problem before. My transmitter RF energy had gotten into my computer when I transmitted and made it do all sorts of strange things....none that were pleasant. So I knew where to begin.

Now what I am about to cover is not rocket science, especially in the ham radio arena. These are simple fixes and will help cure RF interference most of the time.

The problem is that a computer and the associated electronics for driving the stepper motors are usually shielded enough to where NORMAL outside electrical noise does not bother them. Usually they are installed in metal cases and the computer and drive controls have some RF suppression built into them. In this particular case, though, the RF interference is many times greater than what is considered normal. The Plasma cutter (as well as a TIG welder, or any electric welder for that matter) is nothing more than a "big time" RF generator or transmitter. To make matters worse (for the computer and drive electronics) the plasma and TIG machines have high frequency generators in them. This makes it many more times worse than "normal".

The problem can be attacked from two sides. You can try to shield the source of the RF energy being transmitted from the plasma cutter or TIG welder or you can try to shield the computer and drive electronics from the RF "intrusion". We're going to try and do a little of both.

Stopping the RF Energy From Being Propagated

When a plasma or TIG welder is in operation and the arc is running, the RF energy is transmitted through the air and there is not much else you can do to shield this short of welding and plasma cutting inside a grounded metal box. Usually this is impractical......BUT....all of the metal that that is electrically connected to the plasma cutter or TIG welder may be acting like an antenna and propagating this RF energy. This you can do something about. You may not be able to suppress the RF energy that is radiating into the air directly from the arc, but you can help suppress all of the antennas that are connected to the arc. First you get an 4' copper coated ground rod. These are cheap and can be had a Radio Shack (p/n 15-530 for $12.99). See link below.

http://www.radioshack.com/product/index.jsp?productId=2103057

I used (and highly recommend) an 8ft copper coated rod. RadioShack no longer carries the 8ft ones, but Grainger carries them.

http://www.grainger.com/Grainger/categories/electrical/conduit-fittings/ground-rods

Other links that may work for 8ft ground rods.

http://secure2.data-comm.com/servlet/RBIS/-strse-195722/Ground-Rod-Dia-5-fdsh-8/Detailhttp://www.yourbroadbandstore.com/product.php?pid=002949

 

Drive that sucker into the ground (may take a while if you have real hard ground....try wetting it first and leaving it overnight). You need to sink this into the ground AS CLOSE TO YOUR EQUIPMENT AS POSSIBLE. Don't rely on cold water pipes, power line grounds and other stuff. None of them goes down deep enough to really ground the way the 8' rod does. Next, I sank a 12" piece of PVC around it so my weed eater didn't have problems cutting the grass around it (or whacking away at the ground wires).

Then you want to "home run" short pieces of mesh shielding from your RF "antennas" to the ground rod. Let me explain a couple of things. "Home Run" means that the wiring goes from the source DIRECTLY to the ground. What you want to avoid is "daisy chaining" any of the cables from the antennas to ground. In the case of the plasma cutter, you want to run a cable between the plasma cutter case to the ground rod, and you want to run a separate cable from the plasma gantry table to the ground rod. Do not run a cable from the plasma machine case to the gantry table and then to the ground rod. You want to "home run" a separate cable from the "antenna" to the ground rod. This will be the most effective. In the case of the TIG welder, you want to run a cable from the TIG welder case to the ground rod, and from the welding table (if possible) to the ground rod. The pics below are from my Miller Syncrowave 250 book (section 8). I would have pointed you to download it and look at section 8, but at the risk of getting into trouble with posting Miller safety stuff, I listed it below. This will give you an idea of all the stuff that can be an antenna while welding or plasma cutting. (the 2 photos below are property of Miller Welding)

Making the grounding cables

RF energy travels along a wire in what is known as a "skin effect". This basically means that the RF energy tends to travel along the outside area of the conductor as opposed to traveling through the entire conductor. Since it travels on the surface of the conductor better, it travels easier on braided or multi-conductor wire better than on solid conductor wire. The more conductors, the better. This is because there is more surface on multi-conductor wire than on single strand. For more info see:

http://whatis.techtarget.com/definition/0,,sid9_gci541369,00.html

Now, from my amateur radio days, us hams have always had a bunch of coax cable laying around. Coax cable is that stuff that connects your cable service to your TV. This cable contains a center single wire conductor, a plastic or foam shield around that conductor, a wire braid or mesh (electrical shield) and then a plastic outer jacket. There are all kinds of coax cables and all we are interested in here is the ones that have a copper braid or shield. Stay away from the really cheap stuff that has an aluminum shield. Here is one of many places that sell it.

http://www.elexp.com/cbl_rg62.htm

I had some old (copper braided) cable that came from some computer rooms (old mainframe rooms) that I worked at years ago. It is flexible stuff and has good copper braid. Below are pics of cables I made into grounding cables. The stuff I had was RG62, but any flexible cable with copper braid will do.

Once you have your cable, take the end and strip the outer plastic jacket off about 6 to 8 inches up. Be careful not to cut the copper braid inside. I use an X-acto knife and bend the cable to put a strain on the jacket then cut carefully around the perimeter. Remove this outer jacket and discard it. Next I "skin" back the copper braid to expose the plastic or foam dielectric and center conductor. Cut this off (discard it), leaving only the braid...then stretch it back out to its original length. Then solder on a ring lug that is the size you need to bolt this to the "antenna" and to the ground rod. This makes a very flexible, good RF conductor. See pics below.

Now that you have made your grounding cables, run one from each point of "antenna" source that you can to the ground rod.  You want to find a screw (or drill a hole and mount one) that is connected to the frame and cover of your equipment. I used a cover screw on my computer and had to drill and mount one on my drive rack. Be sure and clean ANY paint of coating that is underneath the screw in order to have a good electrical connection. Also be sure to use a "star" washer to go between the case frame and the lug. This star washer will "dig" into the frame or case and make a good connection. See pic below

 Installation of these cables would include your welder or plasma cutter, your welding table, your plasma gantry, etc. You cannot overdo this!

Just like a radio/tv transmitter has an antenna to transmit and radio/tv  receivers have antennas to receive, we are grounding or shorting the "transmitter antennas" that are connected to our plasma and welder. Next we will be grounding or shorting our our "receiver antennas".

Stopping the RF Energy From Sneaking Into Your Computer and Drive Electronics

There are two things I use to stop the RF energy from getting into my computer and drive electronics. The first, as you can guess, is the grounding cables. I have them going from my computer to the ground rod and from the drive electronics to the ground rod. See pics below.

As you may have noticed in the pics, there is mention of "chokes". Even though I grounded the computer case and the drive electronics case, there are "antennas" that are connected to each of these that let RF energy into the system. These "antennas" are the power cable to the computer and drive electronics, the mouse and keyboard, the monitor vga cable, the DB25 (25 pin printer type) cable between the computer and the drive electronics and any other cable that carries data or power. Chokes are made of a ferrite powder that is compressed into a grey metal-like substance, and this ferrite material or core, when clamped around a cable, will "choke" most RF energy that is trying to get into the computer system by way of that cable (and the conductors inside). These chokes (or split ferrite cores) are inexpensive and I would suggest using them freely. You can't hurt anything by using too many. They are always available at Digi-key. The p/n I have had good luck with is 240-2076 at about $2 (or less if you order 10 or more). Search for that p/n at:

http://www.digikey.com

You can also find these at surplus houses. They are usually no more than about $2.00 each. As of 5/7/2009, All Electronics has several from $1 up, but they are surplus and not guaranteed to be there when you need them. Buy a few extra to keep on hand. Got to link below and click on "Ferrites" on left.

http://www.allelectronics.com

Radio shack also still carries one:

http://www.radioshack.com/product/index.jsp?productId=2103222

Anyway, I use them on every cable I can coming out of my computer and any cable going from my drive electronics to the computer. In the case of the cable running from the computer to the drive electronics (db25), I use one on each end. The cables going from you drive electronics to the stepper motors are not usually susceptible to RF intruders. See pics below.

Once I had these clamps and ground cables installed (nearly 5 years now), I haven't had ANY problems running my plasma cutter while running my computers (I have 3 in the lab) or sensitive gear (such as oscilloscopes and test gear) or my drive electronics. Recently, I purchased a Miller Syncrowave 250DX TIG and have noticed that it is effecting some of my equipment when it runs. I am installing a new ground rod near the welder, running braided ground cables and using ferrite chokes where needed. I will have more pics when I am completed.

If you want more info on this, there are Amateur Radio Handbooks that have been published every year since 1926. Many of the "not so new" ones are on ebay and they are often cheap. The handbooks go into more detail about shielding and grounding. If you get one, I would try and get one that is less than 20 years old. There are surely places on the web with more info and if someone will email me them, I'll post them here.

I hope this helps some of you to cure your RF interference problems associated with welding, plasma cutting and computer gear. Please email me at:

vendor@randyray.com

if you have any corrections or web pointers that I should post.

Randy Ray

8/29/2004

Last updated 5/08/2009

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