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Degauss of LCD

Degauss of LCD

Degauss of LCD

I'm looking for a way to degauss an LCD panel.

The issue is that either the material handling equipment for the panel or the stamping process has caused residual magnetism in the metal. For our product we have to pass a DO-160 magnetic effects test for compass deviation.

What would be the best way to degauss the sheet metal of the panel?

So far I have found I can reduce it using a bulk tape eraser and wave it around the panel and gradually increase the distance. I have also made a coil that I plug it into 110 VAC start at a distance from the LCD and pass the LCD thru the coil and shut the coil off after the LCD exits the coil about 2 feet away. That just made matters worse.

I'm really looking for a better or more repeatable method to be used in production. Better yet a source to send the unit out to for degaussing.

Any suggestions?

RE: Degauss of LCD

I'm surprised the coil around the LCD didn't work.  Is your coil large enough to allow you to insert the face of the LCD through the bore of the coil?  That would simulate the way degaussers work in cathode ray tubes.

Another idea is to find a working cathode ray monitor, place the face of the LCD screen onto it and run the degaussing routine on the cathode ray monitor.  Perhaps it'll be strong enough to degauss the LCD.

RE: Degauss of LCD

They use a large coil and a 'ringing circuit'.
You tune the inductance and capacitance of the coil circuit so that you energize it with 60Hz and a fairly high field and then you remove power and the AC field gradually decays.
You shouldn't need much field so this should be easy to do.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Degauss of LCD

"What would be the best way to degauss the sheet metal of the panel?"

Make it out of aluminum?

RE: Degauss of LCD

Unfortunately we can't make it out of aluminum since the panel is an off the shelf item for us. We integrate the panel into an assembly to sell to the OEM.

Could a variac be used to lay the coil on the back of the panel and start out with it at full power and decrease it to zero and accomplish the decaying field of the "ringing circuit"?

RE: Degauss of LCD

It's really simpler to just physically move the panel away from the degaussing coil, which you could do continuously by putting the coil around a simple nonmagnetic conveyor comprising a fabric belt running on a waxed board.  Or just carry the panel by hand, but move slowly.  For production, I'd automate it to get consistent results.

Your experiment with moving the panel away from the coil was flawed because you removed power from the coil while the panel was still near enough to be influenced by the collapsing field.  

It was probably erratic because of the AC.  To see what I mean, try this:  You can use the bulk tape eraser to magnetize a screwdriver, just by switching it off when the screwdriver is nearby; this works because the odds of switching it off on a zero crossing are small.

Yes, a Variac will also work, but the operators typically get impatient and crank it down too fast.  I think it's easier to automate physical movement of the panel than to automate the variac; easier to verify, too.

Mike Halloran
Pembroke Pines, FL, USA

RE: Degauss of LCD

The orientation of the coil would have some significance. CRT's have the coil wound around the picture frame of the tube making the field perpendicular to the face of the tube. This coil is usually connected to the mains power through a varistor. When power is first turned on, the cold varistor has low resistance so coil current is high. This quickly heats the varistor and its resistance becomes very high, thus reducing the current almost to zero. The varistor will not cool down again until power is removed. Thus degausing briefly occurs every time power is applied.    

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