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Author Topic: Source of shielded power cord: dead window AC units  (Read 334 times)

WA2ISE

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Source of shielded power cord: dead window AC units
« on: June 07, 2021, 02:14:29 PM »

Air conditioners lately come with power cords with the hot wire with a shield, the neutral a separate shield, and no shield on the ground wire.  These shields are not grounded, but are meant to sense if a grounded metal object (like the AC unit's metal case) cuts into the cord.  The sensor is in the power plug.  I've salvaged these cords off AC units people have tossed. 



I take off the power plug, and using a regular 3 prong power plug, tie the shield drain wires to the ground wire and the ground pin.  This should act like an RF shield to confine RFI in the hot and neutrals, or keep RFI out of these wires.   This should help more if your house uses BX housewire or conduit (which should also act like shielding)

This might be useful to keep your RF from getting into the rig's power supply. Or keep the switching power supply crud from leaking out to your antenna.   
« Last Edit: June 07, 2021, 02:18:10 PM by WA2ISE »
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K6AER

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Re: Source of shielded power cord: dead window AC units
« Reply #1 on: June 07, 2021, 06:11:51 PM »

it has been my experience that unless  a wire is 100% intrinsically shielded that a little shielding has little or no effect. The rest of the house wiring is unshielded.
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N2AYM

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Re: Source of shielded power cord: dead window AC units
« Reply #2 on: June 08, 2021, 05:53:47 AM »

Air conditioners lately come with power cords with the hot wire with a shield, the neutral a separate shield, and no shield on the ground wire.  These shields are not grounded, but are meant to sense if a grounded metal object (like the AC unit's metal case) cuts into the cord.  The sensor is in the power plug.  I've salvaged these cords off AC units people have tossed. 



I take off the power plug, and using a regular 3 prong power plug, tie the shield drain wires to the ground wire and the ground pin.  This should act like an RF shield to confine RFI in the hot and neutrals, or keep RFI out of these wires.   This should help more if your house uses BX housewire or conduit (which should also act like shielding)

This might be useful to keep your RF from getting into the rig's power supply. Or keep the switching power supply crud from leaking out to your antenna.   

So what is that brown wire to the left in the image above? There is no mention of it.
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W1RKW

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Re: Source of shielded power cord: dead window AC units
« Reply #3 on: June 08, 2021, 06:13:52 AM »

looks like it is a shadow of the green wire
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W9IQ

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Re: Source of shielded power cord: dead window AC units
« Reply #4 on: June 08, 2021, 06:49:03 AM »

Something to keep in mind when thinking about this type of shielding is the skin depth of the RF within the material.

Common household aluminum foil is up to 15 μm thick - probably thicker than the average wire shielding foil. At 80 meters, one skin depth in aluminum is about 47 μm while at 10 meters, it is about 16 μm.  Then recall that one skin depth is where the RF current is 37% of what it was on the surface. So there can be substantial RF current flowing on the outside of this relatively thick foil across the entire HF band. In reality, the situation is more complicated than this simple analysis. So while it will most likely have some attenuation effect, its effectiveness as a shield is questionable.

It is always better to kill the noise as close to the source as possible.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WB4SPT

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Re: Source of shielded power cord: dead window AC units
« Reply #5 on: June 08, 2021, 07:01:49 AM »

...
It is always better to kill the noise as close to the source as possible.

- Glenn W9IQ

Very true.   Once the evil genie is out of the bottle, it becomes very difficult.   I'm working on a yacht solar controller EMC issue.  Its taking a ton of external ferrites, where a PCBA fix would be so much cleaner and more effective. 
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WA2ISE

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Re: Source of shielded power cord: dead window AC units
« Reply #6 on: June 08, 2021, 04:22:18 PM »

The rest of the house wiring is unshielded.

True if your house is wired with romex.  Metal Conduit should be a decent shield, and maybe metal armor BX cable.  Though the spiral armor of BX cable may be not great for RF. 

The foil shield of the wire has a braid over it, albeit about as thin as Radio Shack coax.  :D
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K6BSU

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Re: Source of shielded power cord: dead window AC units
« Reply #7 on: June 08, 2021, 05:14:53 PM »

Don't understand why an AC unit would cause RFI.  It's basically just a motor, and the motor is inside a sealed can pumping Freon (or something)
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N7EKU

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Re: Source of shielded power cord: dead window AC units
« Reply #8 on: June 08, 2021, 05:31:36 PM »

Don't understand why an AC unit would cause RFI.  It's basically just a motor, and the motor is inside a sealed can pumping Freon (or something)

It's because AC's now come with GFCI or LCDI plugs, so the sheild is needed as part of the cable damage detecting circuit.

73.
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Mark -- N7EKU/VE3

WA2ISE

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Re: Source of shielded power cord: dead window AC units
« Reply #9 on: June 09, 2021, 02:47:38 PM »

Don't understand why an AC unit would cause RFI.  It's basically just a motor, and the motor is inside a sealed can pumping Freon (or something)

It might have a microcontroller. 

I'm repurposing the cord removed off dead AC units. 
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