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Joined: Jan 2005
Posts: 354
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pdh Offline
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This is a common problem with computers and other devices connected with ungrounded wall warts. The cause is that the adapter has a transformer that is not shielded and the secondary is not grounded. It CAN'T be shielded/grounded because these adapters usually do not have a ground pin. The end result is the secondary, and thus the DC, is floating between 0 and 120 volts. Odd as it may sound, connect it to 240 volts (be sure the adapter is rated for it ... but most are) to fix it. Then the system will be floating between the two hots. Unless you are on 2 legs of three phase, the buzz should be virtually gone. If you are on three phase connecting it to 208, at least the buzz will be reduced.

Another fix is to wire up a transformer to give you a split phase 60/120 system, with the secondary center tap properly grounded. You'll have 2 hots at 60 volts, but that will be 120 volts between them. If you can find an UN-polarized 1-15R somewhere, use that so that polarized plugs can't go in (most computer power adapters have unpolarized 1-15P). This would be unsafe for some devices such as lamps with screw bulbs since both conductors will be hot (60 volts is still somewhat dangerous).

Read up on NEC 647 so you know what kind of power this is.

Personally, I'd prefer the 240 volt method (ignoring 210.6(A)(2)) as that also gives me brownout protection.

FYI, I can feel the 60 volt potential on the metal surface parts of my laptops/netbooks when plugged in. I touch the surface very lightly and stroke the finger tip across and feel the 60 or 120 Hz vibration. It's the same frequency no matter the speed I move the finger. It goes away when on battery.

I had some fun with this at a local chain store selling computers. I could feel the same thing in a couple laptops on display. When one of the "geeks" came by to help me, I showed him what I was doing and asked him if he could feel the electricity, too. He did. Then I said "I think you have an electrical short in your power somewhere and something's not grounded right ... and if you unplug the power this will be safe again." (I specifically used those words to imply this is scary stuff). He unplugged it and behold, the problem went away. He plugged in back in and it was back. Not all the computers did that and I explained that the plastic ones were safe when something's wrong with the power. He got the manager and showed him, and the manager shut all the computer displays down. I managed to hold off laughing until I returned to my car. I haven't been back there since.

Last edited by pdh; 01/03/12 02:17 AM.
Joined: Mar 2011
Posts: 98
V
Member
Originally Posted by pdh
I had some fun with this at a local chain store selling computers. I could feel the same thing in a couple laptops on display. When one of the "geeks" came by to help me, I showed him what I was doing and asked him if he could feel the electricity, too. He did. Then I said "I think you have an electrical short in your power somewhere and something's not grounded right ... and if you unplug the power this will be safe again." (I specifically used those words to imply this is scary stuff). He unplugged it and behold, the problem went away. He plugged in back in and it was back. Not all the computers did that and I explained that the plastic ones were safe when something's wrong with the power. He got the manager and showed him, and the manager shut all the computer displays down. I managed to hold off laughing until I returned to my car. I haven't been back there since.


Ha Ha, funny, NOT!

I wonder how much money they lost until it was discovered they didn't have an issue.......

Funny thing about those power supplies, some of the major name brands still have 2-wire cords and charge $90-$120 for them, but if you buy a cheap Chinese knock-off power supply, many of them come with 3-wire cords.

I replaced a faulty one recently, the OEM guys wanted $95, I got one shipped to my door for less than $15 and it showed up in 3 days. I'm seeing the same thing going on at work too with different brand names.

Joined: Jul 2004
Posts: 9,788
Likes: 14
G
Member
You can just go with the isolation transformer pair and make the problem go away. I used 600x600 telephone transformers from a surplus outlet like Hosfelt in a shallow Carlin box with the appropriate in and out connectors.
In my case the problem was between a TV and a PC and there was no easier fix.

[Linked Image from gfretwell.com]


Greg Fretwell
Joined: Jul 2004
Posts: 787
L
Member
I thought "line in" and "line out" signals were designed for about 10 K ohm impeadances? I am sure a 600 / 600 ohm transformer works fine though.

Joined: Jul 2004
Posts: 9,788
Likes: 14
G
Member
I didn't see much insertion loss when I plugged it in. I suppose the extra load may have just bled off the common mode hum but it worked and I moved on.


Greg Fretwell
Joined: Dec 2011
Posts: 7
L
New Member
Originally Posted by pdh
This is a common problem with computers and other devices connected with ungrounded wall warts. The cause is that the adapter has a transformer that is not shielded and the secondary is not grounded. It CAN'T be shielded/grounded because these adapters usually do not have a ground pin. The end result is the secondary, and thus the DC, is floating between 0 and 120 volts. Odd as it may sound, connect it to 240 volts (be sure the adapter is rated for it ... but most are) to fix it. Then the system will be floating between the two hots. Unless you are on 2 legs of three phase, the buzz should be virtually gone. If you are on three phase connecting it to 208, at least the buzz will be reduced.

Another fix is to wire up a transformer to give you a split phase 60/120 system, with the secondary center tap properly grounded. You'll have 2 hots at 60 volts, but that will be 120 volts between them. If you can find an UN-polarized 1-15R somewhere, use that so that polarized plugs can't go in (most computer power adapters have unpolarized 1-15P). This would be unsafe for some devices such as lamps with screw bulbs since both conductors will be hot (60 volts is still somewhat dangerous).

Read up on NEC 647 so you know what kind of power this is.

Personally, I'd prefer the 240 volt method (ignoring 210.6(A)(2)) as that also gives me brownout protection.

FYI, I can feel the 60 volt potential on the metal surface parts of my laptops/netbooks when plugged in. I touch the surface very lightly and stroke the finger tip across and feel the 60 or 120 Hz vibration. It's the same frequency no matter the speed I move the finger. It goes away when on battery.

I had some fun with this at a local chain store selling computers. I could feel the same thing in a couple laptops on display. When one of the "geeks" came by to help me, I showed him what I was doing and asked him if he could feel the electricity, too. He did. Then I said "I think you have an electrical short in your power somewhere and something's not grounded right ... and if you unplug the power this will be safe again." (I specifically used those words to imply this is scary stuff). He unplugged it and behold, the problem went away. He plugged in back in and it was back. Not all the computers did that and I explained that the plastic ones were safe when something's wrong with the power. He got the manager and showed him, and the manager shut all the computer displays down. I managed to hold off laughing until I returned to my car. I haven't been back there since.


Thanks for this! laugh

You said to connect my AC adapter to 240 volts? how do I do this :P


Joined: Jan 2005
Posts: 354
P
pdh Offline
Member
If the adapter has 1-15P, you'd have to power a 1/5-15R with 240 volts, and that certainly has the obvious hazards. Unless one can find a split-phase output step up transformer or isolation transformer, then the option I see is rigging up a box like gfretwell did for audio, but for power instead with power rated transformers. But if you are going to do that, you could just as well do it with a 60-0-60 volt output (split phase with 120V line to line). Be sure to use non-polarized 1/5-15R. Recycle an old T-slot that's still in good condition?

Joined: Jan 2005
Posts: 354
P
pdh Offline
Member
If by chance your power adapter has a C14 inlet, then ...
http://www.stayonline.com/detail.aspx?ID=16477

Otherwise add this ...
http://www.stayonline.com/detail.aspx?ID=6628

Joined: Dec 2011
Posts: 7
L
New Member
to be honest... that all seems greek to me! sorry smile

Also: I tried to look at the links but i get "Service Unavalibe". Ill check back later

THANKS!

Joined: Mar 2007
Posts: 404
Member
Noise on the audio is a very common problem with laptops, especially cheaper ones. Aside from the power supply problems discussed here, some are just made with cheap audio chipsets and pick up all kinds of stray noise from the electronics and whatever EMI may wander in. For "professional" use, a Firewire audio interface or similar is usually used.

For casual use, a USB headset should do the trick... If you've got a headset that you're absolutely married to, you can also get a USB adapter on eBay or elsewhere, though cost often has an impact on your audio quality (though EMI is usually not a problem).

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