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David Ashton
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Re: power strip and ground plug adapter
David Ashton   8/13/2014 6:17:14 PM
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@Wnderer....sorry, only noticed this post after I posted mine above.  Great little gadget, much easier than my idea if you can get them easily.

David Ashton
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Re: Maybe...
David Ashton   8/13/2014 6:14:31 PM
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@Max...sorry to rain on your parade (or pee on your fire) but I would say 100% of all the brick type laptop and other power supplies I have ever taken apart (and there have been a few) have been switching types with the output isolated from the mains.

A mini-tower desktop computer might be different - those power supplies usually have a connection between the mains earth and the case of the PC....but I am not sure whether this will extend to the USB shield, I will check mine when I can.

Your solution might be to just switch your power supply off rather than unplugging it.  Of course you should also check if the power supply ground goes all the way through to the output.

The best solution would be to get a mains cable with a proper 3-pin plug,  cut the IEC end off (preferably while it is NOT plugged in :-), strip it back a bit, and cut off and insulate the white and black wires (which is what I think you have in the US) leaving the green earth wire only - and connect that to your anti-static mat.  This does assume you have a spare power outlet where you are working, and that G the G does not want to use said outlet while you are electronicking...

PS to avoid the white and black wires shorting, cut them off different lengths.  Then get some pliers and pull the insulation so you don't have wire ends sticking out.  Then insulate them with tape.   As the Irish would say....To be sure...to be sure...

Max The Magnificent
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Re: Braided shield
Max The Magnificent   8/13/2014 5:41:37 PM
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@MeasurementBlues: Congratulations Max, you've just created a pigtail, a wonderful source of radiated emissions.

Thank-you-very-much ... I'll be playing here all week :-)

MeasurementBlues
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Google has found you
MeasurementBlues   8/13/2014 5:36:17 PM
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@Max, I get a Google alert for "electrostatic discharge" and you've been picked up.

MeasurementBlues
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Braided shield
MeasurementBlues   8/13/2014 5:30:59 PM
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Congratulations Max, you've just created a pigtail, a wonderful source of radiated emissions.

I'll be posting an article to my EDN blog about how those pigtails are evil.

The braided shielding -- shown pulled back from the inner wires, twisted together, and sticking out of the right-hand side



MeasurementBlues
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Re: Wear the wristband
MeasurementBlues   8/13/2014 5:26:26 PM
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As you surely know, never ever float the scope.

zeeglen
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Re: Wear the wristband
zeeglen   8/13/2014 5:24:09 PM
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@Martin So if your circuit is floating and you're connected to the low side of its power supply, you should be OK.

Also a good idea to connect a scope probe ground to a floating circuit as the first connection so that the circuit is no longer floating.  I've zapped MOSFETs by connecting the probe tip to the gate first before the ground.

MeasurementBlues
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Wear the wristband
MeasurementBlues   8/13/2014 5:13:00 PM
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@Max,

The purpose of the wrist band is to provide ahigh-resistance (leakage path) from you to whatever you're working on. So if your circuit is floating and you're connected to the low side of its power supply, you should be OK. If the power supplye is indeed connected to the ground lead of the AC mains, then that's better.

Some years ago, I was installing a new hard drive in my desktop PC. When I powered it up, the power sypply fan came on, but nothing else. I had zapped the motherboard and needed a new one. Since that day, I always wear an ESD wrist strap.

ESD wrist straps whould be part of everyone's EMC troublshooting kit.

MeasurementBlues
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What ground is actually good for.
MeasurementBlues   8/13/2014 5:01:27 PM
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MeasurementBlues
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Ground? What ground?
MeasurementBlues   8/13/2014 4:58:10 PM
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Having just returned from the EMC Symposium, I'm reminded that there is, as Bruce Archambeault will tell you, there's no such thing as ground except perhaps in this case, for safety.

Too often, we use the term "ground" when we really mean "return" as in current returning to its source. Ground is a reference voltage. Current shuld not flow through a ground wire at all. When it does, you have samety and EMC problems.

Read what Dr. Todd Hubing said last week about "ground" at an EMC fundamentals tutorial.

 

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