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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.

 

Wnderer
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power strip and ground plug adapter
Wnderer   8/13/2014 4:07:48 PM
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Get a powerstrip and one of these $7 ground plug adapters. With a power strip you'll have a circuit breaker and switch to help shut down things quickly if there's trouble.



ninja'd by elizabeth

elizabethsimon
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Re: Maybe...
elizabethsimon   8/13/2014 4:04:07 PM
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I did something similar when I had to set up on a table that was inconvineiently far from a wall socket. Since I needed a multi lead outlet strip with a long cord to power the equipment, I attached the cable from the antistatic mat to a ground on the outlet strip (which was located on the far side of the table). I then attached the lead from the wrist strap to a banana jack at the grounding point on the antistatic mat (which was on the edge near the front of the table). This minimized the stretching of the wrist strap cord and the whole thing could be moved fairly easily by unplugging the outlet strip from the wall and rolling it (and the cables) inside the antistatic mat.

 

Max The Magnificent
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Re: Maybe...
Max The Magnificent   8/13/2014 3:55:44 PM
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@mhrackin: ...and i noticed that they have a TWO-WIRE UNGROUNDED (and unpolarized) AC plug.

1,000 curses-- you are of course 100% correct -- in fact Ivan came bouncing into my office about 30 minutes ago saying the same thing -- and as soon as he said it I remembered that my notepad also has a 2-pin power plug.


Bummer ... watch this space...

mhrackin
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Re: Maybe...
mhrackin   8/13/2014 3:55:43 PM
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Max, the use of the USB and the use of the power strip are NOT mutually exclusive!  It would make sense to use the power strip regardless, just to reduce the excess bending over (although perhaps the exercise is beneficial!). I'd still vote for plugging directly into the U-ground hole rather than depending on the combined continuity of the series chain of alligator clip to USB shield wire to PC USB connector through PC internals to DC ground of power jack to power supply, thence (maybe) to the U-ground of the power cord into the AC outlet!  Far too many points of failure there (and of the most unreliable system element: interconnections!).

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