AC power adapter grounding

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Why is it desirable for laptop computer AC power adapters with 3-wire AC co=
rds to have their AC and DC grounds separate?  The ground on the DC side is=
n't connected to the ground prong of the AC cord.  With desktop computers t=
he metal case gets earth grounded, but with a laptop not connected to any e=
arthed peripherals, wouldn't it be possible for leakage through the AC adap=
ter's transformer to cause a shock hazard?  Should the DC side be connected=
 to the AC ground?

I'm referring to 3-wire laptop power adapters included with Dell computers =
and made by Lite-On, not eBay junk, like those with AC ground prongs that a=
ren't connected to anything, i.e. no-name model SPP34-12.0/5.0-2000.

Re: AC power adapter grounding

<larrymoencurly@my-deja.com
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Why is it desirable for laptop computer AC power adapters with 3-wire AC
cords to have their AC and DC grounds separate?

** Improved safety.


The ground on the DC side isn't connected to the ground prong of the AC
cord.

** So if the ground pin ever becomes live, the computer itself does not
since the output of the PSU is safety isolated from the AC side.


With desktop computers the metal case gets earth grounded,


** It has only basic insulation, so must be fully earthed.


but with a laptop not connected to any earthed peripherals, wouldn't it be
possible for leakage through the AC adapter's transformer to cause a shock
hazard?


** Nope.

You would probably have to soak the PSU in salt water for that to happen.


...   Phil



Re: AC power adapter grounding

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So if the power adapter leaked from the high voltage side to the low voltage
side and I stood barefoot on the ground and touched the metal shell of a
connector on the laptop, that wouldn't cause a shock?




Re: AC power adapter grounding


larrymoencurly@my-deja.com>
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** Do you bother even reading what I wrote  ?




...   Phil



Re: AC power adapter grounding
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Yes, and that's why I still don't understand why it's safer to keep the gro=
unds separate.  If the AC adapter's transformer leaks to the secondary but =
not to ground, but then a person touches bare metal on the laptop computer =
with one hand and earth ground with the other hand, won't he get shocked be=
cause the power adapter's DC ground isn't connected to its AC ground?


Re: AC power adapter grounding
On Sat, 11 Aug 2012 08:43:56 -0700 (PDT), larrymoencurly@my-deja.com

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  You need to learn the concept and real world application of the term
ISOLATION.

  Also, most DC dongle adapters do NOT have transformers in them.

  Now, figure out why an old AC powered drill motor DOES have a three
wire, ground fault protected cord which carries a fault back to ground,
and why the new, PLASTIC case jobs do NOT.

  Then apply that learned knowledge to a PLASTIC cased DAC powered DC
power supply device for ANY human held product.

  Now, go stick your finger in an open light bulb socket standing
barefoot on a concrete floor.  Make sure the switch is on.

  Usenet line length is 72 characters.

Re: AC power adapter grounding
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What is a "DC dongle adapter"?  Because I've never seen an
AC power adapter that didn't contain an isolation transformer,
so is a dongle adapter something different?

A Class II double insulated drill isn't required to have all
its exposed metal isolated with double insulation, and
usually the front 1/4 of the case is an aluminum casting
that the user often holds during operation, and inside the
casting are gears that come into contact with the motor's
metal shaft.  So if there's a breakdown in the motor's
insulation, where's the benefit of the double insulation
here?  OTOH an AC power adapter's isolation transformer is
supposed to protect against high voltage leakage to any
exposed metal.
 

Re: AC power adapter grounding


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** Fraid that is not so.

All exposed metal must have "double insulation" between it and live parts -
and that insulation must stand up to foreseeable events like the motor
windings burning.


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** Generally there is a plastic coupling or gear wheel to isolate the head
of a class 2 drill.

 Methods that prevent the windings in the drills motor from ever overheating
are also possible.

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**  You are inventing stuff.


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** And does so very nicely under nearly all conditions.

Salt water bath not withstanding, as I already pointed out.

Some folk are incapable on comprehending the benefits of Class 2 insulation.

Like the fuckwit OP here.


...  Phil



Re: AC power adapter grounding

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My old Sears Craftsman consumer grade double insulated drill
measured from about 20-1800 ohms whem measured between the
rear of the motor shaft and the chuck, depending on how hard
I pressed a piece of copper wire against the shaft or rear
bearing.  The resistance between the chuck and aluminum
housing at the front was about zero ohms.  Here's a photo:

  
http://img690.imageshack.us/img690/1597/doubleinsulateddrill.jpg

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We had a Eureka vacuum with a steel housing and 2-wire AC cord,
and that cord was sliced all the way through its insulation in
a few places by sharp edges on the cord reel.  

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I'm not worried about being shocked through the plastic but
through exposed metal on the computer connected to an
ungrounded AC adapter that isn't safety certified by UL or CSA.




Re: AC power adapter grounding
larrymoencurly@my-deja.com expressed precisely :
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But you never checked the standard of the complying insulation between
the Electics and all the metal parts you found were connected together.

And who said it was double insulated anyway? I cannot see a double
insulated standard mark on the name plate.
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--
John G



Re: AC power adapter grounding

"John G"

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  ** The FW would not even know what to look for.

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 ** " DOUBLE INSULATED  " is written on the name plate.

 It also carries the UL logo,  so it seems kosher.

 WARNING:

 Do no get ambushed by an individual example that  YOU  cannot investigate.

 If somehow the drill fails to actually meet class 2 requirements, that is
NOT a reflection on the standards.



....   Phil



Re: AC power adapter grounding
Phil Allison formulated the question :
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Of course you are correct.
I was looking for the Symbol and missed the wording (age has its
dificulties).
As he says "His Old ...." may predate the standard symbol.
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Absolutely.

--
John G



Re: AC power adapter grounding
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Just in case it was a fluke for the first drill to measure 20+
ohms from chuck to shaft, here's another drill, made by One
World Technologies, the producer of many Ryobi handheld tools:

  
http://img62.imageshack.us/img62/8872/doubleinsulateddrill3.jpg

The meter read 4.7 ohms.  I saw no electrical isolator between
the metal shaft and gearbox (visible gears were metal) or between
the gearbox and chuck.  I had to remove the plastic cover to
measure the resistance.



Re: AC power adapter grounding
larrymoencurly@my-deja.com Inscribed thus:

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If you look at the photo you can see the double insulation !  Anyway you
need a megger to check that there is no leakage between the prongs on
the plug and any accessible metal parts.

--
Best Regards:
                          Baron.

Re: AC power adapter grounding

"Baron"
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** To have any idea of the drill is safe, the owner needs to stall the motor
at full setting and see what happens.

He has never done this and is basing his conclusions on  PURE  ASSUMPTION  !

BTW:

Any ( genuine) class 2 transformer will pass a similar test with no
possibility of the core, frame or secondary windings becoming live at supply
voltages.

This is fairly easily achieved and incorporated in HUNDREDS of  MILLIONS of
units.



....  Phil




Re: AC power adapter grounding

<larrymoencurly@my-deja.com

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** READ what I wrote-  FUCKHEAD !!


" ** Generally there is a plastic coupling or gear wheel to isolate the head
of a class 2 drill.

Methods that prevent the windings in the drills motor from ever overheating
are also possible.  "







Re: AC power adapter grounding

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Not in the only two consumer grade double insulated drills I
checked, including a drill from one of the leading makers of
handheld power tools.

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Is it inherent protection, or does it require a high temperature
cutoff?  Some Sears drills and vacuum cleaners included a bimetal
switch on the motor that would open when overheated, while almost
the same models sold by other companies lacked that.






Re: AC power adapter grounding

"Bob Boblaw"
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** Could be either.

With a class 2 transformer: apply the worst case overload and if the
windings do not reach a high enough temp to damage the insulation materials
used  -  then there is no problem.

With most class 2 transformers, this normally means using a thermal cut out
embedded in the primary winding.

With a " universal " motor, it may well be that the self inductance is high
enough to limit the stall current to a safe number.

This would be entirely consistent with the "  AC ONLY  " marking on the name
plate of the Sears drill.


...  Phil





Re: AC power adapter grounding

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Sorry to nit pick (I made the same mistake myself). Class 2 appears to
refer to some US regional NEC specific issues (e.g. limiting power
levels to 100 VA) etc.

If I understand correctly, we are discussing about IEC 60950 Class II
double isolated issues.


Re: AC power adapter grounding

[snip]
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Most do. SMPS wall warts/adapters have transformers that operate at high
frequencies rather than 50/60 Hz. But they are transformers with the same
isolating properties nevertheless.
  
--
Paul Hovnanian     mailto: snipped-for-privacy@Hovnanian.com
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