Why do stereo components lack ground plugs?

[J. Tyler]

Does anyone know the main reason computers have 3-pronged plugs but
most consumer stereo gear doesn't? Is it less vulnerable to surges,
even with all the microprocessors in modern receivers?

JT


P.S. I notice laptops rarely have ground plugs either. Must be a clue
there.

 

[Kevin McMurtrie]

Does anyone know the main reason computers have 3-pronged plugs but
most consumer stereo gear doesn't? Is it less vulnerable to surges,
even with all the microprocessors in modern receivers?

JT


P.S. I notice laptops rarely have ground plugs either. Must be a clue
there.

It's for the switching power supplies and ESD. Large switching power
supplies need strong grounding to avoid radiating strong ultrasonic AC
currents. The low voltage chassis is earthed because the few VAC that
you'd get from neutral grounding wouldn't be good enough to protect I/O
ports.

TVs and some stereo equipment can have multiple levels of isolation that
can eliminate the need for a third prong. In this case, the power
supply chassis will be grounded to neutral and a separate low power
chassis will be isolated except for maybe a ~10M Ohm resistor to neutral
for electrostatic charge bleeding. Their ports aren't so critical about
being touched that they need a true earth ground.

 

[Henry Mydlarz]

Many years ago I read a technical article which stated that in a situation
where each component in a stereo is grounded, then there is the ground of
the interconnecting signal cables, that can create a "ground loop" with a
resulting hum. Some years ago when I used to service ATMs, such a dual
grounded communications adapter box caused some interesting intermittent
communications dropouts.

Henry

 

[Todd H.]

P.S. I notice laptops rarely have ground plugs either. Must be a clue
there.

Laptops differ from desktop computers in that their power supplies are
isolated from the AC mains via battery isolation essentially. And
rare is the power supply for a laptop that has a metal case.

I don't know the answer to your interesting question though, but I
think laptop power supplies having a 2 prong plug might be a red
herring.

 

[Richard D Pierce]

Laptops differ from desktop computers in that their power supplies are
isolated from the AC mains via battery isolation essentially.

No, they are not. The battery provides NO isolation at all when
the power supply/line cord is attached to the wall.

Laptops don't have ground plugs because they have the internal
design necessary to meet isolation standards.

 

[mike]

No, they are not. The battery provides NO isolation at all when
the power supply/line cord is attached to the wall.

Laptops don't have ground plugs because they have the internal
design necessary to meet isolation standards.

That's not the case either, except in the case of models that plug
direclty into the wall. Most laptops use a wall wart. The wall wart
is double insulated and is not required to have a ground pin.

Using a wall wart relieves the computer manufacturer from all manner of
regulatory problems. They've been taken care of by the wart vendor.
As long as you keep the output voltage below 42.5V, your regulatory
issues are very much easier to handle.

mike
--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $550
Police Scanner, Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Michael Black]

Laptops differ from desktop computers in that their power supplies are
isolated from the AC mains via battery isolation essentially. And
rare is the power supply for a laptop that has a metal case.

I don't know the answer to your interesting question though, but I
think laptop power supplies having a 2 prong plug might be a red
herring.

Considering that someone might be plugging that laptop supply
into any available outlet, I suspect convenience is one reason
it would have a two-prong plug.

Probably a lot of consumer equipment is done that way too for
the same reason.

Of course, that doesn't really address why some equipment routinely
uses a third prong, only that there's a practical reason for some
equipment having only two prongs.

Michael

 

[Richard Crowley]

Does anyone know the main reason computers have 3-pronged
plugs but most consumer stereo gear doesn't?

Partly tradition. And I suspect somewhat to avoid ground loops.
Pro audio equipment tends to use 3-wire power cables, but then
it is assumed that the user is better equipped to handle shielding
and grounding problems like ground loops.

Ground loops are rarely a problem with computers (except when
we interconnect them with sound equipment!)

Is it less vulnerable to surges, even with all the
microprocessors in modern receivers?

Doesn't really have anything to do with surge protection.

P.S. I notice laptops rarely have ground plugs either.
Must be a clue there.

Most equipment that use wall-warts use 2-pin power plugs.
Isolation is provided within the wall-wart. Laptop computers
are little different than any other equipment that uses external
mains power supplies/chargers.

 

[Richard Crowley]

"rstlne" wrote ...

A AC Ground adapter that removes ground? ..
I dont think that's verry wise..

Most law-making jurisdictions agree. Those ground-lifting adapters
are illegal in most places these days. Doesn't prevent them from
being useful in some situations, however. I used one to break
a ground loop going to a video projector last summer. Note that
if there weren't already TWO grounds, there wouldn't be a ground
LOOP (by definition!)

Maybee take an isolation transformer with you..

That also interrupts the ground.

The battery probably provides isolation because it isnt
connected to the same ground that the power company
uses..

The "battery ground" is on the isolated side of the wall-wart.

Ever held a battery in your hand?
ever notice how a car battery wont shock you when
your cleanign either terminal?

Low-voltage batteries don't have enough voltage to push
any kind of current through your high-impedance epidermis.
I don't see the connection with the current discussion about
power line grounding.

I mean .. okay.. I might not be the brightest lightbulb but
your statement seems a little silly to me so maybee I am
reading it wrong

You appear to be mixing several different issues together.

If your removing the earth line from electronic equipment (on a
professional basis) then I would say that your probably liable
for damages if anyone gets shock'd, but that's just me

Conventional wisdom, petty bureraucrats, and plantiff's attorneys
all agree.

 

[GregS]

"rstlne" wrote ...

Most law-making jurisdictions agree. Those ground-lifting adapters
are illegal in most places these days. Doesn't prevent them from
being useful in some situations, however. I used one to break
a ground loop going to a video projector last summer. Note that

The problem here is obviously the lack of isolation between signal
ground and chassis ground which in most cases should always be separate.
Don't forget, most commercially wired isolation transformers are sold with the
ground tied to one leg of the output. This does not isolate ground, but does cut
noise way down.

greg

 

[J. Tyler]

Many years ago I read a technical article which stated that in a situation
where each component in a stereo is grounded, then there is the ground of
the interconnecting signal cables, that can create a "ground loop" with a
resulting hum.

With CAR stereo ground leads (I've never had noise problems with
interconnects) that's usually fixed by grounding all components at the
same chassis location. That's DC though, and the components depend on
grounding to complete their power circuits. It makes sense that you'd
want to totally isolate AC-powered components.

JT

 

[Rusty Boudreaux]

Does anyone know the main reason computers have 3-pronged plugs but
most consumer stereo gear doesn't? Is it less vulnerable to surges,
even with all the microprocessors in modern receivers?

It's purely a design/economic decision.

Having the earth connection can:
1. Make meeting emission limits easier since you can shunt
common mode noise.
2. If low voltage secondaries are earthed then it reduces safety
clearances (actually creepage distances) by half.

If the product is designed for international use then reinforced
insulation is required anyway so earthing the secondary doesn't
reduce margins. Unless there's a good reason to earth the
secondary (like ensuring SELV) the only advantage of having the
third wire is for meeting emissions. If the product can be
designed to meet SELV and emissions without the third wire the
manufacturer can save money on the cord. Those molded IEC
connectors are much more expensive than a hard wired cord with
grommet.

 

[H. R. Bob Hofmann]

Sometimes I wonder if anyone has a clue about this issue.

The non-use of a grounding plug implies that the object to which it is
connected is "double-insulated" so that a double fault must occur
before the user can get an electrical shock (short of going swimming
with the device plugged in - then the GFI circuit near the swiming
pool or bathroom or whatever is supposed to protect the dumb fool that
goes swimming). In the case of a computer, as noted earlier, the
wall-wart provides double insulation for the electrical dc output that
goes to the computer. If stereos, tvs, etc used wall-warts, they could
also be provided with no grounding plugs. In fact, the tv sets use
insulating techniques inside the tv sets that qualify them for the
"double-insulating" ratings, as far as I know. It would be helpful if
someone from UL was on the newsgroup.

H. R. (Bob) Hofmann