Why 3-prong plugs

Why do laptop power supplies, the ones with black rectangular things in the middle of the cord, have 3-prong plugs when everything you can touch is plastic or "rubber"?
And, I believe, when there's nothing more dangerous than a lamp underneath the plastic and the rubber? Less dangerous because some light bulbs ger really hot!
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On 3/7/17 4:29 PM, micky wrote:

I have a Toshiba Netbook with a small "black rectangular thing" that is only 2-prong.
The Mac Mini has a large white thing that is 2 prong.
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On Tuesday, March 7, 2017 at 5:00:39 PM UTC-5, Retired wrote:

Same here with a Dell notebook, it has a two prong power supply.
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wrote:

If the power supply were to short power could get through the laptop, and out the USB or headphone, or whatever jacks.? Anyway, some require grounding by code, others don'r.
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On Tue, 07 Mar 2017 20:27:10 -0500, snipped-for-privacy@snyder.on.ca wrote:

It's a good question, since the OP is right that a power supply seems pretty well isolated by all that plastic encasement, and the output is low DC at relatively low amps (< 10 or so amps most of the time).
There's no power transformers anymore due to the switching power supply design.
And yet ... ... ...
I wonder if the electronics of the switching power supply require a really good ground?
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says...

The switchers do have transformers in them. They are usually after some of the switching circuit and are very small compaired to what they are if they were on the 60 HZ side.
The electronics will work fine without a ground. The ground is for safety, and it does help with the filtering of the hash the switchers generate that is fed back on the power wires. It may also pay a small part to help prevent surges coming in from the AC line.
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My Lenovo laptop has a 2-prong plug and the converter is rated for 240 volts. I guess I am in mortal danger - if "the power supply were to short" ... :-) John T.
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snipped-for-privacy@gmail.com says...

(?)_/¯

They have done that for many years, or in some cases like priners and modems the power supply is seperte so they can just ship seperate supplies depending on the country.
Many computers and some other devices have a switch for 120 or 240 volts and that universal socket on the device. The suplies are really not even that particular about the voltages, just get into the ball park with them. Going 20 or 30 volts either way is fine.
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On Wed, 8 Mar 2017 11:01:06 -0500, Ralph Mowery

These days the switchers are very wide mouth and can take anything from 100-250v at any frequency, even DC although they are not labeled that way. The first thing that happens is the input is converted to DC immediately anyway. Then it is chopped to something in the 10s of thousands of hz, fed through a small toroid transformer, regulated, rectified and comes out at the desired voltage. When went to New Zealand, all I needed was a plug adapter to run every electronic device I had (laptop, phone, camera, MP3 player, tablet etc) All of the chargers were 100-250v.
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On Wed, 8 Mar 2017 11:01:06 -0500, Ralph Mowery

Power supplies with 120/240 switches are SO 1990.
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On 03/08/2017 08:16 PM, snipped-for-privacy@snyder.on.ca wrote:
[snip]

WHAT is that supposed to mean? It can't mean that year, since there was nothing special about 1990.
Also, I have a PC power supply from last year (2016). It has a 120/240 switch.
--
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http://notstupid.us/
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wrote:

The design is from 1990. I haven't seen an ATX supply that was not "auto switching" AKA wide mouth and I have a dozen of them in my parts cabinet.
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wrote:

SO OLD. Virtually everything in the last20 years anyway is "universal" or "auto switching" in the small SMPS world. Virtually all my laptop powes supplies and switch mode wall warts are listed as 100-240 or the odd one 85-250 volts The odd PC power supply still has a switch (Del and HP up until at least a few years ago) but Acer has been full auto for at least that long. I've got8 nyear old acers that are full automatic, for sure

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The ac can use ground as part of a filter, capacitor to ground. I have seen ground fed to common on laptop, sometimes causes ground loop problems.
Greg
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Tue, 07 Mar 2017 21:29:46 GMT in alt.home.repair, wrote:

Filtering perhaps? Limited surge supression components? Redundant safety...I've got a few laptops around here that have the 3 prong and a couple that don't. I prefer the ones that dont, because, sometimes, the 3 prong ones generate unwanted background hum when fed to external audio amplifiers while plugged into their chargers.
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wrote:

That implies that the ground is connected to the DC common. Not sure why they would want to do that unless it is just for RF filtering.
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snipped-for-privacy@aol.com

I've suspected the issue is a ground loop based on prior experience with that...I haven't tried to open one of these and take a look inside, so it's entirely possible it is connected, but, due to some filtering circuitry in between, I'm unable to do a simple continuity test on it. By unable, I mean that it doesn't read as completing a circuit when tested in that manner.
http://www.channld.com/hum.html
This works well to remedy the problem...
(Amazon.com product link shortened)
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wrote:

Reading with a meter you will be looking through the filter so YMMV on what kind of continuity you see but at 60hz there will be some pass through. I have had that problem with desk tops that feed amps, particularly if there are other things in that loop like a TV.
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snipped-for-privacy@aol.com wrote:

Same here. When I routed a friends computer thru a mixer board into a stereo system, we had the ground loop hum issue to deal with. Oh the joys of a semi digital recording studio.
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