#302955 - 11/10/2007 17:39
What good is a polarized AC plug?
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Two Conductor AC plug (USA). Once upon a time, both blades were the same size and shape. I should know, my old house is full of those kind of plugs. I have some that are polarized and grounded, but many are still the old symmetrical ones. Nowadays, everyone has polarized plugs, where one blade is bigger than the other. Unless I dremel down one of the blades, those plugs don't fit in many of my outlets. So I'm frequently dremeling down blades, cutting off ground pins, or using a ground adapter (with one of the blades dremeled down so it will fit a non-polarized plug). They all work and I have noticed no ill effects from doing any of the above. Anyway. My point being... If the AC power is just alternating current, what's the point of polarizing the blades on a two-conductor plug? All the appliances and equipment I've tried works fine the other way around. I can't imagine a situation where it *wouldn't* work the other way around. Is there a such thing as an appliance that must be plugged in with the correct polarity?
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#302956 - 11/10/2007 17:52
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 10/06/1999
Posts: 5916
Loc: Wivenhoe, Essex, UK
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I don't know about two prong mains plugs (we don't have them here) and I don't know much about the US system. But in the UK we have three prong plugs, with Live, Neutral and Earth. The potential between live and neutral tends to be around about zero, whereas the potential between live and earth is around 220V.
When you are dealing with an appliance with a two prong figure-of-eight plug then polarity clearly doesn't matter, as you are free to plug it in any way around, but of course those devices aren't earthed.
I am always nervous when I use US or European plugs, they feel so unsafe compared to the chunky UK ones (whether there is any actual practical difference in safety I don't know). The US ones in particular feel like the prongs are made of tin foil, compared to the UK ones.
_________________________
Remind me to change my signature to something more interesting someday
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#302957 - 11/10/2007 18:01
Re: What good is a polarized AC plug?
[Re: andy]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
When you are dealing with an appliance with a two prong figure-of-eight plug then polarity clearly doesn't matter, as you are free to plug it in any way around, but of course those devices aren't earthed.
The US plug I'm referring to is the equivalent of that one. Yet modern ones are polarized.
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#302958 - 11/10/2007 18:02
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 30/10/2000
Posts: 4931
Loc: New Jersey, USA
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I can't answer your quesiton, but I do have the same problem in my circa-1950s house. About half the outlets are 2-prong. Most/all are polarized, but some have been painted over so many times that the holes have become smaller. It turns out that much of Japan uses non-polarized 2-prong outlets very similar to ours. Cisco part number 72-1925-01 is an IEC cord with 2-prong connector and a grounding wire that's about 4" long. It has come in handy on a few occasions and they're not too pricey.
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-Rob Riccardelli
80GB 16MB MK2 090000736
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#302959 - 11/10/2007 18:05
Re: What good is a polarized AC plug?
[Re: robricc]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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*Nod*. I don't have any problem getting things plugged in, I'm just wondering what the point of polarizing an ungrounded AC power source is.
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#302960 - 11/10/2007 18:07
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Rather than destroying the safety features of all of your corded devices, perhaps instead you should simply be replacing those obsolete electrical outlets as need-be. A new outlet fixture costs about 0.79 each locally, or cheaper in bulk packs.
One purpose of the polarized 2-prong plugs is to allow devices that would otherwise have required a heavier 3-prong cord to use "double-insulated equivalency" construction along with the 2-prong plug.
The larger blade (that you've been destroying) is the neutral wire, and the smaller one is the hot wire.
If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.
Cheers
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#302961 - 11/10/2007 18:11
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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One rather obvious safety feature of a polarized plug, is that it ensures that the outside screw conductor of a standard light bulb socket will not be connected to the hot voltage wire.
People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.
Cheers.
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#302962 - 11/10/2007 18:19
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.
It would be interesting to know if there is any truth to this one. If so, I would consider it critical for the equipment in my basement recording studio.
Quote:
People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.
So you're saying that touching one of the prongs will electrocute me, while touching another prong would be harmless?
How is that possible in an ungrounded AC system?
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#302963 - 11/10/2007 18:20
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
I'm just wondering what the point of polarizing an ungrounded AC power source is.
Ahh.. One side of your single-phase AC power soure *is* grounded at the source, meaning it literally is connected to the dirt outside your house. The other side is the actual AC supply, known as the "hot" or "live" wire. Only one of the two wires is hot.
Originally, electricity used a single wire distribution system in many areas, and each house had their own grounding rod(s) to complete the return loop back to the supplier. But house grounding systems were (and are) often suspect, so now a pair of wires comes in from the utility company: the new wire is the "neutral" wire, which is actually grounded out on the utility company end of things.
But just to continue with the safety trend, we now have three wires, where the hot and neutral are now supplemented by a local earth ground, which is connected to grounding rods at the utility pole and at the house electrical panel -- often just connected to a copper water supply in older homes, or to grounding plates/rods in newer construction.
Cheers
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#302964 - 11/10/2007 18:25
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Quote:
People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.
So you're saying that touching one of the prongs will electrocute me, while touching another prong would be harmless?
How is that possible in an ungrounded AC system?
The system *is* grounded.
You can only be electrocuted by inserting yourself into the current flow loop. Like standing on the kitchen sink in bare feet to change a light bulb --> perfectly safe when things are properly polarized because you are unlikely to stick your finger deep inside the empty bulb socket.
But if not polarized, then the hot wire could be on the outside shell of the socket instead of the inside contact, and you'll get fried.
Our neighbours here once called me over to their house next door, because they were getting sparks from their dining room light fixture. Turns out the hot and neutral wires were reversed on it, and electricity was flowing down the decorative metal hanging chain instead of through the correct wiring path.. Which is where the third, earth, wire comes in. It provides a mostly failsafe path for current to find the ground (mother earth) when the neutral wire is broken.
Cheers
Edited by mlord (11/10/2007 18:45)
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#302965 - 11/10/2007 18:28
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Ahh.. One side of your single-phase AC power soure *is* grounded at the source, meaning it literally is connected to the dirt outside your house. The other side is the actual AC supply, known as the "hot" or "live" wire. Only one of the two wires is hot.
Here's your homework for tonight: Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).
Cool, eh? Tell us what voltage you measured. I'll go out and do it here right now.. gimmeasec..
Cheers
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#302966 - 11/10/2007 18:32
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Effectively, it's not that the hot wire forces electricity for 1/60th of a second and then the neutral wire does, it's that the hot wire forces electricity for 1/60th of a second and then sucks it back for 1/60th of a second.
In other words, if you stick a multimeter in an electrical outlet, you get 120V between hot and ground and between hot and neutral, but you get 0V between neutral and ground. You can stick your tongue in the neutral side of a socket and leave it there all day long. (Note: I do not suggest this.)
The reason that it has to be plugged in the right way around is for safety. If the 2-prong device you're using fails and shorts, it needs to make sure that its neutral connector is connected to ground, as that's how it's designed. If it tried to ground to the hot wire, it would fail to trip the fuse/circuit breaker.
_________________________
Bitt Faulk
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#302967 - 11/10/2007 18:34
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Here's your homework for tonight: Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).
Cool, eh? Tell us what voltage you measured. I'll go out and do it here right now.. gimmeasec..
Here ya go. Red probe plugged into a single (hot) conductor of a 120VAC outlet, and black probe just stuck into the wet earth outside our house. You can read the meter voltage..
Edited by mlord (11/10/2007 18:36)
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#302968 - 11/10/2007 18:41
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Here ya go. Red probe plugged into a single (hot) conductor of a 120VAC outlet, and black probe just stuck into the wet earth outside our house. You can read the meter voltage..
A variation on that experiment may also be instructional.. take a table lamp outside, and wire it with only one conductor going into the hot side of an outlet, and the other conductor stuck into the ground (do this with it switched OFF!!). Then turn on the switch, and watch what happens..
For an older home, the light will probably illuminate nice and bright.
With a newer home, probably nothing will happen.. can you guess why?
Edited by mlord (11/10/2007 19:02)
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#302969 - 11/10/2007 18:51
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).
Cool, eh? Tell us what voltage you measured.
Then repeat the experiment, connecting the voltmeter to the neutral socket instead of the hot socket on the outlet. It will measure 0.0V --> that's the one you can safely lick, if (like Bitt) you're into that sort of thing. 
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#302970 - 11/10/2007 18:56
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Quote:
One purpose of the polarized 2-prong plugs is to allow devices that would otherwise have required a heavier 3-prong cord to use "double-insulated equivalency" construction along with the 2-prong plug.
I don't think that answers the question of why the 2-prong plug must be polarised. In the UK all items with 2-conductor leads must be double-insulated, but many such items (e.g. laptop PSUs) have non-polarised "figure-of-8" connectors (which I've just found out Wikipedia calls C7, having no evidence of their being called "Telefunken" as I had thought).
Peter
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#302971 - 11/10/2007 18:59
Re: What good is a polarized AC plug?
[Re: peter]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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No, the main reason is is that it ensures that the outside screw conductor of a standard light bulb socket will not be connected to the hot voltage wire.. Ditto for ensuring that switches cut the hot wire before the device in just about any product, and that internal fuses are on the incoming hot wire.
Cheers
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#302972 - 11/10/2007 19:00
Re: What good is a polarized AC plug?
[Re: mlord]
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old hand
Registered: 17/01/2003
Posts: 998
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Very good explanations. Cool that you got your shoes to light as well. I hope you didn't get them dirty. The wife will be mad.
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#302973 - 11/10/2007 19:04
Re: What good is a polarized AC plug?
[Re: Redrum]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
Very good explanations.
Cool that you got your shoes to light as well.
I hope you didn't get them dirty. The wife will be mad.
Heh! SWMBO took that photograph!
We love those garden clogs, and it will be a sad day when they eventually wear out. All of the new ones have these stupid holes in them to let the garden muck dirty one's socks/feet, which kinda defeats the entire purpose of garden clogs..
Cheers
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#302974 - 11/10/2007 19:10
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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No, the main reason is so that an internal short will go to ground.
Your chandelier example is a fine example of this. The chandelier was designed so that the neutral connector was connected to the metal body of the chandelier. This is required by electrical code (or some other law -- who knows?). If there was a short that connected the metal body to power, you need that power to go to ground, which the neutral conductor provides. If it is connected backwards, that short will remain live and people could easily get killed.
Of course, having the easily-touchable part of a lightbulb as neutral is not a bad side-effect of this.
_________________________
Bitt Faulk
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#302976 - 11/10/2007 19:13
Re: What good is a polarized AC plug?
[Re: wfaulk]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
No, the main reason is so that an internal short will go to ground.
No, it will still do that even if the two wires are reversed. The purpose of polarization is to allow things like switches to be connected to the correct wire, so that when "off", there is no hazardous voltage present in the product beyond the switch. And etc..
Quote:
Your chandelier example is a fine example of this. The chandelier was designed so that the neutral connector was connected to the metal body of the chandelier. This is required by electrical code (or some other law -- who
No, neither the hot nor the neutral should ever be connected to the body of any fixture. That's what the third "earth ground" is for.
-ml
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#302977 - 11/10/2007 19:13
Re: What good is a polarized AC plug?
[Re: Robotic]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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A fine point.
s/60/30/g
_________________________
Bitt Faulk
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#302978 - 11/10/2007 19:26
Re: What good is a polarized AC plug?
[Re: mlord]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
The purpose of polarization is to allow things like switches to be connected to the correct wire, so that when "off", there is no hazardous voltage present in the product beyond the switch. And etc..
Ah. This makes the most sense of all the reasons.
All good explanations in this thread, thanks everyone.
Follow up question:
So then why do things seem to work fine no matter which way I plug in the device? Why aren't old style plugs (or the English plugs) polarized?
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#302979 - 11/10/2007 19:40
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 29/08/2000
Posts: 14493
Loc: Canada
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Quote:
So then why do things seem to work fine no matter which way I plug in the device? Why aren't old style plugs (or the English plugs) polarized?
Because AC current flows both ways, so the electronic stuff doesn't care at all. It's purely a safety issue.
-ml
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#302980 - 11/10/2007 20:03
Re: What good is a polarized AC plug?
[Re: tfabris]
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carpal tunnel
Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
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Think of it as pressured gas piping. On the power company end, there is a piston that continually moves back and forth in the pipe, forcing air out and then sucking it back in. On the other side, there's a giant balloon. At your outlet, there's a valve. Normally that valve is closed. Since the power company can't pull back and create a vacuum, and because it's far easier for it to just be sucking the air from other places where it is connected to the reservoir, nothing happens while the valve is closed. But when you plug something in, it connects the two pipes together, running through your device. Now all of a sudden, both pipes have air rushing back and forth. But if you were to manually open the power company side, you'd have air rushing out and being sucked back in. On the other hand, if you opened the balloon side, nothing would happen.
_________________________
Bitt Faulk
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#302981 - 11/10/2007 20:10
Re: What good is a polarized AC plug?
[Re: mlord]
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old hand
Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
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Quote:
If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.
Switch mode PSUs take their bite equally on each half of the mains cycle so on a single phase supply it makes no difference...
They employ a full wave rectifier across the mains supply that feeds a big capacitor. The capacitor supplies a switching circuit that chops this rectified raw mains, at around 20kHz, into a step-down transformer. The capacitor is there to ensure the fast switching circuit has sufficient power in between the crests of the mains 50/60Hz sine wave.
Now, getting to my point here: I suspect you were thinking about power factor (PF)? This relates to where on the mains sine wave you draw your power. The utility co.'s want us all running with a PF ratio of 1 - an example of such a device is a simple resistive electric heater. With a PF of 1 we are presenting an even load throughout the cycle, anything less than 1 means were are 'unevenly' loading the supply. PF<1 is bad because it is more expensive to produce and distribute and it 'looks' to our electricity meters that we are using more kWH.
Switch mode PSUs are inherently bad here - they only draw current at the crests of the sine wave when the mains voltage exceeds the voltage in the capacitor. Natively, they have a PF of around 0.5 but nowadays internal correction components tend to make them around 0.7 or higher.
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#302982 - 11/10/2007 20:24
Re: What good is a polarized AC plug?
[Re: tfabris]
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old hand
Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
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Quote:
Why aren't old style plugs (or the English plugs) polarized?
But they are polarised? The 13A plugs can only be inserted one way and they have assigned Live and Neutral pins (link).
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#302983 - 11/10/2007 20:59
Re: What good is a polarized AC plug?
[Re: AndrewT]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Quote:
But they are polarised?
I wouldn't know. I was just going by what Andy said above.
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#302984 - 11/10/2007 21:07
Re: What good is a polarized AC plug?
[Re: tfabris]
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old hand
Registered: 16/02/2002
Posts: 867
Loc: Oxford, UK
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Ah, right. He was talking about the figure of 8 "radio cassette" leads. No matter what mains connector you fit at the plug end, they are reversible at the appliance end.
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