Switching Wall Warts, Isolated Grounds?

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I searched a few words and threads but did not seem to fine what I was looking for.

In "the old days" wall wart power supplies had a transformer in them, and one could power separate portions of a circuit with two or three wall warts, and they were all isolated, floating, and independant of each other. The grounds were not tied to each other back through the 120 VAC building wiring.

With the newer, physically smaller and more effieicent switching wall warts, are the V+ and Gnd leads isolated from the 120 VAC terminals, and hence floating with respect to earth ground, and building neutral?

A simple test with an ohm meter indicated that they were, but I'm not familiar with the circuitry used inside them.

For a quick testbed project it would be easier if they truely floated with respect to each other.

Any thoughts?

JC

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Quote:
are the V+ and Gnd leads isolated from the 120 VAC terminals, and hence floating with respect to earth ground, and building neutral?
I would sincerely hope so or you would have many more fried patients in the ER ward. :lol:

Usually there is a bridge with a big cap smoothing/storing the rectified mains, then there is the switch mode regulator feeding a transformer which supplies the DC to your circuit. Again usually there will be an optocoupler for feedback to the regulator to keep the voltage stable.

You may want to have a look at a site like "power integration" and you should have many examples in the data sheets. www.powerint.com

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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John,

Got it!
Thank you for the link, also.

Your description makes perfect sense, but I wanted some guidance before I fried something, (like myself :shock: ).

JC

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All care taken but no responsibility accepted. :)

When I lived in Italy we had 2 pins power plugs. To save costs things like TVs and Radio had autotransformers which meant that 1 leg of the mains went to chassis (metal). It was meant that the neutral went to ground but the plugs are reversible, so one learned very quickly to either have a neon light screwdriver that would indicate a live chassis or stand on your heels and quickly brush the chassis with your hand.

A tingle would mean "reverse the plug"!!

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:
All care taken but no responsibility accepted.

I think you are safe... If I die I won't be around to complain!

I remember working on some of those old AC/DC tube radios with live chassis long ago. In retrospect I'm lucky I didn't kill myself a time or two.

It was some of those memories, and others, that elicited the original question above. I saw, on the link you provided, several data sheets with exactly the circuit you described, (a transformer included which provides isolation), and a few others without transformers, (scary thought).

I appreciate your insight!

JC

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DocJC wrote:
With the newer, physically smaller and more effieicent switching wall warts, are the V+ and Gnd leads isolated from the 120 VAC terminals, and hence floating with respect to earth ground, and building neutral?
Not always.

The low-voltage and the high-voltage parts are isolated, but

a) At least in the EU authorities regularly find and withdraw wall warts from the market where the required isolation, creepage, etc. was not observed.

For a thrill, you can go to http://ec.europa.eu/consumers/dy... and type "charger" or "electric" into the search field. Note the country of origin of most violators.

Because of the country of origin I assume that similar wall warts could be on the market in the US.

b) Even when within regulations, the high-voltage and low-voltage side need not the be "100%" isolated. The use of capacitors to connect the low-voltage and high-voltage ground (there is a high-voltage ground, because the AC is rectified into DC on the "primary" side) is permitted under certain circumstances (usage of "Y" class capacitors etc. required)

Generally speaking, medical equipment must not be powered from consumer-graded wall warts because of these possibilities. Medical-equipment grade power supplies require more isolation (double? tripple? sure depends on local regulations) for a good reason. Consumer.grade wall warts, especially the budget ones, don't provide the isolation.

Consider using batteries.

Stealing Proteus doesn't make you an engineer.

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ArnoldB wrote:

For a thrill, you can go to http://ec.europa.eu/consumers/dy... and type "charger" or "electric" into the search field. Note the country of origin of most violators.

With 1 Bn. ppl , they might think they can afford to loose a few :wink:

/Bingo

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Hi Arnold,

Thank you for the info.

This is just for a quickie weekend project, not for a "real application", or for anything medically oriented. (An AVR ammeter to watch another AVR PCB...).

Hi Bingo,
I just wanted to make sure I was not going to be one of the individuals zapped!

JC

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js wrote:
A tingle would mean "reverse the plug"!!

American guitar amplifiers from the tube era (showing my age here) often had a 2 pin plug and a toggle switch on the back that would connect the chassis to one side or the other of the 110V supply, for hum cancellation. Allowing one's lips to touch the microphone would generally reveal whether the switch needed reversing, if you survived. Mortality in the Hamburg clubs was further enhanced by the German practice of using red for the ground wire, whereas the British used red for live, before the brown-blue-green/yellow convention was adopted. I still have a Schaller reverb unit with a red ground.

Regarding switch mode supplies, I have known equipment with multiple PSUs in it to fail compliance testing because the combined ground leakage current was too high. Changing to medical grade supplies fixed it, though at substantial extra cost. The leakage current has to go somewhere, presumably into the low voltage equipment in the case of a wall wart, and thence to ground through someone's body, but the current won't be hazardous unless a fault develops.

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I had the same question as you. I decided to use a wall wart with transformer.

Using batteries is a good idea when practical. But my handheld DMM that I converted to use a wallwart is one of my best inventions. It's been on for a year now and may last a lifetime. It's generally monitoring the current draw of AVRs. What a relief not having to change the bleepin' 9 volt battery.

You do bring up an interesting question. I have a dozen or so of the transformer type wall warts, but if I ever have to buy one, how can I be sure it has a transformer?

I guess I would avoid regulated ones. Maybe the AC output ones would be most likely to have a transformer.

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Weight is usually a good indicator ;)

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ArnoldB wrote:

Quote:
b) Even when within regulations, the high-voltage and low-voltage side need not the be "100%" isolated. The use of capacitors to connect the low-voltage and high-voltage ground (there is a high-voltage ground, because the AC is rectified into DC on the "primary" side) is permitted under certain circumstances (usage of "Y" class capacitors etc. required)

This is a point I have made many times regarding the 2 pin variety of Plug Packs / Wall Warts.

The capacitor if fitted will capacitively couple the mains to the output.

Please take the time to measure either output terminal to Earth with an AC meter. Then with a CRO.

Then You decide if the supply is suitable for your application.

Most dammage occurs when connecting power supplies to grounded circuits.

SMPS power supplies that are grounded, have these capacitors between the output and earth, Not output to rectified mains, so usually dont pose any problems.

Ron.

 

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I had developed one SMPS from mains to 5V and 12V once. In fact, many regular (except some low power) SMPS's have a little cap (Y class safety type) that connect a 'low noise point' on the HV side to a 'low noise point' on the LV side for conducted noise supression. A poor design can lead to some little AC, 'HF' (low KHz) to pass to the LV side. Check PowerInt app notes (they have a lot!!) about that. Sizing this cap is usually done by "succesive approximation".

Isolation transformers also have some low capacitive coupling between both sides that can affect it.

Anyway, for many hobbyist applications at my home, I used these kind of supplies, but my preferred for +5V supply is the USB bus from PC's.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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JC,

You can perform the test on a switch mode supply yourself using a DVM. You may be surprised at the results and at what performance is actually legal.

1. Measure the AC volts between DC output and ground. You could find approx. half mains voltage here.

2. Measure the AC leakage current between DC output and ground. On many examples I've tried I find approx 110uA.

If your project involves any ADC work, then, if that leakage current flows in signal lines significant errors can result.

Nigel

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Hi everyone,

I appreciate everyone's insight.

Ron and Nigel have both suggested simply measuring the AC potential, and leakage. I haven't done that yet, but will, either this evening or tomorrow.

I sure hope not to find Vmains/2 on the plug. That would suprise me, but then that's why I started the thread. I don't know much about switching supplies, or these wall wart modules.

As it turns out it is for an ADC, (ammeter), circuit. I ended up using two ADC inputs, measuring across the current sense resistor with respect to ground, instead of just one ADC, (single ended), across the sense resistor. This avoided the entire issue of having a secondary supply floating with respect to the device under test.

Either way, it is still a good topic for me to learn about, as I am sure the subject will present itself again.

Hi Guillem. USB supplies are convienient , but not for this particular project. I'll take a look at those app notes, too.

Hi Steve. Yes. I, too, do not wish to keep replacing batteries!

Hi Peret,
I can't imagine using Red for Ground connections...Maybe there is a reason for Standards after all!

Quote:
Weight is usually a good indicator
JayJay, is that a reference to the Wall Wart, or the Designer :)

JC

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The wall wart :D

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Quote:

sure hope not to find Vmains/2 on the plug. That would suprise me, but then that's why I started the thread. I don't know much about switching supplies, or these wall wart modules.

If you use a high-impedance Voltmeter (or a high
impedance scope) you may well find Vmains/2.

One explanation is the following: In many
designs the output ground is connected
to the both input lines with a Y capacitor
in the order of a few nF. This
capacitors are a voltage divider with respect
to the mains, and the midpoint is connected
to the secondary. So you have a open loop
voltage of Vmains/2 with a internal impedance
of somehundred kOhms.

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Hi Ossi,

ArnoldB mentioned the Y cap above, also, but I had not tracked down the reference, yet.

Your explaination is great.

Thanks,

JC

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wow, gosh ... now I TOTALLY know why we have to connect ground to ANY metal chassis in germany. ZAPPP there goes the fuse.

Quote:
Anyway, for many hobbyist applications at my home, I used these kind of supplies, but my preferred for +5V supply is the USB bus from PC's.

Is there any over current protection for USB? I know, that USB slaves have to request to use the full 500 mA scale. But some devices do it without asking.

I use a PC power supply for experiments and it saved my circuits at least two times. I produced shorts between Vcc and GND and my supply simply turned off^^

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Nephazz, then call yourself lucky :!:
The use of a PC PowerSupply as lab-supply has been discussed several times, and the general conclusion was: don't !

Back to Doc's question: I also wondered what these SMPS's give as a leakage current. So I took a bunch of them and did measurements. Preliminary results can be found here:
http://www.aplomb.nl/SMPS_leakage/Doc_ie.html

I will add more info as I collect it.

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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whenever unsure, use an isolation transformer. In short, for a lab bench, you should always have an isolation transformer available to plug your mans supplied circuitry into until you know it is safe.

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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Check http://www.powerint.com/sites/de...

The infamous cap is C4. Too big, and you will 'feel the power of the dark side'. Too small, and you don't pass EC EMI compliance tests.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Guillem wrote:

Quote:
Too big, and you will 'feel the power of the dark side'

:lol: :lol:

I tested that "in vitro" on the bunch of power supplies I posted about a few posts back: they appear to be of the right size. Got 70 uA as max, 40 uA in most of the cases. The transformer-versions are the ones to use for experiments.

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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HI Nard,

Nice job on the photos, testing, and results presentation.

It would be interesting to include at least one "old" transformer type, (non-regulated), wall wart, just to see what the leakage current was under the same test setup and measurement conditions, just for a comparison.

Sometimes the simple things that one takes for granted, like a wall wart, can be more complex than one at first appreciates!

JC

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JC, that's what I did: you can find them on sheet/page 3. I tested 2 old-fashioned transformer-type wall warts. Very safe results, as you can see !

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Opps, :oops:

Got it. I was going back to look at all of the numbers more closely, and the difference is obvious!

Need more caffeine... :shock:

JC

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Quote:
Need more caffeine... :shock:

Nah .... take some more sleep instead. That's what I would recommend if I was a doc ;)

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Plons wrote:
JC, that's what I did: you can find them on sheet/page 3. I tested 2 old-fashioned transformer-type wall warts. Very safe results, as you can see !
Bzzzzzzt, No.

As you have shown, the ones with the capacitor between the primary and secondary side don't kill you - if done right. The same argument holds for traditional transformers: they don't kill you - if done right.

Transformers have to be winded observing particular isolation requirements. Capacitors are to be build observing particular isolation requirements (the Y class).

To decide which one is in principle safer you would have to have statistics about breach of isolation. I don't have access to such statistics. However, the power that be deem the capacitors good enough (safe enough) for the consumer mass market.

Stealing Proteus doesn't make you an engineer.

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Quote:
The same argument holds for traditional transformers: they don't kill you - if done right.
True. But what more can we do ? Stop designing circuits that require to be powered from the mains ? I mean .... where to draw the line ?
For medical electronics we use(d) either battery power or certified transformers and/or certified isolated DC/DC converters. What do you suggest for our freaks-circuits ?

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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One solution if we wanted a simple bench power supply would be to wire the power supply output to some terminals and have the - side earthed.

For PC power supplies - due to the large current availability (>20A), I'd suggest using a fuse or polyswitch device to limit the current to say, 1A to minimise the risk of bits of wire going up in smoke and causing a fire.

I have noticed the leakage in mainly dvd played and set top boxes where they have a metal chassis but only a two pin plug. If you're earthed and touch the case of these devices when they're floating ( electrical wise) you'll get a tingle that will make you jump.

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ArnoldB wrote:

Quote:
s you have shown, the ones with the capacitor between the primary and secondary side don't kill you - if done right. The same argument holds for traditional transformers: they don't kill you - if done right.

Ron:-->Agreed.

Kartman wrote:

Quote:
One solution if we wanted a simple bench power supply would be to wire the power supply output to some terminals and have the - side earthed.

Yes: This is the sort of thing that I have been trying to say.

The main problem with switchmode plugpacks with a capacitor from output to input rectifier is not necessarily the leakage current but the instantanious voltage on the capacitor when the plugpack is connected to one of our applications when there is a pre-existing grounded connection such as a CRO or PC Serial port.

Kartman's solution is valid and removes my hatred of un-earthed SMPS plugpacks. The transformer based variety have much less input to output capacitance and rarely provide a problem.

While I note that while Nard has performed extensive measurements and posted the results, (Thanks), I am still at a loss that no other poster has put their multimeter or CRO on their mobile phone charger to see what there is there. Its a 30 second test, not too much to ask, is it ?

I'm happy to see that for the first time in years, there there are some AVRfreaks willing to look into this pet issue of mine a bit deaper.

This all stems from loosing an expensive old programmer when one of my previous staff members connected a live SMPS plugpack instead of the transformer based one into the programmer which was already connected to a PC.

The programmer was irreplaceable.
Thanks, Ron.

 

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Later today I will put some more SMPS's to the test, and will focus on mobile-phone chargers and other low-cost wall-warts.

I want to let you (all) know that I am very pleased with the thorough approach to this prblem.

Nard

Edit: I got my first "tickles" from one of the SMPS-wall-warts. And I need to have a closer look at the multimeter I am using for this: sometimes odd results. I will update when I sorted that out.

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Nice report on your part Nard.

Thanks for your time & effort.

This does shed a different light on what suitable testing equipment is required.
The oscilloscope wins hand down with visual feedback.
Then again no reason a black box cannot do the same thing & transfer it optically to a PC for the result.

I have seen few wall wart units using like audio transformer inside as part of their switchmode setup.
These do not have the "C Tick" Australian Standard & we do not use them.(cheap & comes from Ch---)

Ken

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I've put my scope at the 12V line of my pc power supply. While the GND was connected I meassured nothing. If I leave the GND of the scope away I get something like a 50 Hz sine and it's magnitude is way too high to measure. Like more than 400 V.

I get the same signal if I put the probe at the 12V-GND. (always leaving scope gnd in the air)

I don't know how to think about this. Is this only some "caught in voltage" from "long" wires? Or is it some serios commom mode leakage?

PS: my multimeter has only an "auto V" option. So no possibility to get an AC meassurement :(

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The problem with ground, earh and similar appear when one wants to measure voltages of electronic systems enclosed in a metallic box. That is translated as the shield of the 'scope probe being earthed (law issues...), while some other part of the metallic enclosure of the device under test is also earthed (PC SMPS).

That leads to these high voltages measured with the 'scope, and that is only solved properly grouding both. The term 'properly grounding' can mean the use of isolation transformers, differential probles, isolating earth, etc.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Ok, I took the Pepsi challenge.

Just got a Dlink router to hack around with so I measured its plug-pack(wall wart etc).
- to earth 105vac
- to earth 130uA

This one has an earth pin, but I'm not sure it does anything.

Another router power pack - this time from a Edimax BR6204Wg

- to earth 98vac
- to earth 71uA

This one only had two power pins.

I've been going router hack mad lately. On my way to becoming an embedded Linux guru.

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Kartman wrote:

Quote:
I've been going router hack mad lately. On my way to becoming an embedded Linux guru.
Enjoying yourself ? :lol:

Your measurements confirm what I have found so far.

Since the scope-pictures tell so much more, I tested 4 selected wallwarts. Results:
http://www.aplomb.nl/SMPS_leakage/Doc_ie.html#August_7_scoped

It ain't half hot mum .... as it's about 30 degrees C in the shack :-(

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Impressive job, Nard!!

That confirms that for precisse instrumentation, classic transformer rocks much more than wallwarts. Anyway, that Samsung one also impresses me, and leads the path to follow.

Mmmmm, that makes me think about to test certain SMPS I have at home, but I wouldn't post any results if they are bad, may be I will be ashamed.

Oh, and all this also makes me wonder about the current output SMPS that showed me "the paths of the dark side" for my first time. It left me breathing like Darth Vader, and seeing everything black for few minutes.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Thanks !

I have a question for the experts (which I am not):

What is the purpose of that Y-class capacitor ? EMI regulations have been mentioned, but what is the explanation .... what does it do ?
And what are the risks of removing it ?

And please don't shoot me for asking this ! I simply want to learn.

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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I think that's a good starting point:

http://schmidt-walter.eit.h-da.d...

My EMC-book (from the dutch expert Jasper Goedbloed)
is in my workplace-office. No access
before monday.

Y capacitors are called y capacitors because
nobody knows y . :lol:

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IIRC Class Y caps are guaranteed/required not to fail into a dead short.

Isn't the capacitor required to shunt HF noise to somewhere? One PCB designed had an isolated area with DC/DC converters and optocouplers. I used one single capacitor between the grounds and that reduced noise on the isolated side enormously.

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Quote:
I used one single capacitor between the grounds and that reduced noise on the isolated side enormously.

Aha, I snap it (Dutch joke)
Without that Ycap, the capacitive coupling between primary and secundary winding, at the high frequency as used in SMPS's, is large enough to make the secondary circuit to jump up and down at the switching frequency, resulting in noise.
Hmmm, pretty odd sentence, but the intention is good.
Tomorrow I will give it a try ... live !
Thanks Jeroen.

Ossi, very interesting piece of documentation. I will keep it with other treasures. It explains very well and clear the construction for SMPS's with a SafetyGround connection. And from there it's a small step to SMPS's without a SG connection.

In the live test I will use a radio to get an impression of the effect.

You guys rock :)

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Google for 'stitching cap'.

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y

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y¬

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I did the radio-test on one of the SMPS's: There is an audible difference with the Y-cap in versus open. I have no equipment to quantify the difference ... :(

Video: http://www.aplomb.nl/SMPS_leakage/Doc_ie.html , in the top of the page is the .mpg

Edit: added a second video with effect visible on oscilloscope

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

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Greetings Nard, and A/all
Nard wrote:

Quote:
What is the purpose of that Y-class capacitor ? EMI regulations have been mentioned, but what is the explanation .... what does it do ?
And what are the risks of removing it ?

This link is most informative:
http://www.powerint.com/sites/default/files/product-docs/an15.pdf
Nard, Thanks to you and others for putting in the time and effort on this subject, and for publishing your results. It has been enlightening to many AVRfreaks, created good discussion and has opened up the benifits and pitfalls of using SMPS plug packs for our AVR projects.

One thing I have to ask is why not make 3 pin SMPS plugpacks? i.e. ones with a ground to return the HF noise to?
Surely a third pin on a plug would be a better solution for both EMC and Safety?

Ron.

 

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Because not every socket in the world has an safety earth so it would be completely impractical :D And as most, if not all, chargers are completely plastic and are double insulated they are class B and don't require a safety earth.

edit:
In NL since 2003 or so are new home electrical installations required to provide safety earth on every socket; before 2003 it was only required in the kitchen and bathrooms. Old installations don't need to be upgraded, so in most homes you only find sockets with safety earth in the kitchen and bathroom, the latter usually not there either. Washing machines in the bathroom need a hard wired connection.

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That's a good point. I guess the ubiquitous cheap switching power supplies will have two pins for a long time.

But there might be a market for three pin supplies also. Computer power supplies around here use a 3 pin power cord, and in fact have a switching supply.

One problem with 3 conductor cords is they are thick, stiff and clumsy. It hasn't helped the situation that in the U.S., apparently many power cords must have thicker wires than in previous times. I've given up trying to find an extension cord using 18 gauge wire. They are all 16 gauge.

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Ron, there is hope: http://www.aplomb.nl/SMPS_leakage/Doc_ie.html#There_is_hope

This was a usefull exercise and a pleasant thread.

Cheers

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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