Can Power Brick serve as Isolation Transformer?

**PlainBill** · 05-08-2009, 04:53 AM

Re: Can Power Brick serve as Isolation Transformer?

While I haven't thought of this, it certainly makes sense. There are several points on every notebook that UST be isolated from AC (USB ports, ethernet port, printer and serial ports, etc).

The obvious danger that I would like to point out is that while the notebook would be isolated from the AC line, it wouldn't be too cool if the exposed metal on the notebook was at some elevated voltage (165VDC!!) relative to ground because the equipment you were scoping wasn't isolated.

PlainBill

**i4004** · 05-08-2009, 09:10 AM

Re: Can Power Brick serve as Isolation Transformer?

why do you think isolation trafo will block ground loop?
it's 1:1 device so anything getting in should also be getting out, right?

as for laptop power adapters, no they don't have 1:1 isolation trafo inside, they are usual smps psus.

what they do have is a capacitor between primary and secondary side(why don't you take one apart?)...at least those that i took apart had it...adapters for external usb hdd trays atc.

and the interesting thing is this: removing that cap won't prevent ground loop from occuring: why should it, ground loop is ac, and ac can pass thru the trafo regardless of that capacitor which probably has some protective prupose, or so...

>So,,, I took some of my power bricks and measured each AC connector to each DC connector and in every possible combination they are all open.
[No connection from AC to DC.]

well, if you had a capacitor between primary and secondary it would also look open...to the multimeter you're using...
try measuring it with esr meter.
depending on the capacitor value it may or may not show something; i just got reading of 20ohm between ground connector on mains plug and minus of laptop ac/dc adapter....

open the adapter and see what's inside.

but overall, why WOULDN'T YOU wanna see ground loop on the scope if one exists?

**PCBONEZ** · 05-08-2009, 01:18 PM

Re: Can Power Brick serve as Isolation Transformer?

Originally posted by i4004

why do you think isolation trafo will block ground loop?
it's 1:1 device so anything getting in should also be getting out, right?

as for laptop power adapters, no they don't have 1:1 isolation trafo inside, they are usual smps psus.

The ground loop is DC ground in circuit to earth ground.
There is no DC path through an isolation transformer.
-
Isolation Transformers aren't all 1:1 that's just the most common type.
The better [or at least more expensive] ones have multiple secondaries and a switch or different terminals to choose different output voltages.

Originally posted by i4004

blah blah capacitor blah blah blah

well, if you had a capacitor between primary and secondary it would also look open...to the multimeter you're using...
try measuring it with esr meter.

No, not even, it would show a charge and discharge on DMM.
Did check with ESR meter.
- ESR meter shows dead open circuit Pri-Sec just like DMM.
[And yes, the adapters work.]

Originally posted by i4004

but overall, why WOULDN'T YOU wanna see ground loop on the scope if one exists?

The point is to not CREATE one through the test equipment. - DUH!

.

**i4004** · 05-08-2009, 03:01 PM

Re: Can Power Brick serve as Isolation Transformer?

>The ground loop is DC ground in circuit to earth ground.

what sort of current does ground loop introduce, ac or dc?

>No, not even, it would show a charge and discharge on DMM.
Did check with ESR meter.
- ESR meter shows dead open circuit Pri-Sec just like DMM.
[And yes, the adapters work.]

well it depends how big the cap is, obviously. those are not lytics, but typically very small capacitance...

>The point is to not CREATE one through the test equipment. - DUH!

well if you don't have g.l. problems overall at that place i doubt you will create one in that way...

**PCBONEZ** · 05-08-2009, 04:06 PM

Re: Can Power Brick serve as Isolation Transformer?

Originally posted by i4004

>The ground loop is DC ground in circuit to earth ground.
what sort of current does ground loop introduce, ac or dc?

If you will read what I said I wasn't talking exclusively about ground loops I was referring to that TYPE of problem in general.
As to ground loops and what they are and do I'm NOT going around with that with you again, please go find previous discussion on that and read it.

Originally posted by i4004

well it depends how big the cap is, obviously. those are not lytics, but typically very small capacitance...

Small non-lytics have high ESR, not low.
But meter did not read out of range high, it read dead OPEN.

Originally posted by i4004

well if you don't have g.l. problems overall at that place i doubt you will create one in that way...

One doesn't KNOW what problems they have in the first place when testing deranged, damaged, or non-working equipment.
If you had a concept of what is goes on in TSnR you wouldn't have said that.

~~~~
My question [this thread] is about the nature of Power Bricks.
Stop polluting my subject with your opinions on the need for an Isolation Transformer when using test equipment.
That much is a GIVEN in the question and anyone that knows what they are doing already knows why.
.

**i4004** · 05-08-2009, 06:03 PM

Re: Can Power Brick serve as Isolation Transformer?

uh, don't i just love discussing ground loops with you...boy oh boy...
last time we discussed it we just figured you don't really know what it is...
(by the end of this page
https://www.badcaps.net/forum/showth...ht=ground+loop )
and here you're only confirming it.

there is one comforting thing, though; if you don't know what it is, it means you don't have it, because if you did, you would probably research it and know exactly what it is.
symptoms are easy to spot both on audio and video systems.

>One doesn't KNOW what problems they have in the first place when testing deranged, damaged, or non-working equipment.
If you had a concept of what is goes on in TSnR you wouldn't have said that.

why do you wanna fight somebody trying to help you?
i said that YOU know what YOU have in YOUR place.
ok?
you know if plugging something into particular wall outlet produces ground loop or whatever....if you have your hearing and you can see, you know...

you said you were gonna use it at your place.
some places have ground loops, others don't.
most don't.

if you don't have it, don't bother with removing it.
but get isolation trafo for your safety.

>My question [this thread] is about the nature of Power Bricks.

and it was answered, wasn't it? i told you those are small smps supplies.
so it can(or can't) serve as isolation trafo as any other usual smps psu can or can't.
heh

>Stop polluting my subject with your opinions on the need for an Isolation Transformer when using test equipment.
That much is a GIVEN in the question and anyone that knows what they are doing already knows why.

true, but it seems you don't.

isolation transformer you tak about is device to increase your safety(ie to prevent electrocution). as such, it's excellent.
otoh, as a means of reducing ground loop and all other sorts of interference...well....most devices already have some sort of mains transformers, so that would presume such problems cannot occur....yet they do...

so overall i don't see this isolation transformer as a mean to remove ground loop and other types of interference that spread via mains...
you should have isolation transformer(of some sort...surely not the same type u use for mains) in the signal path, and that should prevent ground loop....

dunno, maybe i'm wrong.......perhaps plainbill knows more...

**PCBONEZ** · 05-08-2009, 06:09 PM

Re: Can Power Brick serve as Isolation Transformer?

What didn't you understand?

**PCBONEZ** · 05-08-2009, 06:16 PM

Re: Can Power Brick serve as Isolation Transformer?

If you want to help then do so. - Discussion about what an Isolation Transformer is or is used for is off the topic, polluting the thread, and wasting my time.
The need for it is a given...

The topic is: "Does a Power Brick negate the need for [fulfill the purpose of] an Isolation Transformer."
.

**i4004** · 05-08-2009, 07:11 PM

Re: Can Power Brick serve as Isolation Transformer?

how will you fix things if you can't deduce that from the data i gave you about what's inside the "power brick"(as you call it)?

also, are you not aware your question is completely ridiculous?
you're comparing mains-voltage/low dc voltage device to a device with 1:1 windings that's ment not to alter the voltage, and instead just to serve as isolation from the ground so that you can't accidentally electrocute yourself.

how the hell will you use one instead of the other?

how will you supply the tv-set (you're servicing) from that laptop power-brick?

to that extent you didn't get what plainbill said:
"The obvious danger that I would like to point out is that while the notebook would be isolated from the AC line, it wouldn't be too cool if the exposed metal on the notebook was at some elevated voltage (165VDC!!) relative to ground because the equipment you were scoping wasn't isolated."

he told you that equipment you're scoping should be isolated by isolation trafo.
don't worry about the freakin laptop and bloody ground loop....worry about the device you're inspecting: IT is the potential source of voltage that can kill you. IT should be connected via iso.trafo.

>Discussion about what an Isolation Transformer is or is used for is off the topic, polluting the thread, and wasting my time.

if you knew what it was, you would never ask this question.
then you bitch at me when i tell you what it is, and how power brick is not isolation transformer...

just swell.

and you see, i bother with you, i try.

do you now have explanation why your brick is not isolation trafo and why you need real isolation trafo with 1:1 windings to connect the equipment you're measuring to the mains?

**Krankshaft** · 05-08-2009, 08:02 PM

Re: Can Power Brick serve as Isolation Transformer?

Originally posted by PCBONEZ

The topic is: "Does a Power Brick negate the need for [fulfill the purpose of] an Isolation Transformer."
.

Yes the notebook power brick would isolate the laptop from ground. As for some SMPSs will isolate and others won't that's bunk.

ALL laptop SMPSs I've opened use a transformer. Transformers no matter what their turns ratio provide isolation from ground.

Yes you can engineer a SMPS with an inductor instead of a transformer which wouldn't isolate but you will never find such a design on a notebook or most any PSU.

After it's all said and done though the point Plainbill made is valid you also need to make sure the equipment you're probing with the scope is isolated as well.

If you're probing a transformer powered circuit you're all set (like a circuit powered off a wall wart). If you're working on line operated equipment like a TV or PC PSU then a 1:1 mains isolation transformer will be required. For the device under test.

Here's the one I use works great and didn't cost an arm and a leg:

http://www.mcmelectronics.com/product/72-6670

**PCBONEZ** · 05-08-2009, 09:05 PM

Re: Can Power Brick serve as Isolation Transformer?

i4004, I rarely even read you nonsense anymore when it comes to anything technical.
If I said "the sky is blue" you'd argue the point and I don't have time for bullshit lately so just go away.

Krankshaft
I think I saw you post that before but I couldn't remember who, when, or where I saw it and I was looking for it. - Thanks for popping in.

I already have Isolation for the equipment by way of a 1.5kVA 220-115v step down Iso Xfmr that's hard-wired through the benches to dedicated sockets and I already have a portable one for the 7633 Tektronics I use.

I was thinking I needed yet another one for the laptop but it appears not.
[Laptop and Tektronics may be used at same time.]

Somewhere I think I have a dead [same model] laptop brick I intended to tear apart in the name of science but it's been a while since it died [like, since before I moved here] and I don't remember where it is. [Or even if I still have it....]

I figure here at BNC 'someone' has dissected some of these before.
.

**Wizard** · 05-08-2009, 09:36 PM

Re: Can Power Brick serve as Isolation Transformer?

Main two reasons for isolation transformer use One: were for safety (number one for consumer and measuring devices measuring for any reasons the device gets isolated from main power). It does not have to do with the ground pe se.

Second reason was for techanicans' and measuring devices that's is grounded there was a need to float the unit under test because direct main connected still have hot section that we need to clamp ground clip onto without blowing the fuse or damage the tool or the unit.

Early electronics years ago was totally HOT chassis literally and ONLY isolation was plastic and wood cabinet and insulating knobs even isolating ceramic couplings for the shafts and switches linkages (!!). But we still see many intances of this in modern stuff.

Nowdays, if the power brick has a ground pin on the main plug, it will be still isolated but the ground is directly connected at the cold ground. If the power brick do not have grounded mains plug, this will be also isolated and any grounding is by chance via anything plugged to the said powered device.

The grounding on TV still happen via the RF cable but this is totally unreliable. Good ones have grounded mains plug connected to the cold ground of the chassis.

Ungrounded Hot chassis still exist in some devices, but not as frequent. Good example is hair dryers, electric drills, line-powered electric motors, fans and very rare few electronics. I have a phone xenon flasher box that is total hot chassis. The isolation is in tiny transformer for the phone ringing voltage to trigger the xenon trigger circuit. Xenon capacitor is directly driven by voltage pump up directly powered by the ac mains power. That small transformer for phone plug and plastic casing is what provides the isolation.

PS: reason we break the ground in the safety isolation transformer for our service is to prevent ground loops that will distort the scope's waveforms at some times.

Cheers, Wizard

**i4004** · 05-08-2009, 09:48 PM

Re: Can Power Brick serve as Isolation Transformer?

something i mentioned earlier
http://www.epanorama.net/documents/g...isolation.html

>i4004, I rarely even read you nonsense anymore when it comes to anything technical.
>If I said "the sky is blue" you'd argue the point and I don't have time for bullshit lately so just go away.

nope, i only correct you when you're wrong, but for some reason you can't tolerate being wrong. most nonsense in the thread was produced by you.

>Yes the notebook power brick would isolate the laptop from ground. As for some SMPSs will isolate and others won't that's bunk.
>ALL laptop SMPSs I've opened use a transformer. Transformers no matter what their turns ratio provide isolation from ground.

uhm...it's not about that....transformers don't matter if you have a circuit which puts half the line voltage on metal case of the pc psu....and they all do, so if you don't have grounded wall outlet you better not touch your pc case if you're barefoot on concrete...

as for laptop providing isolation from ground....meh....bollocks...and here are pix to prove it:
http://picasaweb.google.com/i4004b3/...su_2009_05_09#

you can see it's adapter for 100-240v which means it's worldwide model..just like many smps psus are....

you can see that when measuring with ac(with esr meter) resistance(between minus of the 20V connector and ground pin of wall plug) is about 20ohm

AND you can see voltage betwwen ground(water-tap) and minus pole of that 20V output is half line voltage....just as with all pc psus on their metal case

when you see the trafo inside the psu it doesn't tell you anything about what's it putting on the metal shield....or on output ground(ie minus of the connector)

here are pix from the inside such psus
http://picasaweb.google.com/i4004b3/...69133845610754
notice blue capacitor on the side of the trafo; it connects low side ground(minus) to hi voltage ground...

same thing here
http://picasaweb.google.com/i4004b3/...34355766982978
above the "qc 10" sticker you can see the blue capacitor...

in the lower right angle of that pic you see 2 caps that provide half line voltage on the smps metal housing(when measured against ground), when pcb is in such housing(like in pc psus)

in this diagram
http://s183.photobucket.com/albums/x..._200wsmps1.gif
those are c2 and c3.

here

they are cy (twice)

>If you're probing a transformer powered circuit you're all set (like a circuit powered off a wall wart). If you're working on line operated equipment like a TV or PC PSU then a 1:1 mains isolation transformer will be required. For the device under test.

you wanted to say there's danger when working on the primary side of any line powered power supply, wether it's smps or linear...
those portions are usually marked "hot" on the pcb itself.

**PlainBill** · 05-08-2009, 10:02 PM

Re: Can Power Brick serve as Isolation Transformer?

i4004 IS correct about one thing: an ESR meter may not show the existence of a capacitor linking the input and output of a 'power brick'. Decades ago I used an ohmmeter to verify a Lafayette FM tuner was isolated from ground. Then I discovered there were 500pF caps from each side of the AC line to the chassis, so no matter which way the cord was plugged in, the METAL chassis was at 60 volts.

I've seen three different styles of AC cords for power bricks. Some use the standard IEC? three wire cord used on virtually every computer power supply and on most monitors. Others use either a polarized two wire or non-polarized two wire cord.

The chance that those with two wires have any coupling between line in and dc out is vanishingly small. A mis-wired outlet or someone clipping the wide blade of the power cord down would create a laptop with potentially lethal voltages on the ports.

I'm less confident about those using a three wire cord. IF the 'power brick' in question uses one of those I would definitely check voltage AND resistance between ground and any exposed metal on the laptop.

PlainBill

**PCBONEZ** · 05-08-2009, 10:17 PM

Re: Can Power Brick serve as Isolation Transformer?

I'm not wasting time with i4004's comments. I already asked him to go away.
If you say he's right about that one thing then congrats i4004.
Except it's still N/A because they don't float laptop chassis' on AC.

Most of mine are 2-wire non-polarized with the 'figure 8' AC->Brick connector.
The only I have with 3-wire are higher output than I need for these laptops and they won't be used for this anyway.

.

**i4004** · 05-08-2009, 10:23 PM

Re: Can Power Brick serve as Isolation Transformer?

>Nowdays, if the power brick has a ground pin on the main plug, it will be still isolated but the ground is directly connected at the cold ground. If the power brick do not have grounded mains plug, this will be also isolated and any grounding is by chance via anything plugged to the said powered device.
The grounding on TV still happen via the RF cable but this is totally unreliable. Good ones have grounded mains plug connected to the cold ground of the chassis.
Ungrounded Hot chassis still exist in some devices, but not as frequent.

exactly...but here's the catch; if for some reason wiring of the house doesn't have ground connected properly(ie ground pin is not really connected to "earth") then you have tingling when you touch metal portions of the cold ground...
all you need is good path to ground to feel it...
it won't kill you, for sure, but it's not pleasant at all...

**PCBONEZ** · 05-08-2009, 10:36 PM

Re: Can Power Brick serve as Isolation Transformer?

I'm not wasting time with i4004's comments.
I already asked him to go away because he can't stay on topic.
If you say he's right about that one thing then congrats i4004.
Except it's still N/A and off-topic because they don't float laptop chassis' on AC.

Mine are 2-wire non-polarized with the 'figure 8' AC->Brick connector.
.

**i4004** · 05-08-2009, 11:31 PM

Re: Can Power Brick serve as Isolation Transformer?

it is easy to establish if you have half of the line voltage on laptop's cold ground:
multimeter to ac volts (put it at 200v range, just incase), one probe to something grounded (if water supply pipes are metal and go to the ground that will do) another to minus of the laptop power brick output....plug it in....
any voltage there?
be carefull, don't touch ANY metal with your bare hands while measuring!!! wear rubber gloves if you can't be disciplined

>I already asked him to go away because he can't stay on topic.

this is all perfeclty on-topic: and topic is your safety.
i told you early on that there is a cap, and plainbill confirmed it.
what i simply cannot believe is that you wanna be fixing something, but you're scared to take apart the measly laptop power-brick...

what sort of fixer are you anyway if insides of that thing doesn't interest you?

>The chance that those with two wires have any coupling between line in and dc out is vanishingly small.

hehe...
http://picasaweb.google.com/i4004b3/...34355766982978
this if from a vcr with 2 wires line hookup...you can obviosuly see the blue capacitor connecting input and output...

it doesn't really matter wether it's 2 or 3 pins prong.
except that 2 prongs are actually more dangerous, because excess voltage has no obvious way to ground like in 3 prong design...
i think those caps are essentially a part of EMI filter and anything complying to fcc must have them.
(i can provide the pics of two-prong power brick having this cap, if it's needed...)

btw. am i correct just because you tested it decades ago, or because it's the true?
<wink>

on to scenario with 2 prongs mains connector: live (phase) is connected to hot ground over those two y capacitors...so you always have half of the line voltage on the hot ground....
this hot ground is connected to cold ground via mentioned capacitor that sits between primary and secondary of transformer...
now you have half the line voltage on any metal part of laptop that is connected to cold ground....just like with plainbill's old receiver...

so what is protecting us from this half-line voltage in 2 prong devices? (with 3 prong devices we are protected by the ground connection in the wall socket! any excessive voltage goes to ground directly)
well, mainly that floors are mostly pretty well isolated from ground, and that devices(laptops, vcrs, tvs etc.) are well insulated with their plastics...we don't get to touch their metal parts...

untill we take them apart.

as bonez is so damn clumsy and dense that he can't understand any of this (<wink>) he REALLY should buy the isolation trafo krank linked, and use it to plug device under test...[b]AND he should buy another one(to connect laptop to)[b], in case he's bare-foot in the stone-floor shop and is doing everything he can to touch some metal part of the laptop...hehe

so that's my recommendation to him, and one with which any safety-aware forum member will agree: buy 2 isolation trafos, one for device under measurement, and one for measuring instrument, in this case laptop u're using.

it makes sense(i just explained why laptop can be dangerous) and hey, fins use it!

"A general recommended paractice for electronics device repair shops in Finland is to use at least two mains isolation transformers in their repair environent. The first one is an isolation transformer whcih isolated all the test equipments (like oscolloscope) from the mains ground and the other one is the one used for isolating the equipment which is being tested or repaired. This arrangement is very good on both safety and noise reasons. "

with such setup, i bet even this car-mechanic will succeed in not being electrocuted while scoping something!

**PCBONEZ** · 05-09-2009, 12:06 AM

Re: Can Power Brick serve as Isolation Transformer?

i4004
As before, I didn't bother to read it.

- - Thank you so much for being a complete ASS.

I am no longer subscribed to this thread.

Can Power Brick serve as Isolation Transformer?

Can Power Brick serve as Isolation Transformer?

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