Can Power Brick serve as Isolation Transformer?

[i4004]

Re: Can Power Brick serve as Isolation Transformer?

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

if i thought you were the only audience for this theme i would send you pm or email...
<wink>

some excellent suggestions about safety in this thread...

>I am no longer subscribed to this thread.

you never were...you were just subscribed to insulting me.
apparently subject doesn't interest you at all.

[PCBONEZ]

Re: Can Power Brick serve as Isolation Transformer?

i4004

I AM the one that asked the question.
It's quite a simple question.

Rambling on about what an Isolation Transformer is and does IS COMPLETELY ILLELEVANT to the question I asked.

I asked you to stick to the topic or go away.
- You did neither.
- You continued filling space with IRRELEVANT information.

That was nothing less than RUDE, and you are an ASS HOLE.

If you want to talk about something else go start your own thread.

.

[i4004]

Re: Can Power Brick serve as Isolation Transformer?

you're aware that your question was answered and that you got recomendation on what to do, right?

you're also aware that krankshaft(and plainbill) got it wrong when it comes to these power bricks, right?
krank didn't pay attention to caps, and plainbill thinks 2 prong layout is somehow different than 3 prong. at bricks i have and other 2 prong supplies i didn't see any difference.

but that doesn't mean krank's suggestion about what iso.trafo to buy is not valid.
it is.

lastly, to teach you what's the main purpose of iso. trafo in thread that has that verx word in its subject line is good for you.
it is also good to tell you not to expect full ground loop removal via such trafo alone, just like the link i gave explains.

let me answer your question:

>Can Power Brick serve as Isolation Transformer?

no, don't be cheap ass and buy 2 isolation trafos, one for measuring gear(laptop in your case) and another for gear under test.
further up ther theme you have all explanations on how and why is this so, incase you're ever interested.

also, i will encourage you once again to take that brick apart and inspect what's inside.
should make things much more clear than all the words we wasted on you in this thread(yes, mostly me..but what can i do, i love you.
<wink> ).

oh yeah, there's some slight chance you might prove me wrong if you open that brick and don't find the cap(who knows, perhaps layout is different in usa?).
do it just for the hell of it!
heheh...

[PCBONEZ]

Re: Can Power Brick serve as Isolation Transformer?

Not read.. I repeat..

i4004

I AM the one that asked the question.
It's quite a simple question.

Rambling on about what an Isolation Transformer is and does IS COMPLETELY ILLELEVANT to the question I asked.

I asked you to stick to the topic or go away.
- You did neither.
- You continued filling space with IRRELEVANT information.

That was nothing less than RUDE, and you are an ASS HOLE.

If you want to talk about something else go start your own thread.

[Topcat]

Re: Can Power Brick serve as Isolation Transformer?

Alright, lets all take a breather and be friends again....

[Huckfinn]

Re: Can Power Brick serve as Isolation Transformer?

PCBONEZ:

Yes, a power brick will serve as a good isolation transformer for your laptop-scope application. The secondary DC side of the brick is isolated from the AC side by the ferrite core SMPS transformer, and the feedback to the primary side "chopper" circuit is also isolated by either the simple feedback winding of a transformer, or the more elegant opto isolator feedback technique. The limiting factor in this setup is the insulation breakdown spec of the transformer. If 10 Meg ohm dual scope probes are used in differential mode [provided the PC DSO supports this mode], then some substantial additional isolation exists between the different elements of the measuring setup. This will work well for most common scoping needs. Of course, the scoping of high voltages and/or hazardous voltages requires an additional set of considerations.

Huck

[i4004]

Re: Can Power Brick serve as Isolation Transformer?

>Yes, a power brick will serve as a good isolation transformer for your laptop-scope application. The secondary DC side of the brick is isolated from the AC side by the ferrite core SMPS transformer, and the feedback to the primary side "chopper" circuit is also isolated by either the simple feedback winding of a transformer, or the more elegant opto isolator feedback technique.

nope, it is not. you know, repeating the wrong doesn't make it right:
read, re-read, and read again
https://www.badcaps.net/forum/showpo...0&postcount=14

if you have pix that show otherwise, post them.

but untill then, there IS A CAPACITOR between primary and secondary, and as a result touching something grounded (or being on bare ground) and ground plane of the laptop(if some bare metal parts are exposed), or just it's minus pole on the power jack, can provide a minor electroshock...

anytime anybody says power brick is isolation trafo i will say this and ask for proof it really is. untill proven wrong, my pictures are valid.
(or this one by scenic, for that matter
http://bambooz.pytalhost.net/omg-wtf-psu/DSC07256.JPG
blue cap next to the capacitor....)
you know, it is about your safety, and safety of measuring gear; if he wants safety, he needs isolation trafo.

also, you've just repeated what krankshaft has said.

[PCBONEZ]

Re: Can Power Brick serve as Isolation Transformer?

THANK YOU Huckfinn.
That's what I figured but by asking I don't have to tear one apart.






PCBONEZ - Member, i4004 Ignore Club.

[PlainBill]

Re: Can Power Brick serve as Isolation Transformer?

OK, this nonsense has gone on long enough!!! I've done a few TESTS and have some hard data.

The first test was done on a Dell XPi notebook. This was of interest because the power brick uses a 2 wire NON-POLARIZED AC cord. It would appear that i4004 is right - there is a capacitor between 'hot' and 'cold' sides. It would also appear i4004 is wrong. The capacitor is about .001uF, at 60 hz it's reactance is 2.5 Megohms. I will leave it up to the student to calculate the current it will pass at 60 Hz and 120VAC.

The second test was using an HP DV6645US. This uses a 3 wire power cord. As expected, the ground prong carries through the power brick to the exposed metal on the notebook.

PlainBill

p.s. About a decade ago that was a fierce debate going on in (among others) the sci.electronics.repair newsgroup. The debate was about the residential electric meters supplied by the power companies. The issue - did the measure the true wattage consumed, or did they measure the current and voltage and then derive the wattage. (This is of concern when you are dealing with highly reactive loads - like fluorescent lights, electric motors, or computer power supplies).

The debate dragged on for months and hundreds of postings. NEITHER side would perform a simple experiment. SOMETIMES it pays to actually measure what is happening. Debate without verifiable facts is all well in good in politics and religion. It is pointless in science.

PB

p.p.s. 48 microamps

pb

[PCBONEZ]

Re: Can Power Brick serve as Isolation Transformer?

Thanks. - That helps.

I was asking if someone KNEW, presumably because they'd worked on one.
I can speculate all by myself.

[PlainBill]

Re: Can Power Brick serve as Isolation Transformer?

Thanks. - That helps.

I was asking if someone KNEW, presumably because they'd worked on one.
I can speculate all by myself.

Above all, DON'T trust my measurements. Check the power brick and laptop you are going to use. I know that my (ancient) Dell P2-233 could be used. On the other hand, if the cap in yours is of a much higher value (or shorted), i4004 could have the last word.

PlainBill

[i4004]

Re: Can Power Brick serve as Isolation Transformer?

>It would appear that i4004 is right - there is a capacitor between 'hot' and 'cold' sides. It would also appear i4004 is wrong. The capacitor is about .001uF, at 60 hz it's reactance is 2.5 Megohms.

true.

>p.p.s. 48 microamps

true again. on my lenovo power brick it's about the same.
not much indeed, but it's not the
http://en.wikipedia.org/wiki/Galvanic_isolation
that isolation trafo is.
<wink>

it is still much better than pc psu that produces about 0.5mA at its metal portions incase 3rd prong is not earthed.(i guess i was counting that same cap is used in power brick as those in pc psus....)
few such psus (in such unearthed place) and current goes near 1mA(earth leakage currents of each particular smps psu add up) and you can't work with that if you're, say, holding the shield of coax. cable while installing satellite dish.... gotta disconnect it (from the mains supplied gear) to continue working...

>SOMETIMES it pays to actually measure what is happening.

true. measuring ground<->earth current early on would drastically shorten this thread...i made a mistake of looking at the voltage alone....
seeing half the line voltage i automatically figured it should have same current as pc psu(in unearthed situation), but i was wrong, as capacitor is not the same in these two places (one being link between primary and secondary, and other being cy caps in emi filter...)

oh yeah, if that cap is shorted, and we have 2 prong mains connector(or a 3 prong connector but with no connection to earth in the house), whole chassis is on half line voltage and currents that can tingle and be unpleasant...

[PCBONEZ]

Re: Can Power Brick serve as Isolation Transformer?

PCBONEZ <- Member, i4004 Ignore Club.

[i4004]

Re: Can Power Brick serve as Isolation Transformer?

yawn

[Huckfinn]

Re: Can Power Brick serve as Isolation Transformer?

I have worked on a large number of SMPS power supplies over the years, including the "power brick" variety. I have even carefully sawed apart brick style supplies for laptops that were sealed shut with glue or ultrasonically welded together [for the purpose of analyzing failure modes]. I have not yet seen a single SMPS brick style supply intended for laptops that connects the "hot" side to the "cold" side with a capacitor. Such a thing maybe out there, as per what other respondents are claiming, but I have not seen it. I have seen capacitors and/or high value [read 5 meg ohms] resistors used to provide a virtual "RF" ground reference to the "cold" side by connecting the two together with the resistor and/or capacitor in the configuration where the SMPS circuit is not in a brick, but rather included as part of the circuitry on a "main board" , as in a TV set for example. In giving the info I had based on my experience, I did not imply that I had dismantled every brick style SMPS supply known to mankind.

Huck

[Huckfinn]

Re: Can Power Brick serve as Isolation Transformer?

An additional note for the folks who are worried that a rather high breakdown voltage isolation capacitor might break down in shorted mode, leaving one with no isolation:

Your piece of equipment labeled "dedicated stand alone isolation transformer" might also break down and develop a short between its primary and secondary windings.

Heck,...fourteen isolation transformers all hooked up in one happy series string might all short out at the same time, leaving one with no isolation [not very likely, but it might happen].

Huck