Let's teach Keri how to use an Oscilloscope!

[Bob Parker]

Re: Let's teach Keri how to use an Oscilloscope!

There are plenty of really smart people here who I'm sure will give you some ideas to check out.

See? I was right. This forum's full of experts.

Now I'll just sit back, watch and smile.

[PCBONEZ]

Re: Let's teach Keri how to use an Oscilloscope!

Comment as per a previous discussion.
Stingray's sample rate (the real one, not the 'created one') is adequate but borderline.
I forgotten the exact specs an I'm not looking it up tonight.
As I recall it will get 10 samples on 200k but PSU's are upping the switching frequencies lately and soon you may be losing sample points.

[Oklahoma Wolf]

Re: Let's teach Keri how to use an Oscilloscope!

Yeah, probably. A new SunMoon is higher up the priority ladder right now though

[PCBONEZ]

Re: Let's teach Keri how to use an Oscilloscope!

Ask for a Government Bailout.
[Everyone else is...]

[Gianni]

Re: Let's teach Keri how to use an Oscilloscope!

About isolator transformer I use it both at home and job only for safety reason.

At job, I have a 1000VA for PSU, I test PSU up to 300W. In this case they are fed by the trafo and they are connected also to earth.
The scope is connected directly to the outlet.
This is for safety reason because when i have to check the primary side, I can't get electric shock touching anything on my desk (computer, solder...etc).

At home I have only a 100VA isolator transformer, in this case I isolate my scope since it is a 60W IIRC.
This connection is not safe but I can avoid problem of to make short circuit on the line.
Why is not safe? Because in this way if you are testing the primary side with the scope isolated, you bring the scope potential to the point where you connect your probe.
Let's make an example: if you place your probe on the input, you have the scope at input potential (110Vac or 220Vac) and if you touch any metal parts of your scope and something which is connected to earth, you get an electric shock.
This is why when I use my scope at home I pay a great attention to not touch anything contemporary to scope/probe when I investigating live part.

About differential probe: I don't use them and only thing I know is that they are expensive. So the only thing I can say, but maybe I'm wrong, is that if you need to make a measurement on floating signals and for this you need a differential probe, probably (but I repeat I could be wrong) you can use two scope channel and only with the tip probes and using the math function of the scope you can make the measurements.

Ciao
Gianni

[KeriJane]

Re: Let's teach Keri how to use an Oscilloscope!

Thank you Gianni!


But don't Differential Probes have a sensing element(s) right at the probe tip(s) so as to prevent ringing and other cable-related woes?

I have two identical probes and an available "Math Function" so maybe I'll try that.

So far as Isolation Transformers:
I just had a thought.... My Mac Pro (980w PSU) is on an APC SmartUPS 1500 which is a pure sine wave uninterruptible power supply. It can run a loaded Mac Pro with 4 internal drives and an external RAID enclosure plus the display for about an hour on batteries. For a PSU or even an entire PC system, that runtime would be much longer.

So.... Could a "Pure Sine Wave" UPS function as an isolation transformer for the duration of battery charge?

Have Fun!
Keri

[i4004]

Re: Let's teach Keri how to use an Oscilloscope!

>Somebody can actually understand all that?
I give up
Maybe I'll go back to Terry Pratchett novels.

i just wanted to illustrate how far the sickness can go...hehe...
but they're doing it because they need to measure noise on rather hi freq. signals, yadda-yadda...blah-blah...

>You thought they were using a great scope too.

what do you mean?
you misunderstood something i said, because i know what they're using as i was reading discussions about stingray being not enough....and i saw issue of bandwidth being raised...

so no, i never said it's great, i said it'll suffice, and their site proves it does.
NOW,(note the comma)
you bring the question of the need for increased bandwidth for next generation of smps devices...and that's fair, only...that was never the issue...
let's first have such devices, and i think after that their site will adapt...they can get some of those picoscopes i usually mention when talking about usb scoping.
and you've just heard it yourslef they have "bigger" scope there against which they can test the other scope, as "small one" is handy for taking screenshots for the web.

hehe..."great scope"....you're very naughty boy!
---------------

oh yeah, special thanks to oklahoma wolf for accepting my invitation to help here.

--------------

btw. did we have any sort of consensus about the issue, or should i fire up my scope and see what am i getting?
to remind you all, the biggest difference i see is the load(gianni and bob mentioned this early on): sites use load testers, while keri is using mobo.

so keri, what about using some other load?
offcourse, that also tough, as more load=more ripple, so if you put just(say) hdd you probably don't see any ripple as load is so small...
huhm...

--------------
about safety and isolation trafo: i just don't touch any metal parts anytime i do anything with device powered.
don't have the trafo.
if i had it i would still not be touching anything...
as it happens i'm afraid of electricity in bare metal...anybody that got bitten by it should be.
<wink>

[Gianni]

Re: Let's teach Keri how to use an Oscilloscope!

Hi Keri

in this page you can find a better explanation than mine about using 2 normal probes for differential measurements.

About UPS: I really don't know, if you have the manual you can check if the output is isolated. For sure if you disconnect it from outlet and it runs only on battery, you can use it to isolate the scope or EUT but remember to attach only one thing and pay attention not to touch scope/probes/EUT and something else simultaneously.

About load: one way to build a cheap load is to use low voltage lamp, for instance car lamps, they are rated at 12V and are available from 5W up to 50W. You can connect them in parallel or series to adapt voltage or current to the PSU you want to test. The only problem is that no all PSU can support the initial load because when they are off, the impedance is very low.
Another problem is that they emit light so you have to protect your eyes.

That's all for this year, see you in 2009.

Happy New Year to all memebrs of Badcaps

[PCBONEZ]

Re: Let's teach Keri how to use an Oscilloscope!

>Somebody can actually understand all that?
I give up
Maybe I'll go back to Terry Pratchett novels.

PCBONEZ <-- Likes Terry Pratchett novels.

.

[Bob Parker]

Re: Let's teach Keri how to use an Oscilloscope!

PCBONEZ <-- Likes Terry Pratchett novels..

Wonderful. Everyone who loves Terry Pratchett novels can go to http://www.terrypratchettbooks.com/c...ltimatebb.cgi? and discuss them with all of the other Terry Pratchett fans.

[PCBONEZ]

Re: Let's teach Keri how to use an Oscilloscope!

Keri

I think your idea of using battery power by way of a legit sine wave UPS has merit as long as it isn't plugged into AC. (And thereby grounded to the building's ground.)

.

[PCBONEZ]

Re: Let's teach Keri how to use an Oscilloscope!

Thanks Bob!

Keri:
This is what I had in mind when I mentioned the ISO-Xfmr.
... Before the Pratchett hater butted in and confused everyone. [teasing]

[i4004]

Re: Let's teach Keri how to use an Oscilloscope!

>... Before the Pratchett hater butted in and confused everyone. [teasing]

teasing is fine, but this

">Somebody can actually understand all that?
I give up
Maybe I'll go back to Terry Pratchett novels. "

was me quoting keri. so nobody expressed hate to terry here.
i don't even know who he is.
(btw. bonez have you been drinking a bit too much lately?
above-line is me, and it's pure teasing.. <wink>)

the loop? yeah, i still didn't post my case about floating ground and loop etc.
i should as on my own i wasn't able to decipher it fully.
but something i saw on the scope: loop looks like 50hz sine with "overtones"(higher harmonics) that go all the way to switching frequency.
waveform looks essentially like 50hz sine(at usa should be 60) with thickness in the 'vertical' portions of the wave(those are the harmonics).

but yeah, that scoping could of been totally failed, because of all considerations mentioned here. also, el. installation here is not grounded at all, so....
either way, i think ground loop should be detectable on the scope, if it's present.
it is essentially mains frequency with garbage thrown-in at it.
so if it's that, she should be able to spot it.
(meh...i'm in xp now, and picasa is not installed here so i can't easilly locate photos i took when scoping that...later...)

oh yeah, still like your drawing...
btw., i dunno if you can point the path of the ground loop current, because it's alternating current.

[i4004]

Re: Let's teach Keri how to use an Oscilloscope!

here

http://picasaweb.google.hr/i4004b3/I...su_2008_10_19#

that's the current flowing thru the shield

[KeriJane]

Re: Let's teach Keri how to use an Oscilloscope!

Ok....

So maybe current flowing through a ground loop (Earth loop?) could account for this strange reading? (see drawing 1)

So before I try this....

Does the idea illustrated in Drawing 2 look safe?

Should I leave the separate UPS case ground attached?


Have Fun,
Keri
Guilty of invoking not only Terry Pratchett but Mary Poppins!

[i4004]

Re: Let's teach Keri how to use an Oscilloscope!

i dunno how relevant THAT test is; i mean that ground connection is bound to carry tons of noise so it's not a surpise to see it there...

rather check +5 and +12v.
well, while there you can also repeat your test, but i think you'll see same noisy thing...as every piece of noise is travelling down that ground...

yes, it looks safe, it's essenztially 0v to the scope...
0v usefull signal and many milivolts of noise...heh...

btw. could you post image of your probe. and what caps u used.

[KeriJane]

Re: Let's teach Keri how to use an Oscilloscope!

Hi i4004.

Ok, I'll go ahead and try drawing 2 sometime. (just did an emergency New Years PSU replacement for someone, so it won't be tonight)

Here's a pic of my capped probe tip. It doesn't make sense to me but it appears to function as intended.

Oh! DUH! If Oaklahoma Wolf stops in, maybe he'd like to look at my Mac Pro PSU thread with some pictures of the insides of the 980w Delta proprietary Apple PSU. It's over here:
https://www.badcaps.net/forum/showthread.php?t=6264

Have Fun,
Keri

[KeriJane]

Re: Let's teach Keri how to use an Oscilloscope!

Hi PCBONEZ...

OK, so currents flow through the scope probe alligator clip thing and possibly mess up the acquired signal.

Couldn't one just clip the scope's front panel ground to the ground of the circuit being tested? Wouldn't this shunt the ground loop current away from the probe? I did try this with no noticeable effect.

Here's a drawing of what I had tried....

Have Fun!
Keri

[i4004]

Re: Let's teach Keri how to use an Oscilloscope!

as bonez said
"I think your idea of using battery power by way of a legit sine wave UPS has merit as long as it isn't plugged into AC. (And thereby grounded to the building's ground.)"

by running it off the batteries you'll know if it's ground loop or not.
that way you'll know if you need to remove ground loop(ie if its there at all).

Wouldn't this shunt the ground loop current away from the probe? I did try this with no noticeable effect.

yes, it would.
that's one more thing that points away from the ground loop explanation that many here jumped on.

something to speak in favour of my scoping of the loop:
"Low current wiring is particularly susceptible to ground loops. If two pieces of audio equipment are plugged into different power outlets, there will often be a difference in their respective ground potentials. If a signal is passed from one to the other via an audio connection with the ground wire intact, this potential difference causes a spurious current through the cables, creating an audible buzz at the AC mains base frequency (50 or 60 Hz) and the harmonics thereof (120 Hz, 240 Hz, and so on), called mains hum."

just like i said(after seeing it on scope) ground loop is mains freq.(50 or 60hz) with higher harmonics. and did you see anything simillar to images here
http://picasaweb.google.hr/i4004b3/I...su_2008_10_19#
?
(offcourse, i didn't have any capacitors on my probe when i took those..if you indeed have loop perhaps those caps are preventing you to see it(?))

if not(if you don't see such things without caps on the probe), i don't really think you have a ground loop, but just the noisy ground, thing gianni suggested, ie not the loop bob mentioned first(did someone say
"butted in and confused everyone"....hehe).

that should be somewhat "testable": what about OTHER devices?
ie, do you get same noise only on mobo, or on EVERYTHING else hooked to that wall plug?

what do you get if you make same configuration on, say, output of some ad/dc converter? does it have same spikes?
if not, then it's the mobo making this noise(and channeling it all thru the shield, just like shield of tuner inside the tv-set that gianni mentioned).

which, rather nicely, brings us back to thing i originally said; all these sites have the particular load for a good reason.
if they were to attampt same thing, they would probably get same amount of noise as you did, because mobo ground is as noisy as hell.

---------------------
let me just show (for everybody to see) jonny's probe, ie what's it placed into..you have original pic, and pic with my description of what's what


i was about to make the thing, but then i remembered i didn't buy sunmoon tester last time i was at the supermarket...hehe...hihi....

[KeriJane]

Re: Let's teach Keri how to use an Oscilloscope!

Hi i4004

Actually it's with two different PSUs but not a third one. Also, not with an LCD TV PSU.

So... I'm inclined to think that it's noise that these two PSUs generate. One is a recapped Antec TruePower 2.0 350, the other a brand-new EarthWatts 380. The TruePower is ChannelWell Technologies built, the EarthWatts was built by Seasonic.

The third one which does not generate the massive spikes and noise is a 6 year old featherweight cheapie that I pulled from an upgrade. It says "300w" on the sticker, but it's weight says maybe 150w. On a good day, and don't sneeze! Oklahoma Wolf loves blowing this kind of PSU up.

I feel that maybe the low power of this PSU minimized the spikes, or maybe it runs stuff at different frequencies?

Have Fun!
Don't Sneeze!
Keri