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Re: An unusual Oscilloscope phenomenon


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Unless the shield slowly commences a faint red glow, followed by some
"amperage scent" wafting through the room ...

Seriously, I've seen that happen.


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Yup. OTOH that provides a good business opportunity 8-D

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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Yup. I've seen some ghastly wiring, some done by licensed
electricians.

The most 60 Hz I've experienced (between outlet ground pins, in an
office building) was a few volts RMS, and plenty stiff. Messed up some
printer signals pretty good.

The real way to pipe signals around is fiberoptics. A kilovolts of
common-mode is no problem.

Hey, time for our engineering meeting; Japanese food today!

John


Re: An unusual Oscilloscope phenomenon



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I wish... I'm working on interfacing a current sensing coil from a
welding transformer. During welding there is a couple of hundred volts
everywhere. This project starts to give me a headache :-)

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: An unusual Oscilloscope phenomenon


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Try differential transfer, shielded twisted pair or Twinax soldered to
the burden resistor, from there to your measurement box, place a 2nd
signal transformer there. Make sure the twisted pair is source- and/or
end-terminated properly, usually 100-120ohms. And never, never use this
connection as the burden resistor even if there is a fire extinguisher
at hand ...

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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I already went for a differential input. Solves most of the problems,
but not all but I'm pretty sure the inputs where overdriven as well. I
fixed that, but didn't field-test it. A 2nd signal transformer won't
fly because the pulses may be quite long (up to 100ms).

The problem is that nobody seems to know what the pulses look like
exactly. The transformer is pulsed by a 1000Hz PWM signal. This makes
me assume the current sensor sees pulses as well. But the shape
probably depends on the induction in the welding circuit while
carrying many kA...

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That might be worth something to explore. The burden resistor (1k is
specified) is now on the measurement board itself. I'd like to keep it
there though to keep the installation procedure easy. The wiring in
the test setup is pretty short (2 meters) but bad (not twisted,
unshielded). I'll order some shielded twisted pair tomorow. I already
figured this must be part of the problem.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: An unusual Oscilloscope phenomenon


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[...]

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Recipe against headache in cases like this: Proper matching of literally
everything, shielding, a glass or two of Beerenburger.


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100msec? Is it a DC welder? Anyhow, 100msec is feasible if you use a
small doorbell transformer, the kind with two compartments where you can
scrap one of them out with wire cutters and without having to
disassemble the core. Calculate it so it's well under the saturation
limits at 10Hz. Commercial current transformers can also go that low if
you use one that's totally oversized for your current.

Unless the input diff-amp has a huge common mode range even for fast
stuff such as spikes I am not sure you can do it without another
transformer at the receiving end. Or rather, I've seen cases where it
didn't work.


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If it's fed 1kHz then 1kHz should come out.


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Not good. Increases noise, big time, because the cable is mismatched
because even regular cord won't have a Z of 1000ohms. And if the cable
ever comes off ... tungggg ... phssssss ... *PHUT*


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Ook niet zo goed. In an environment like this you can't work without
shielded twisted pair, definitely not across two meters. That's like
trying to listen to a Mozart concerto on a Harley-Davidson.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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Shielding sounds doable. The Beerenburger is out of the question
though :-)

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Yup. It's a DC welder. I already tried a small mains transformer but
that doesn't work out very well in a test. Besides that would exceed
budget and space constraints.

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I think I have a pretty decent diff-amp. It can handle about 800V p-p
common mode.

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The cord coming off doesn't seem to be a problem. All similar systems
I've seen so far have the burden resistor on the measurement board,
not on the welding transformer. But I'll try it anyways. The old ways
are not necessarely the best ways :-)

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: An unusual Oscilloscope phenomenon


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Every CT I've built has had the resistor as close to the transformer (i.e.,
on it) as possible.  I've had quite awful results putting the resistor on
any kind of length.  Come to think of it, that may be more due to the
resistor itself having length, which would be a problem.  Still, better safe
than sorry...

'Course, I wind my own, encapsulated in masking tape, so embedding a burden
resistor is no big deal.  ;o)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: An unusual Oscilloscope phenomenon



Tim Williams wrote:
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 'Course, I wind my own, encapsulated in masking tape, so embedding a
burden
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--
You can't have a sense of humor, if you have no sense!

Re: An unusual Oscilloscope phenomenon


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I liked this stuff ;-)


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A doorbell transformer blows the budget on a welder? Man, and here I
thought I was uncle Scrooge.


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In a welding environment 800V may not be good enough. Plus you'd have to
check the CMRR for fast pulses, like when the arc starts and stops.


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Often they sure aren't. A burden resistor at the other end of a cable is
usually a bad idea.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon




 
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Which "welding" environments did you have in mind?

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days.



Re: An unusual Oscilloscope phenomenon


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The usual. >100 amps, electrodes touching, arcing, getting stuck once in
a while, thick cables floating around the room, ground potential of
equipment bouncing, and so on. I once told someone to get his ham radio
gear out before we start welding a broken gate. "Nah, this stuff is
tough" ... "Better get it outta here" ... "No, really, it's ok, just get
cracking on the gate" ... "Ok then" ... bzzt, bzzt, bzzzzzzzzzzt ...
beep ... *PHUT*

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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Umm, have you ever put a volt meter across the cables?
I was under the impression that that stuff matched the telephone and was
in the vicinity of 40volts.

I politely suggest that if it was around 800volts, then black and crispy
would be a welder who made the wrong circuit.

Hint, it is the current that causes the field.





--

Great advances in Debian Linux; post a bug report and get spam in three
days.








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days.



Re: An unusual Oscilloscope phenomenon



terryc wrote:
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   Its the collapsing field that generates the HV spikes that can, and
does knock careless welders on their asses.


--
You can't have a sense of humor, if you have no sense!

Re: An unusual Oscilloscope phenomenon


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Hint: There's lots of inductances involved, wanted ones as well as
undesired ones. Inductance -> current -> current suddenly stops ->
inductance says "I don't think so!" -> tsssk ... *PHUT*

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And mains connected power supplies. That's what croaked in the radio.
The spikes on a mains circuit of sufficient length (and thus impedance)
look rather terrifying.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon




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If you live anywhere near high voltage power lines, be very afraid if
they ever collapse. The induced voltages in length of cable are
interesting.


--

Great advances in Debian Linux; post a bug report and get spam in three
days.



Re: An unusual Oscilloscope phenomenon


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I remember a report and a photo where some guy installed a cable TV
hookup at a house near power lines. It cinged the whole side of the house.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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The problem is that there is an existing board. The new board must be
more or less budget neutral. I blew most of the budget on 0.1%
resistors and instrumentation amplifiers.

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Its for a resistive spot welder. There is no arc. Just tens of
kiloamperes! An arc would be very bad for the particular application.

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I hope I can get some testing on a real rig this week.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: An unusual Oscilloscope phenomenon


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Then there's cost reduction potential :-)

No way to clamp anything so you don't need all these precision parts?


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Until the spot contact lets go at a not so good moment, or one of the
panels to be welded had a bad spot there, or a cable breaks, and so on.
Never assume things to remain as perfect as they are on paper or in the
engineering lab.


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Good! I'd test for faulty conditions as well, like the ones above.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon


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Can you get decent yakisoba in SFO? I also like plain ol' ordinary
fried rice, but I can make that at home; I wish I had the recipe
that the Java Curry Shop in Shinjuku used - best curry I've ever
had! :-)

Yes, Japanese curry. :-)

Thanks!
Rich


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