audio recording on IC -help wanted

[Michael A. Terrell]

Lostgallifreyan wrote:

Arlowe wrote:

The evil thing about Voltmeters with very high impedance is they will
read induced voltages that analog meters wouldn't.
It makes a voltmeter useless for checking for live circuits in a
crowded panel.




Why? If the source has a low resistance/impedance then you're ok as soon as
you have good probe contact. If not, then you could always twist the probe
leads together a bit to try to make a balanced line to cancel induced noise.
Crude, but it would help if the signal to be read was DC or audio AC.

Capacitive coupling between live and open circuits will show close to
line voltage no matter how many twists you put into the leads.


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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in news:dfKdnZsd8vZ-
qu7UnZ2dnUVZ_h6dnZ2d@earthlink.com:

Capacitive coupling between live and open circuits will show close to
line voltage no matter how many twists you put into the leads.

Ok. I just took 'induced' to literally mean induction there. Which would
normally mean some low resistance loop, I know, but I assumed that if twisted
pairs can reduce noise into high impedance inputs, as is standard use, then a
crude emulation of that would help a bit. Got to ask, why would the
capacitative coupling not also cancel if it's affecting both leads equally?

 

[Michael A. Terrell]

Lostgallifreyan wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in news:dfKdnZsd8vZ-
qu7UnZ2dnUVZ_h6dnZ2d@earthlink.com:

Capacitive coupling between live and open circuits will show close to
line voltage no matter how many twists you put into the leads.


Ok. I just took 'induced' to literally mean induction there. Which would
normally mean some low resistance loop, I know, but I assumed that if twisted
pairs can reduce noise into high impedance inputs, as is standard use, then a
crude emulation of that would help a bit. Got to ask, why would the
capacitative coupling not also cancel if it's affecting both leads equally?

Even if the test leads were as long as the electrical circuit, it
would reduce it less than half, because there are usually bundles of AC
lines grouped together, or runs in conduit with a higher capacitance.
At that point it becomes a capacitve voltage divider.


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your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:OsmdnXmGwI2lFe7UnZ2dnUVZ_hWdnZ2d@earthlink.com:

Lostgallifreyan wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:dfKdnZsd8vZ- qu7UnZ2dnUVZ_h6dnZ2d@earthlink.com:

Capacitive coupling between live and open circuits will show close
to
line voltage no matter how many twists you put into the leads.


Ok. I just took 'induced' to literally mean induction there. Which
would normally mean some low resistance loop, I know, but I assumed
that if twisted pairs can reduce noise into high impedance inputs, as
is standard use, then a crude emulation of that would help a bit. Got
to ask, why would the capacitative coupling not also cancel if it's
affecting both leads equally?


Even if the test leads were as long as the electrical circuit, it
would reduce it less than half, because there are usually bundles of AC
lines grouped together, or runs in conduit with a higher capacitance.
At that point it becomes a capacitve voltage divider.

Ok, but then isn't the meter just telling it like it is? These are external
conditions. I was branching off from someone's assertion that the meter would
be no good for measuring voltages on a board, but in that case there's
usually a collection of interlinked and closed circuits. In a case like that
the twists would help a bit if running the two probe leads separately was
causing bother. Not that I think it's significant, if running separate causes
that much bother, something more significant needs to be done. I guess many
people don't notice because most meters aren't that high in impedance, and
they'll be measuring circuits that aren't either. I think in most circuits
I've seen that do need a high impedance point to give a signal out, it's
locally buffered to avoid this sort of problem.

 

[Lostgallifreyan]

Also, I seem to have got the original quote interpreted differently:
"It makes a voltmeter useless for checking for live circuits in a crowded
panel."
I took that to be live traces on a crowded PCB.

I agree that in a larger system of conduits and cables there's enough
coupling to be far more awkward.

 

[David L. Jones]

"Lostgallifreyan" <no-one@nowhere.net> wrote in message
news:Xns9B975D699D86zoodlewurdle@216.196.109.145...

Arlowe <bare.arsed@gmail.com> wrote in
news:mn.94977d91e2f70128.90583@gmail.com:

on 16/01/2009, Paul supposed :
On Jan 15, 2:19 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:54c2d7cf-c506-4647-b272-17d608c8854a@x8g2000yqk.googlegroups.com...

I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
100Mohm impedance in 400.0mV mode.

Nothing new there, many DMM's have selectable "high impedance" or
"HI-Z"
modes on the mV range. e.g. the Fluke 87.


I've looked at the specs of ~ 30 DMM's today, include a lot of
fluke's, and never seen anything near 14Gohms impedance. Keithley has
an electrometer that's probably higher. Most DMM's are around 10Mohms
(not gigaohms) input impedance. Don't you think 14 gigaohms is a bit
high?

PL

The evil thing about Voltmeters with very high impedance is they will
read induced voltages that analog meters wouldn't.
It makes a voltmeter useless for checking for live circuits in a
crowded panel.




Why?

Because it can give false readings when bad (or no) contact is made. That's
a BAD thing when dealing with high voltage/high energy gear.
That is why proper "electrical" DMM's have LOW input impedance modes. e.g.
Fluke 113 or 110series.

If the source has a low resistance/impedance then you're ok as soon as
you have good probe contact. If not, then you could always twist the probe
leads together a bit to try to make a balanced line to cancel induced
noise.
Crude, but it would help if the signal to be read was DC or audio AC.

Best to just get the right tool for the job.

Dave.

 

[David L. Jones]

On Jan 18, 8:10 pm, Lostgallifreyan <no-...@nowhere.net> wrote:

Arlowe <bare.ar...@gmail.com> wrote innews:mn.94977d91e2f70128.90583@gmail.com:



on 16/01/2009, Paul supposed :
On Jan 15, 2:19 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:54c2d7cf-c506-4647-b272-17d608c8854a@x8g2000yqk.googlegroups.com...

I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
100Mohm impedance in 400.0mV mode.

Nothing new there, many DMM's have selectable "high impedance" or "HI-Z"
modes on the mV range. e.g. the Fluke 87.

I've looked at the specs of ~ 30 DMM's today, include a lot of
fluke's, and never seen anything near 14Gohms impedance. Keithley has
an electrometer that's probably higher. Most DMM's are around 10Mohms
(not gigaohms) input impedance. Don't you think 14 gigaohms is a bit
high?

PL

The evil thing about Voltmeters with very high impedance is they will
read induced voltages that analog meters wouldn't.
It makes a voltmeter useless for checking for live circuits in a
crowded panel.

Why?

Because a false reading due to a bad (or no) contact can be a BAD
thing when dealing with high voltage/high energy circuits.

That's why good purpose designed "electrical" meters have a LOW input
impedance mode. e.g. Fluke 113 or other 110 series meters.

If the source has a low resistance/impedance then you're ok as soon as
you have good probe contact. If not, then you could always twist the probe
leads together a bit to try to make a balanced line to cancel induced noise.
Crude, but it would help if the signal to be read was DC or audio AC.

Much better idea to simply use the right tool for the job.

Dave.

 

[David L. Jones]

On Jan 18, 7:10 pm, Lostgallifreyan <no-...@nowhere.net> wrote:

Arlowe <bare.ar...@gmail.com> wrote innews:mn.94977d91e2f70128.90583@gmail.com:



on 16/01/2009, Paul supposed :
On Jan 15, 2:19 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:54c2d7cf-c506-4647-b272-17d608c8854a@x8g2000yqk.googlegroups.com...

I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
100Mohm impedance in 400.0mV mode.

Nothing new there, many DMM's have selectable "high impedance" or "HI-Z"
modes on the mV range. e.g. the Fluke 87.

I've looked at the specs of ~ 30 DMM's today, include a lot of
fluke's, and never seen anything near 14Gohms impedance. Keithley has
an electrometer that's probably higher. Most DMM's are around 10Mohms
(not gigaohms) input impedance. Don't you think 14 gigaohms is a bit
high?

PL

The evil thing about Voltmeters with very high impedance is they will
read induced voltages that analog meters wouldn't.
It makes a voltmeter useless for checking for live circuits in a
crowded panel.

Why?

Because if you get a reading when there is bad or no contact that can
be a BAD thing in high voltage/high energy circuits.
That's why proper meters for that job have a LOW impedance mode
(Kohms). e.g. Fluke 113, the other 110 series, and the Fluke 289 etc.

If the source has a low resistance/impedance then you're ok as soon as
you have good probe contact. If not, then you could always twist the probe
leads together a bit to try to make a balanced line to cancel induced noise.

Much better to use the right tool for the job.

Dave.

 

[krw]

In article <Xns9B9AE1E6E7B92zoodlewurdle@216.196.109.145>, no-
one@nowhere.net says...>

Daniel <nidan.danny@gmail.com> wrote in news:e8f56b53-9c0c-49cd-b6b5-
e77d8000ad67@i24g2000prf.googlegroups.com:

A craftsman never blames tools for his failures.  Hackers, on the
other hand...- Hide quoted text -

- Show quoted text -

This is an oft used but seldom understood saying. The reason a
craftsman supposedly never blames his tools is because a craftsman
tends to buy quality tools in the first place.


I thought it was to do with a craftsman knowing the limits of his tools, thus
not blaming them for over-reaching.

Not quite. The other side of the coin is that he knows he's the
one who screwed up when things don't go right. Hackers have no
clue, so blame the tools.

Related to that, I find the best definition of a toolmaker is one who
achieves greater accuracy than was provided, and makes a system to easily
repeat that gain.

 

[Spehro Pefhany]

On Wed, 21 Jan 2009 16:12:47 -0600, Lostgallifreyan
<no-one@nowhere.net> wrote:

Daniel <nidan.danny@gmail.com> wrote in news:e8f56b53-9c0c-49cd-b6b5-
e77d8000ad67@i24g2000prf.googlegroups.com:

A craftsman never blames tools for his failures.  Hackers, on the
other hand...- Hide quoted text -

- Show quoted text -

This is an oft used but seldom understood saying. The reason a
craftsman supposedly never blames his tools is because a craftsman
tends to buy quality tools in the first place.


I thought it was to do with a craftsman knowing the limits of his tools, thus
not blaming them for over-reaching.

Related to that, I find the best definition of a toolmaker is one who
achieves greater accuracy than was provided, and makes a system to easily
repeat that gain.

Indeed and engineers *are* toolmakers, in every sense of the word.

And hence the disgreement between the users of tools and the Creators
of Tools.

 

[TutAmongUs@thebarattheend]

On Wed, 21 Jan 2009 16:12:47 -0600, Lostgallifreyan <no-one@nowhere.net>
wrote:

Daniel <nidan.danny@gmail.com> wrote in news:e8f56b53-9c0c-49cd-b6b5-
e77d8000ad67@i24g2000prf.googlegroups.com:

A craftsman never blames tools for his failures.  Hackers, on the
other hand...- Hide quoted text -

- Show quoted text -

This is an oft used but seldom understood saying. The reason a
craftsman supposedly never blames his tools is because a craftsman
tends to buy quality tools in the first place.


I thought it was to do with a craftsman knowing the limits of his tools, thus
not blaming them for over-reaching.

Related to that, I find the best definition of a toolmaker is one who
achieves greater accuracy than was provided, and makes a system to easily
repeat that gain.

Cincinnati Milacron. Tolerances to one one hundred thousandth of an
inch.

Their machines used to be what made America. Everything from unibody
car frame machining to the M1A1 Turret way and gear. All the vocational
schools were filled up with old Cincinnati machines that companies parted
with as they bought newer, larger machines. America used to be about
fabricated goods. Automation did kill us. The computers that operate
automation machinery are the only jobs left for those folks.

As we globalized the world's economy, we took a hit, and it ain't over
yet. We need to close out borders except to paying customers. They treat
us as tourists when we go there. We should give them the same treatment.

Whay comes around, goes around, and we will be getting our prosperity
back eventually. Likely be after I'm long dead though.

 

[Lostgallifreyan]

Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote in
news:ddafn4dna6esjde1ltfah4vse2hhk7n4f1@4ax.com:

On Wed, 21 Jan 2009 16:12:47 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Daniel <nidan.danny@gmail.com> wrote in news:e8f56b53-9c0c-49cd-b6b5-
e77d8000ad67@i24g2000prf.googlegroups.com:

A craftsman never blames tools for his failures.  Hackers, on the
other hand...- Hide quoted text -

- Show quoted text -

This is an oft used but seldom understood saying. The reason a
craftsman supposedly never blames his tools is because a craftsman
tends to buy quality tools in the first place.


I thought it was to do with a craftsman knowing the limits of his tools,
thus not blaming them for over-reaching.

Related to that, I find the best definition of a toolmaker is one who
achieves greater accuracy than was provided, and makes a system to
easily repeat that gain.

Indeed and engineers *are* toolmakers, in every sense of the word.

And hence the disgreement between the users of tools and the Creators
of Tools.

Cool. Someone gets it. krw isn't wrong, but he said only what I already said.
An inevitable logical derivation isn't a new unmade point.

 

[Max65]

Hi guys,
why does it seem "you are writing aloud"?
There are no reasons for that: I think that we should be here just to
discuss our opinions without pretend to be the owner of the absolute
knowledge, don't you?

The original thread was concerning a problem of capacitor charge
sustainment due to the DMM high Z input.
I guessed it's due to the latest methods used to perform the analog to
digital conversion. The older one methods always used to get the input
voltage on a comparator, this should give a good certainty that no
currents flow out from the ADC input to the device connected to it.
It is my opinion, that the capacitive DAC method instead could inject
a current backward to the DUT due to the switching operations of the
inner capacitances of that ADC frontend.
I don't know whether I'm right or not, but (if I am right) we should
consider this behaviour of that kind of ADC in future applications
where Hi-Z analog to digital conversion is required.

Of course (always if I'm right), this states that this kind of DMMs
should never be used for the purpose of determine the parallel
parasitic resistance of a capacitor, because their input Z could be
"so high" to become "negative".

Massimo

 

[David L. Jones]

On Jan 22, 5:57 pm, Lostgallifreyan <no-...@nowhere.net> wrote:

"David L. Jones" <altz...@gmail.com> wrote in news:9f33ec2f-7412-4832-bcb5-
e326c0ad8...@b38g2000prf.googlegroups.com:

From my side I've only been proposing that high impedance meters can
be a problem, and the solution is using the right tool for the job.

Makes sense but do you concede that understanding the conditions matters more
than the meter?

No need to concede anything, I have never said or meant to imply
otherwise. You might be confusing me with some other poster.
I was just saying that if you need a Low-Z meter it's best to use a
meter designed for the job. It's quicker, it's safer, it's less
hassle, and it's more foolproof. But each to their own.

What matters in wiring that is capacitatively or inductively
coupled but not directly so, is that some kind of light load is placed on the
line, and the voltage measured across that load. Only when that load needs to
be a simple resistance can it be assumed that it is the meter's job to
include it. In all other cases it can be assumed that the meter should tax
the circuit as lightly as possible while analysing an external load chosen to
fit the analysis needed. Which in turn directly implies that in the hands of
someone who knows electricity, especially AC and frequency dependent
behaviour, the high impedance meter is the way to go.

You'll get no argument from me on that at all.
FYI, I can't ever remember needing a Low-Z meter myself in the last 25
years, but I can certainly appreciate those who do have a use for one
and find them very useful.

Dave.

 

[Archimedes' Lever]

On Sun, 25 Jan 2009 06:34:53 -0600, Lostgallifreyan <no-one@nowhere.net>
wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:rOWdnQ1TB4W1kOHUnZ2dnUVZ_v7inZ2d@earthlink.com:

Same applies to responses to Stewart, who is speaking from
professional experience.


My experience is a broadcast engineer, (The largest was at a 5 MW UHF
TV site.) industrial electrical work, and specialized electronics that
you'll never see, without going to the International Space Station.


And both extremly impressive too, yet apparently in disagreement.

I don't think that high an experience is needed to understand this anyway, I
learned it at 14 when an aging friend of the family taught me how to build my
own (and first) multimeter.

Ohm's law.

And a bit of awareness of insulation strength when high volts are involved.

If you're using a low resistance input you might have to take it into account
for accurate measurements but on mains, the error is small, so it's worth
keeping inputs resistance low for meters dedicated to such systems, for
reasons plenty of posts have explained, so I won't flog that horse now.

If you have strong insulation, you can probe an HV circuit without trouble,
just make sure you understand what the meter says. If a meter designed to tax
the system as lightly as possible says 83V it means 83V, the problem isn't
the meter, you just have to know enough to interpret the truth it tell you.
(Mike Terrell got this one right). If you also need to know current through
the same meter, you could do it by measuring small voltage across a part of
one conductor, then measuring resistance of that part after removing power.
Most current meters just do this internally anyway, but they 'know' the
resistance of their shunts so they calculate correctly anyway.

So the question isn't who is the most experienced, it's who is right? And
take care, because if two people with real experience start arguing over
something as basic as Ohm's law, they'll do each other's reputation harm, as
well as making it hard for newcomers to trust what they read here.

The problem is an idiot that HEARS a "fact" from someone, and goes with
it as bible whether they know the whys and wherefores or not.

That is why the dope that thinks a DMM is a bad tool to use in certain
setting is too damned sparky retard stupid to grasp the science involved.
Because some other dope told him so, and he NEVER bothered to understand
the deeper reasons as to why the declaration was made or how such a tool
could be used properly in the setting under discussion.

In other words, the arlowe disphit doesn't know the science. He is only
going on what has been related to him over the years, and he has no grasp
as to actually WHY one meter is better than another for a given test
circumstance.

 

[Michael A. Terrell]

Lostgallifreyan wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:rOWdnQ1TB4W1kOHUnZ2dnUVZ_v7inZ2d@earthlink.com:

Same applies to responses to Stewart, who is speaking from
professional experience.


My experience is a broadcast engineer, (The largest was at a 5 MW UHF
TV site.) industrial electrical work, and specialized electronics that
you'll never see, without going to the International Space Station.


And both extremly impressive too, yet apparently in disagreement.

I don't think that high an experience is needed to understand this anyway, I
learned it at 14 when an aging friend of the family taught me how to build my
own (and first) multimeter.

Some people work with electricity their entire life and never really
understand it. There are dozens of basic voltmeter types, and you need
to know enough about them to use them properly. Have you ever used a
frequency selective voltmeter? How about a field strength meter? A
true RMS voltmeter that will display a .01 dB change at up to 20 MHz?
How about a Vector Voltmeter, or a Differential Voltmeter? One of the
old HP AC VTVMs that read voltage well up into the VHF range? There are
lots of specialized meters out there.


What is wrong with pointing out that there are other, and better
tools for the job? I built a radio at eight years old, and really
didn't understand a lot about how it worked, but I did make it work. At
13 I was repairing radios, working part time in a TV shop. I went
straight from high school to working at a TV bench. I have a poorly
built Eico 1000 Ohm per Volt multimeter someone gave me. I'll redo all
the bad solder work some day, but I would never actually use it for
anything. I use a DVM for electrical work, and it is the meters from my
electronics bench. I've used it to troubleshoot things wire pullers
couldn't figure out with their meters. Instead of get a 'thank you', I
was told off. OTOH, they had spent half a day trying to decode a bundle
of wires in a crowded conduit. I identified and tagged all the wires in
less than 15 minutes.


An equivalent to the specialized voltmeter for wire pullers is the
cheap CATV field strength meter with a few LEDs to give a basic Go/No Go
test for cable TV installers. It takes 30 seconds to train someone off
the street how to use it.

Ohm's law.

And common sense.

And a bit of awareness of insulation strength when high volts are involved.

Er work with HV armored cable? You can't see the insulation, because
it is made like Heliax, with a polypropylene filled insulator. A real
bitch to cut, strip and terminate. Do it wrong and you have a
spectacular fire.

If you're using a low resistance input you might have to take it into account
for accurate measurements but on mains, the error is small, so it's worth
keeping inputs resistance low for meters dedicated to such systems, for
reasons plenty of posts have explained, so I won't flog that horse now.

Only for those who don't understand how to read a real meter. That's
most users, or they wouldn't make so many of the crappy things.

If you have strong insulation, you can probe an HV circuit without trouble,
just make sure you understand what the meter says. If a meter designed to tax
the system as lightly as possible says 83V it means 83V, the problem isn't
the meter, you just have to know enough to interpret the truth it tell you.
(Mike Terrell got this one right). If you also need to know current through
the same meter, you could do it by measuring small voltage across a part of
one conductor, then measuring resistance of that part after removing power.
Most current meters just do this internally anyway, but they 'know' the
resistance of their shunts so they calculate correctly anyway.

A clamp on meter is safer and more accurate to read the current in a
working circuit. They are fairly cheap these days, too.

So the question isn't who is the most experienced, it's who is right? And
take care, because if two people with real experience start arguing over
something as basic as Ohm's law, they'll do each other's reputation harm, as
well as making it hard for newcomers to trust what they read here.

The bigger problem is ignorant trolls who infect every thread,
spouting garbage. Unfortunately, these bottom feeders are growing in
numbers.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in news:ab-
dnX9LJ9G4suDUnZ2dnUVZ_jOdnZ2d@earthlink.com:

I've used it to troubleshoot things wire pullers
couldn't figure out with their meters. Instead of get a 'thank you', I
was told off. OTOH, they had spent half a day trying to decode a bundle
of wires in a crowded conduit. I identified and tagged all the wires in
less than 15 minutes.

Reminds me of the teacher who told me at age 7 to show my workings in long
multiplication and division. As soon as I had to do it with their formalism I
stopped being able to do it as fast. Didn't matter that I was right, didn't
matter that maybe a kid who had something good should be encouraged to
develop and explain it even though it didn't fit the model. We're lucky. One
time they'd have screamed BURN THE WITCH and done it too.

 

[Lostgallifreyan]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in
news:ab-dnX9LJ9G4suDUnZ2dnUVZ_jOdnZ2d@earthlink.com:

Ohm's law.

And common sense.

Notoriously rare, allegedly.
In something like 'phantom' voltages in unconnected lines in power
distribution, it might be considered that it goes way beyond ohm's law, as
inductance and capacitance are involved, but my 'common sense' tells me to
try to reduce to Ohm's law where possible. So for simple tracing of lines, a
light load is enough to reduce the values of indirectly coupled voltage. I'd
rather have more control of what load I choose than leave it up to whatever
is supplied in a meter, but that's just my choice. At least this way I have
the option of putting it in series with the meter and measuring current.

And a bit of awareness of insulation strength when high volts are
involved.


Er work with HV armored cable? You can't see the insulation, because
it is made like Heliax, with a polypropylene filled insulator. A real
bitch to cut, strip and terminate. Do it wrong and you have a
spectacular fire.

No, never done that. But I try not to walk blind into situations like that.
The only time I did get a copper vapour deposition of some strength across my
eyeballs was after someone else had gone in and royally blown 100A cables
apart first so the resulting mess made it very hard to figure out what the
state was, on a circuit that could not be isolated within reach. All it did
was confirm what I already knew: that my own assumptions are the best place I
can fix an error before it goes bad. (in this case I assumed that two
separate lumps of brass could not have been welded into one through a thick
thermosetting plastic layer by the actions of a guy who had shorted them with
only a momentary contact with a 2.5mm square wire. To this day I don't know
what demonic persistence he used to manage that much destruction, all I know
is it can't have been momentary, he must have been feeding it in like a
welder).

If you're using a low resistance input you might have to take it into
account for accurate measurements but on mains, the error is small, so
it's worth keeping inputs resistance low for meters dedicated to such
systems, for reasons plenty of posts have explained, so I won't flog
that horse now.


Only for those who don't understand how to read a real meter. That's
most users, or they wouldn't make so many of the crappy things.

True. Electricians do have a point though, I tested this last night with a
Fluke 79 series II. Its input resistance is high, but not the type of high
that prompted the OP to post. I knew that when you have a moderate length of
corridor, say 6 metres, with a lamp that has a switch at both ends, you have
a very standard situation where a line is unconnected in a cable when the
lamp is on. Which line it is depends on which positions the SPDT switches are
in to close the lamp circuit. I noticed that the capacitatively coupled
voltage on both lines was identical, at least, differences were lower by far
than line fluctuations. This is probably the sort of thing that angers the
electricians when someone claims a meter with high input resistance can be
used. In this case it can't, alone. But I'd still rather have a separate load
to apply, than have to use more than one meter. A 200K resistor seems about
right (for circuits up to 500V), as in this case there is only the earthed
switch box to reference to, and any more than a few milliamps would probably
trigger an ELCB somewhere...

 

[Lostgallifreyan]

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9B9F30780727Dzoodlewurdle@216.196.109.145:

the capacitatively coupled voltage on both lines was identical

Too clumsy an edit to let pass... ONE of them was directly coupled.

 

[Spurious Response]

On Fri, 30 Jan 2009 03:45:36 -0600, Lostgallifreyan <no-one@nowhere.net>
wrote:

lasting about half of a second. I'm a civilian, but that's about as
close I know to the experience of being shot at by live rounds at close
range.

Yeah, DUMBFUCK, it was SPATTER.