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Freaky Amazing DMM?!

P

Paul

Jan 1, 1970
0
I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
100Mohm impedance in 400.0mV mode. So I charged a 4.7uF Mylar
capacitor to 36mV DC, and then placed the AM-240 (while in 400mV DC
mode) across the Mylar cap. After 25 minutes and 40 seconds it was
35..2mV. That comes to 14Gohms. So I thought it may be due to bias
current or offset voltage, and reversed the DMM polarity. Same
results. So then I charged the Mylar to 200mV. Same results.

Then, I measured the parallel resistance of my 4.7uF Mylar cap by
charging it to 184.8mV, disconnected the AM-240, and 1050 seconds
later connected the AM-240 and measured 177.1mV. That comes to
5.25Gohms, which is what I would expect from this capacitor. Actually,
for months I've been telling people my guesstimate for this cap is
5Gohms.

Anyhow, what kind of circuit are they using in this AM-240? It
appears as if it *resists* change! When it is disconnected it tends
to somewhat maintain the DC voltage, regardless of polarity. IOW, lets
say it's measuring the DC voltage on the Mylar cap, and it's 180mV.
Then one of the leads is removed. The AM-240 DC voltage decreases a
bit, not too much, but it slows down, and tends to hang around, say
160mV. If I reverse the polarity, to -180mV, the same thing happens
except it hangs around at -160mV. If the AM-240 was measuring say
35mV, and then disconnected, it tends to hand around at oh 20mV to
30mV.

Very interesting DMM. Not sure to like or dislike this.

Thanks for any info.
Paul
 
D

David L. Jones

Jan 1, 1970
0
Paul said:
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.
Anyhow, what kind of circuit are they using in this AM-240? It
appears as if it *resists* change! When it is disconnected it tends
to somewhat maintain the DC voltage, regardless of polarity. IOW, lets
say it's measuring the DC voltage on the Mylar cap, and it's 180mV.
Then one of the leads is removed. The AM-240 DC voltage decreases a
bit, not too much, but it slows down, and tends to hang around, say
160mV. If I reverse the polarity, to -180mV, the same thing happens
except it hangs around at -160mV. If the AM-240 was measuring say
35mV, and then disconnected, it tends to hand around at oh 20mV to
30mV.

The input impedance is so high that the small input capacitance maintains a
charge when you disconnect.
A bit disconcerting when your meter doesn't read zero, but it comes good
when you connect a source.

Dave.
 
P

Paul

Jan 1, 1970
0
Thanks Clifford!  I think you nailed it.  Anyhow, this is a first for
me, and a pleasant surprise to learn of this.

I have no affiliation with Amprobe, but as far as inexpensive ($40)
DMM's go, this one seems like a gem. I was going to take it back today
at Frys Electronics for the PM51A because it claims 1Gohm impedance,
while the AM-240 only says > 100Mohms. Hmmm, 14G is far greater than
100M, lol. I'll keep it.

Paul


BTW, here's a bootstrapped input amp circuit -->

http://web.telia.com/~u43200663/blocks/bootstrapping.htm

Thanks Clifford.

Paul
 
P

Paul

Jan 1, 1970
0
What's it's input capacitance?

RL


I don't see it listed on the spec sheet, but I've seen a lot of other
similar Amprobe meters that are around 30pF input.

Paul
 
E

Eeyore

Jan 1, 1970
0
Paul said:
Thanks Clifford! I think you nailed it. Anyhow, this is a first for
me, and a pleasant surprise to learn of this.

I have no affiliation with Amprobe, but as far as inexpensive ($40)
DMM's go, this one seems like a gem. I was going to take it back today
at Frys Electronics for the PM51A because it claims 1Gohm impedance,
while the AM-240 only says > 100Mohms. Hmmm, 14G is far greater than
100M, lol. I'll keep it.

Looks like you bought a good 'un ! There are some bargains out there if
you search.

Graham
 
M

Max65

Jan 1, 1970
0
Probably the input of the AM240 has a non linear devices (maybe just
the input protection diodes).
If I'm right, when you connect a capacitor the non linear device works
like a RF detector that charges the capacitor to a voltage lesser than
its forward voltage level. The multimeter wires work as the antenna in
this case.
Try to twist the multimeter wires together and see what happen.

Have fun.
Massimo
 
P

Paul

Jan 1, 1970
0
Probably the input of the AM240 has a non linear devices (maybe just
the input protection diodes).
If I'm right, when you connect a capacitor the non linear device works
like a RF detector that charges the capacitor to a voltage lesser than
its forward voltage level. The multimeter wires work as the antenna in
this case.
Try to twist the multimeter wires together and see what happen.

Have fun.
Massimo


The AM240 is not charging the cap per say. So far it has merely tried
to *maintain* the caps charge. So far, regardless of the caps charge,
or its polarity, the AM240 has tried to maintain the charge.

I've never seen a DMM do this before. Maybe it's possessed. ;-)

PL
 
P

Paul

Jan 1, 1970
0
You're underestimating the insulating power of mylar.

Try soaking the capacitor good and full, then letting it sit,
leads in the air, without the meter.  Then, after a good
long time, measure.

Then you'll see if the meter's been charging the cap or
not.  Probably not.

Cheers,
James Arthur


Yeah, Mylars are good, but even they are imperfect. I've spent
probably too much time testing for dielectric absorption over the
years. Although Mylars have hardly no dielectric absorption. I guess
there are caps better than Mylars in terms of dielectric absorption.
An air gap cap, but how big would it have to be to make 4.7uF?

PL
 
J

James Arthur

Jan 1, 1970
0
Paul said:
Yeah, Mylars are good, but even they are imperfect. I've spent
probably too much time testing for dielectric absorption over the
years. Although Mylars have hardly no dielectric absorption. I guess
there are caps better than Mylars in terms of dielectric absorption.
An air gap cap, but how big would it have to be to make 4.7uF?

PL

Sure, mylars leak, and the meter has some bias current too,
however small it might be. If you hit the magic voltage
the two might even cancel.

Tom Bruhns posted some remarkable polypropylene cap leakage
measurements to SED a few years back. Garden-variety 0.1uF
caps had 50-year time constants.

http://groups.google.com/group/sci....hread/7a433a7c2b8f072e/d75ac181536b0aa4?hl=en

http://groups.google.com/group/sci....hread/bddb0ddbcf15eef7/626be43dba1b608d?hl=en

Cheers,
James Arthur
 
P

Paul

Jan 1, 1970
0
Sure, mylars leak, and the meter has some bias current too,
however small it might be.  If you hit the magic voltage
the two might even cancel.

Tom Bruhns posted some remarkable polypropylene cap leakage
measurements to SED a few years back.  Garden-variety 0.1uF
caps had 50-year time constants.

http://groups.google.com/group/sci.electronics.design/browse_thread/t...

http://groups.google.com/group/sci.electronics.design/browse_thread/t...

Cheers,
James Arthur


In the first link he mentions 50 years for a 0.1uF cap. That comes to
1.6e+16 ohms! Here's the datasheet of my Mylar capacitor or very
similar-- don't know the manufacturer of my 4.7uF Mylar -->
http://www.panasonic.com/industrial/components/pdf/abd0000ce23.pdf

The spec shows an insulation resistance of >=10 Gohms (20 °C, 100 VDC,
60 s), and >=2 Gohms (20 °C, 500 VDC, 60 s).

Insulation resistance -->
http://www.murata.com/cap/faq/faq03.pdf

I guess it's possible to make a cap with 1.6e+16 ohms, but I would
tend to first believe it's due to either dielectric absorption or
small signal rectification of electromagnetic signals. As you know,
any two atoms forms a junction. There are a lot of impurities in
capacitors, thus forming diodes, albeit poor diodes. It's not really
possible to have all of the poor diodes counter act each other out,
which is probably why a cap, even a good cap can produce a DC voltage
in a good electromagnetic field. In the post he says that he did not
place the experiment in a shield. From my experience that's normally
unnecessary, unless you're near a wifi or radio station, but you never
know. It's difficult to say, but my best guess, and it's just a guess,
that he was seeing dielectric absorption, and perhaps a bit of DC
voltage produced by rectifying electromagnetic signals.

My quick and dirty test of my 4.7uF Mylar showed 5.25Gohms parallel
resistance, but I didn't spend much time logging the data. It's
probably a lot higher given time to relax. It's possible the
insulation resistance would have increased over time.

Paul
 
J

James Arthur

Jan 1, 1970
0
Paul said:
In the first link he mentions 50 years for a 0.1uF cap. That comes to
1.6e+16 ohms! Here's the datasheet of my Mylar capacitor or very
similar-- don't know the manufacturer of my 4.7uF Mylar -->
http://www.panasonic.com/industrial/components/pdf/abd0000ce23.pdf

The spec shows an insulation resistance of >=10 Gohms (20 °C, 100 VDC,
60 s), and >=2 Gohms (20 °C, 500 VDC, 60 s).

Insulation resistance -->
http://www.murata.com/cap/faq/faq03.pdf

I guess it's possible to make a cap with 1.6e+16 ohms, but I would
tend to first believe it's due to either dielectric absorption or
small signal rectification of electromagnetic signals. As you know,
any two atoms forms a junction. There are a lot of impurities in
capacitors, thus forming diodes, albeit poor diodes. It's not really
possible to have all of the poor diodes counter act each other out,
which is probably why a cap, even a good cap can produce a DC voltage
in a good electromagnetic field. In the post he says that he did not
place the experiment in a shield. From my experience that's normally
unnecessary, unless you're near a wifi or radio station, but you never
know. It's difficult to say, but my best guess, and it's just a guess,
that he was seeing dielectric absorption, and perhaps a bit of DC
voltage produced by rectifying electromagnetic signals.

My quick and dirty test of my 4.7uF Mylar showed 5.25Gohms parallel
resistance, but I didn't spend much time logging the data. It's
probably a lot higher given time to relax. It's possible the
insulation resistance would have increased over time.

Paul


I'd say clean the cap carefully with alcohol, soak it
at voltage for a few days, then measure leakage.

But if you think it's getting charge from the air, a
shielded box would easily answer the question.

Meanwhile, either way you got a nice deal on a DMM
with a very high input impedance.

Ain't nuthin' wrong with that.


Cheers,
James Arthur
 
K

krw

Jan 1, 1970
0
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.
A craftsman never blames tools for his failures. Hackers, on the
other hand...
 
A

Archimedes' Lever

Jan 1, 1970
0
A craftsman never blames tools for his failures. Hackers, on the
other hand...


Apples and oranges.

"A craftsman" works the medium he ends up selling directly and
irrecoverably with the tool or tools involved so his 'toolwork' is his
work product, and one can never blame the tool for the finished product,
only the tool pusher.

"A hacker" doesn't use a 'tool', they hack. When they fail, it is
because they didn't hack long enough or correctly enough. They *ARE* the
tool that failed, and ALL failures are their fault.
 
K

krw

Jan 1, 1970
0
krw explained on 19/01/2009 :

If you work with electricity you had better know the limitations of
your tools or you will find them...the hard way.

That is certainly true (though perhaps your heirs are the ones who
will find you), but doesn't modify my statement. In this case, the
tool *can* be used as long as the one using it knows what he's
doing.
 
K

krw

Jan 1, 1970
0
krw used his keyboard to write :

Ok....everything seems to pass right over your head....

No, your incompetence didn't pass by anyone.
I pointed out a limitation of a DMM and you seem to be inferring that I
am somehow a hack who blames his tools...

It is not a limitation of the tool, only the user. Lower the
impedance if it's too high.
BTW> I am not a "craftsman" I am an electrcian..

I'll take your word that you don't do "craftsman like" work. You
clearly don't know how.
You don't do what I do for as many years as I have without knowing what
the **** you are doing.

*I* know what I'm doing. You've clearly demonstrated that you
don't.
 
R

Rich Grise

Jan 1, 1970
0
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.

Then pick the right voltmeter for the job, duh.

Hope This Helps!
Rich
 
Y

YD

Jan 1, 1970
0
krw used his keyboard to write :

Ok....everything seems to pass right over your head....
I pointed out a limitation of a DMM and you seem to be inferring that I
am somehow a hack who blames his tools...
BTW> I am not a "craftsman" I am an electrcian..
You don't do what I do for as many years as I have without knowing what
the **** you are doing.

What's wrong with using a hi Z voltmeter in a live panel? I do it more
often than I really care for, and never have a problem.

- YD.

--
File corruption detected. Select option:
1 - Call the cops
2 - Call the press
3 - Bribe it

Remove HAT if replying by mail.
 
D

David L. Jones

Jan 1, 1970
0
Late at night, by candle light, Arlowe <[email protected]> penned
this immortal opus:





What's wrong with using a hi Z voltmeter in a live panel?

It's not the right tool for the job.
A meter which gives you a false reading with bad (or no) contact is a
BAD idea in dangerous high voltage/high energy circuits.
That is why proper meters with low impedance (Kohms) modes exist for
the job, e.g. Fluke 113, 110 series, 289 etc.
I do it more often than I really care for, and never have a problem.

Oh dear...

Dave.
 
K

krw

Jan 1, 1970
0
What's wrong with using a hi Z voltmeter in a live panel? I do it more
often than I really care for, and never have a problem.

There isn't anything wrong with it. Analog meters are dead. High
impedance digital meters are only a problem if there is a loose nut
inside the panel, holding the leads.
 
D

David L. Jones

Jan 1, 1970
0
There isn't anything wrong with it. Analog meters are dead. High
impedance digital meters are only a problem if there is a loose nut
inside the panel, holding the leads.

Then I suppose Fluke and others make all those purpose designed Low-Z
electrical meters for loose nuts then?

Dave.
 
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