Freaky Amazing DMM?!

[Paul]

On Jan 15, 3:27 pm, Paul <energymo...@gmail.com> wrote:

On Jan 15, 3:22 pm, "David L. Jones" <altz...@gmail.com> wrote:

"Paul" <energymo...@gmail.com> wrote in message

news:41bfbacc-6f1e-4b44-8aff-

So, what shocked me was that AM-240 help the Mylar retain it's charge.
Polarity didn't matter, which rules out bias current or voltage
offset. I guess it's a bootstrapping circuit.

I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).

Dave.

How could it nearly stop the Mylar from discharging. When the meter is
disconnected from the Mylar, then nothing is connected to the Mylar,
and it discharges at a rate equivalent to 5Gohms. So even if you
connect a DMM that has infinite impedance, it's not going to make the
Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
the Mylars charge. I'm still thinking about this, lol.

PL

Another test. I just connected the AM240 (while in 400.0mV mode) to my
Keithley. It measured no bias current. The Keithley resolution is
10pA. So that's cool. It has 14Gohm impedance, and who knows how much
bias current, definitely less than 10pA.

Paul

 

[Eeyore]

Paul wrote:

On Jan 15, 2:26 pm, Clifford Heath <n...@spam.please.net> wrote:
Paul wrote:
Anyhow, what kind of circuit are they using in this AM-240? It
appears as if it *resists* change!

It's almost certainly a bootstrapped input amplifier, with the
bootstrap gain slightly over unity.

Clifford Heath.

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

 

[Max65]

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

 

[Archimedes' Lever]

On Fri, 16 Jan 2009 03:23:32 -0800 (PST), Max65 <mporzio@tele2.it> wrote:

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

Whenever I read very small voltages I make sure that my meter leads are
twisted.

Twisted pairs on a source and return has amazing noise abatement
capacity.

 

[Max65]

I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo

 

[krw]

In article <e0fcb090-c377-4d8c-92a3-
7e10641717bf@w1g2000prm.googlegroups.com>, energymover@gmail.com
says...>

On Jan 16, 5:35 am, Max65 <mpor...@tele2.it> wrote:
I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo


If the cap is left out in strong electromagnetic fields such as caused
by local radio stations or wifi etc. then it's possible for the cap to
slowly charge. These are issues a researcher learns to overcome. I use
two layers of metal shielding (large, and small) where one is non-
magnetic (for high frequency shielding) and the other is magnetic (for
low frequency shielding). Almost all of the time the Mylar cap will
*not* charge even when left outside by itself, and I live smak dead in
Los Angeles, California. Although it's common to see
electrolytic charge by themselves when left out. Try placing the
electrolytic inside a heavy shielded area (faraday cage) and one may
see a different result. Although it's possible for electrolytics to
charge even inside a shielded area for obvious reasons.

Regards,
Paul

 

[Guy Macon]

Paul wrote:

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. ;-)

If so, that's easy to fix. Just use a Hex Inverter. A 74HC04
should do the job.

I hope this helps...

--
Guy Macon
<http://www.GuyMacon.com/>

 

[GregS]

In article <e0fcb090-c377-4d8c-92a3-7e10641717bf@w1g2000prm.googlegroups.com>, Paul <energymover@gmail.com> wrote:

On Jan 16, 5:35=A0am, Max65 <mpor...@tele2.it> wrote:
I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo


If the cap is left out in strong electromagnetic fields such as caused
by local radio stations or wifi etc. then it's possible for the cap to
slowly charge. These are issues a researcher learns to overcome. I use
two layers of metal shielding (large, and small) where one is non-
magnetic (for high frequency shielding) and the other is magnetic (for
low frequency shielding). Almost all of the time the Mylar cap will
*not* charge even when left outside by itself, and I live smak dead in
Los Angeles, California. Although it's common to see
electrolytic charge by themselves when left out. Try placing the
electrolytic inside a heavy shielded area (faraday cage) and one may
see a different result. Although it's possible for electrolytics to
charge even inside a shielded area for obvious reasons.

In a radio hobby magazine of years back, there someone gave details how to make
a gravity detector. More of a gravity detector of other worlds, planets, etc.
I gave it a try, seems like it was some kind of capacitor integrator
circuit. So I built it and hooked it up to a stripchart. What
I got totally surprised me. I got symetrical waveforms of periods
of minutes or hours. Maybe the damm thing worked.
The theory of the article suggested when objects lined up, they produced
varying fields at the detection point. I went no further, but should have measured
a full day.


greg

 

[GregS]

In article <gkqmpq$c54$2@usenet01.srv.cis.pitt.edu>, zekfrivo@zekfrivolous.com (GregS) wrote:

In article <e0fcb090-c377-4d8c-92a3-7e10641717bf@w1g2000prm.googlegroups.com>,
Paul <energymover@gmail.com> wrote:
On Jan 16, 5:35=A0am, Max65 <mpor...@tele2.it> wrote:
I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo


If the cap is left out in strong electromagnetic fields such as caused
by local radio stations or wifi etc. then it's possible for the cap to
slowly charge. These are issues a researcher learns to overcome. I use
two layers of metal shielding (large, and small) where one is non-
magnetic (for high frequency shielding) and the other is magnetic (for
low frequency shielding). Almost all of the time the Mylar cap will
*not* charge even when left outside by itself, and I live smak dead in
Los Angeles, California. Although it's common to see
electrolytic charge by themselves when left out. Try placing the
electrolytic inside a heavy shielded area (faraday cage) and one may
see a different result. Although it's possible for electrolytics to
charge even inside a shielded area for obvious reasons.


In a radio hobby magazine of years back, there someone gave details how to make
a gravity detector. More of a gravity detector of other worlds, planets, etc.
I gave it a try, seems like it was some kind of capacitor integrator
circuit. So I built it and hooked it up to a stripchart. What
I got totally surprised me. I got symetrical waveforms of periods
of minutes or hours. Maybe the damm thing worked.
The theory of the article suggested when objects lined up, they produced
varying fields at the detection point. I went no further, but should have
measured
a full day.

Gee I think I found it.....

http://amasci.com/freenrg/grav3.html

greg

 

[Paul]

On Jan 16, 5:35 am, Max65 <mpor...@tele2.it> wrote:

I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo

If the cap is left out in strong electromagnetic fields such as caused
by local radio stations or wifi etc. then it's possible for the cap to
slowly charge. These are issues a researcher learns to overcome. I use
two layers of metal shielding (large, and small) where one is non-
magnetic (for high frequency shielding) and the other is magnetic (for
low frequency shielding). Almost all of the time the Mylar cap will
*not* charge even when left outside by itself, and I live smak dead in
Los Angeles, California. Although it's common to see
electrolytic charge by themselves when left out. Try placing the
electrolytic inside a heavy shielded area (faraday cage) and one may
see a different result. Although it's possible for electrolytics to
charge even inside a shielded area for obvious reasons.

Regards,
Paul

 

[Paul]

On Jan 16, 3:23 am, Max65 <mpor...@tele2.it> wrote:

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

 

[Paul]

On Jan 15, 4:04 pm, James Arthur <bogusabd...@verizon.net> wrote:

Paul wrote:
On Jan 15, 3:22 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:41bfbacc-6f1e-4b44-8aff-

So, what shocked me was that AM-240 help the Mylar retain it's charge..
Polarity didn't matter, which rules out bias current or voltage
offset. I guess it's a bootstrapping circuit.
I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).

Dave.

How could it nearly stop the Mylar from discharging. When the meter is
disconnected from the Mylar, then nothing is connected to the Mylar,
and it discharges at a rate equivalent to 5Gohms. So even if you
connect a DMM that has infinite impedance, it's not going to make the
Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
the Mylars charge. I'm still thinking about this, lol.

PL

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

 

[James Arthur]

Paul wrote:

On Jan 15, 4:04 pm, James Arthur <bogusabd...@verizon.net> wrote:
Paul wrote:
On Jan 15, 3:22 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message
news:41bfbacc-6f1e-4b44-8aff-
So, what shocked me was that AM-240 help the Mylar retain it's charge.
Polarity didn't matter, which rules out bias current or voltage
offset. I guess it's a bootstrapping circuit.
I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).
Dave.
How could it nearly stop the Mylar from discharging. When the meter is
disconnected from the Mylar, then nothing is connected to the Mylar,
and it discharges at a rate equivalent to 5Gohms. So even if you
connect a DMM that has infinite impedance, it's not going to make the
Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
the Mylars charge. I'm still thinking about this, lol.
PL
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.


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.electronics.design/browse_thread/thread/7a433a7c2b8f072e/d75ac181536b0aa4?hl=en

http://groups.google.com/group/sci.electronics.design/browse_thread/thread/bddb0ddbcf15eef7/626be43dba1b608d?hl=en

Cheers,
James Arthur

 

[Paul]

On Jan 16, 3:47 pm, James Arthur <bogusabd...@verizon.net> wrote:

Paul wrote:
On Jan 15, 4:04 pm, James Arthur <bogusabd...@verizon.net> wrote:
Paul wrote:
On Jan 15, 3:22 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message
news:41bfbacc-6f1e-4b44-8aff-
So, what shocked me was that AM-240 help the Mylar retain it's charge.
Polarity didn't matter, which rules out bias current or voltage
offset. I guess it's a bootstrapping circuit.
I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).
Dave.
How could it nearly stop the Mylar from discharging. When the meter is
disconnected from the Mylar, then nothing is connected to the Mylar,
and it discharges at a rate equivalent to 5Gohms. So even if you
connect a DMM that has infinite impedance, it's not going to make the
Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
the Mylars charge. I'm still thinking about this, lol.
PL
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.

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.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

 

[Archimedes' Lever]

On Fri, 16 Jan 2009 05:35:11 -0800 (PST), Max65 <mporzio@tele2.it> wrote:

I know the issue you are talking about. HV capacitors charge
themselves when their leads are left free, but I'm not sure you can
state for sure that they grab the "free ions" of the air, since many
of them are not polarized capacitors, so you should answer this
question: why +ions should attach to one lead and the -ions to the
other?
Let me know your point of view about it.
Have a nice day.
Massimo

They wouldn't "attach", however, if a free ion hits a lead, it will

become stored on that node's plate. Same is true if a person charged
with an electrostatic field walks by. Generally both leads will see
equal pressure, but if one 'sees' the field and the other doesn't,
electrons can be added to that plate.

The main cause of capacitor charging up without stimulus is the memory
effect that the dielectric media has. One can discharge a freshly
charged cap to 'zero' volts. Check again in a few minutes and it will
again show some charged voltage.

 

[James Arthur]

Paul wrote:

On Jan 16, 3:47 pm, James Arthur <bogusabd...@verizon.net> wrote:
Paul wrote:
On Jan 15, 4:04 pm, James Arthur <bogusabd...@verizon.net> wrote:
Paul wrote:
On Jan 15, 3:22 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message
news:41bfbacc-6f1e-4b44-8aff-
So, what shocked me was that AM-240 help the Mylar retain it's charge.
Polarity didn't matter, which rules out bias current or voltage
offset. I guess it's a bootstrapping circuit.
I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).
Dave.
How could it nearly stop the Mylar from discharging. When the meter is
disconnected from the Mylar, then nothing is connected to the Mylar,
and it discharges at a rate equivalent to 5Gohms. So even if you
connect a DMM that has infinite impedance, it's not going to make the
Mylar discharge at a slower rate. Somehow the AM240 is *maintaining*
the Mylars charge. I'm still thinking about this, lol.
PL
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.
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.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

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

 

[Max65]

Almost all of the time the Mylar cap will
*not* charge even when left outside by itself, and I live smak dead in
Los Angeles, California.  
Hi Paul, lovely place LA, I had been there a long time ago for a

vacation, and I promised myself to be there again.
I better read your issue (I'm Italian and some time I can
misunderstand, but to be honest when I read your message yesterday I
was at office during the coffe break and I get too little time to give
a "right thought" to your question).
I'm not sure, but read this application note from Maxim about the
switched capacitors ADC.

http://www.maxim-ic.com.cn/appnotes.cfm/an_pk/1080/

If your DMM has this kind of converter, it's probable that in the low-
voltage-high-impedance mode it enters directly into it.
Figure 3 shows how it works, and the related paragraph explains it.
As you can read there, after all the caps are charged with the Vin
voltage, the ADC switches the MSB cap to Vref leaving the other to
GND, so the output logic compute the bit value just comparing the
capacitive divider with 1/2Vref.
The following less significat bit is computed the same way just
removing the previous cap (this is not explained there but it shoud be
done, otherwise it can work) and switching the next largest cap to
Vref.
The Dummy cap is needed to get the LSB.
It's my opinion that the caps maybe charged someways during the
conversion by the Vref applied to the divider depending on the
dispersion characteristic of their values. This could explain the
behaviour of you DMM. Don't you?
Have a nice day in LA.
Massimo

 

[Arlowe]

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.

 

[Lostgallifreyan]

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? 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.

 

[krw]

On Sun, 18 Jan 2009 19:35:00 +1100, Arlowe <bare.arsed@gmail.com>
wrote:

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.

A craftsman never blames tools for his failures. Hackers, on the

other hand...