Freaky Amazing DMM?!

P

Paul

Guest
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
 
"Paul" <energymover@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.
Click to expand...
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.
Click to expand...
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.
 
Paul wrote:
Anyhow, what kind of circuit are they using in this AM-240? It
appears as if it *resists* change!
Click to expand...
It's almost certainly a bootstrapped input amplifier, with the
bootstrap gain slightly over unity.

Clifford Heath.
 
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.
Click to expand...

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
 
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.
Click to expand...
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
 
On Thu, 15 Jan 2009 13:46:48 -0800 (PST), Paul <energymover@gmail.com>
wrote:

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
Click to expand...
What's it's input capacitance?

RL
 
"Paul" <energymover@gmail.com> wrote in message
news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
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.
Click to expand...

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?
Click to expand...
No, it's a good thing, it's like that by design.
Yes, normal meters have a 10Mohm resistor on the input. One ones with "HI-Z"
mode remove this resistor and rely just on the input impedance of the FET
gate and other circuitry which is there. This value varies a *lot* which is
why they typically don't specify it, they just call it "high impedance"
mode. E.g. Fluke do not specify the value on their 87 meter, not even a
minimum (BTW, hold the Hz button when you power-up to get this mode).

When you need this mode, the input impedance can never be high enough! e.g.
when measuring very high impedance circuitry (you can buy Gohm range
resistors for example). Actually, even "normal impedance" stuff causes a
problem with a 10Mohm input. e.g. you can start seeing errors creep in
measuring say >10Kohm stuff.

The cheap Protek 506 & 608 are other meters that have this (not selectable)
on the mV range. They spec it at simply >1Gohm.

Dave.
 
"Paul" <energymover@gmail.com> wrote in message
news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
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.
Click to expand...

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.
Click to expand...
The Agilent U1253A also has it. Spec is ">1Gohm".
There are quite a few meters I've seen over the years that have it too.
I think I even saw it on one of those $10 disposable meters too.

Dave.
 
On Thu, 15 Jan 2009 13:46:48 -0800 (PST), Paul <energymover@gmail.com>
wrote:

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
Click to expand...
Most likely an R-C circuit (maybe 10M and 1uF) going into a CMOS
analog circuit with almost no leakage typically (at room temperature).
 
"Paul" <energymover@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.
Click to expand...
I doubt it, just a high impedance CMOS input circuit (along with the usual
protection stuff).

Dave.
 
"Paul" <energymover@gmail.com> wrote in message
news:e522f235-fe26-4a21-b838-8341fe53f4ff@41g2000yqf.googlegroups.com...
On Jan 15, 3:12 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
On Jan 15, 2:19 pm, "David L. Jones" <altz...@gmail.com> wrote:

The Agilent U1253A also has it. Spec is ">1Gohm".
There are quite a few meters I've seen over the years that have it too.
I think I even saw it on one of those $10 disposable meters too.

Dave.

I think the Agilent U1253A typically lists for $450. That's a bit more
expensive then the $40 AM240.
Click to expand...
Hardly comparing apples and oranges!
If you are purely after "the cheapest meter that has a high impedance mV
range" then that's a different ball game.

Do you have model # for the $10 one?
Click to expand...
No, sorry. These things come and go like the wind.

Dave.
 
On Jan 15, 2:59 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
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?

No, it's a good thing, it's like that by design.
Yes, normal meters have a 10Mohm resistor on the input. One ones with "HI-Z"
mode remove this resistor and rely just on the input impedance of the FET
gate and other circuitry which is there. This value varies a *lot* which is
why they typically don't specify it, they just call it "high impedance"
mode. E.g. Fluke do not specify the value on their 87 meter, not even a
minimum (BTW, hold the Hz button when you power-up to get this mode).

When you need this mode, the input impedance can never be high enough! e.g.
when measuring very high impedance circuitry (you can buy Gohm range
resistors for example). Actually, even "normal impedance" stuff causes a
problem with a 10Mohm input. e.g. you can start seeing errors creep in
measuring say >10Kohm stuff.

The cheap Protek 506 & 608 are other meters that have this (not selectable)
on the mV range. They spec it at simply >1Gohm.

Dave.
Click to expand...

I'll check out that Fluke. BTW, one thing of significance with my test
is that the AM-240 was maintaining the charge on the 4.7uF Mylar! IOW,
if I disconnected the AM-240, then the Mylar slowly discharge at a
rate equivalent to 5.25Gohms, but when the AM-240 was connected, then
the 4.7uF Mylar hardly discharge. Actually, it discharged, but at such
a slow rate, equivalent to 14Gohms.

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.

Paul
 
On Jan 15, 3:12 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
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.

The Agilent U1253A also has it. Spec is ">1Gohm".
There are quite a few meters I've seen over the years that have it too.
I think I even saw it on one of those $10 disposable meters too.

Dave.
Click to expand...

I think the Agilent U1253A typically lists for $450. That's a bit more
expensive then the $40 AM240. Do you have model # for the $10 one?

PL
 
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.
Click to expand...
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
 
On Jan 15, 2:37 pm, Paul <energymo...@gmail.com> 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.

Paul
Click to expand...

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

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

Thanks Clifford.

Paul
 
On Jan 15, 2:54 pm, legg <l...@nospam.magma.ca> wrote:
On Thu, 15 Jan 2009 13:46:48 -0800 (PST), Paul <energymo...@gmail.com
wrote:



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

What's it's input capacitance?

RL
Click to expand...

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
 
On Jan 15, 2:59 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message

news:7f239414-f99c-48ad-90e2-c9b0d0db2424@l39g2000yqn.googlegroups.com...
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?

No, it's a good thing, it's like that by design.
Yes, normal meters have a 10Mohm resistor on the input. One ones with "HI-Z"
mode remove this resistor and rely just on the input impedance of the FET
gate and other circuitry which is there. This value varies a *lot* which is
why they typically don't specify it, they just call it "high impedance"
mode. E.g. Fluke do not specify the value on their 87 meter, not even a
minimum (BTW, hold the Hz button when you power-up to get this mode).

When you need this mode, the input impedance can never be high enough! e.g.
when measuring very high impedance circuitry (you can buy Gohm range
resistors for example). Actually, even "normal impedance" stuff causes a
problem with a 10Mohm input. e.g. you can start seeing errors creep in
measuring say >10Kohm stuff.
[snip]
Click to expand...


That's true, but once you have the kinks worked out it's not too
difficult to measure sources with megaohm impedance. As you know, you
have to use low bias current meters or circuits, like an electrometer
with femto bias current. If say the meter has 10pA, and the DUT
impedance is 100Mohms, then that's 10pA * 100Mohms = 1mV caused just
from the meter itself. If the electrometer is 10fA, then it drops to
1uV. Then you have thermoelectric effects unless you use a balanced
circuit such as an instrumentation op-amp. After a while of working
out the kinks, you can get very stable predictable measurements on
high impedance sources.

You probably know all of that. It can take some time to work out the
kinks, but such low signal high impedance sources is usually no big
deal for a EE who's spent a lot of years working in such a field.

Paul
 
On Jan 15, 3:29 pm, "David L. Jones" <altz...@gmail.com> wrote:
"Paul" <energymo...@gmail.com> wrote in message
Do you have model # for the $10 one?

No, sorry. These things come and go like the wind.
Click to expand...

I have a few $3 DMM's from Harbor freight, but they're 2Mohm impedance
in the 200mV setting.

PL
 
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
Click to expand...
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
 
On Fri, 16 Jan 2009 00:04:57 GMT, James Arthur <bogusabdsqy@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
Click to expand...

A capacitor with flying leads can grab loose ions out of the air, and
will hold them. It takes a while, but it can register.

ALL larger sized HV caps usually are shipped, stored, and handled with
shorted leads.
 
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