9 Jun
2009
9 Jun
'09
1:21 p.m.
David writes:
? Anyone here use an Extech multimeter? This thing seems
? to get confused with certain capacitors. In particular,
? a new 10uF electrolytic register randomly from 1 to 8
? nF whereas a 1-year-old 1uF electrolytic registers fine.
I never trust autoranging multimeters. What's happening is that it is applying a
charging current, seeing a dV/dt response that's in a different range, trying to
switch to a different range, applying a different charging current, seeing dV/dt in some
other range, switching to a different charging current, etc.
The textbook definition of capacitance I = C dV/dt works great in a truly linear world but
old electrolytics have leakage, soakage, nonlinearities, and other things messing up the
textbook definition.
Soakage is always fun. Take a big electrolytic, short it out and remove the short, let it
sit for a while on the bench, then put a hi-impedance voltmeter on it. It will not read
0V.
Tim.
9 Jun
9 Jun
1:48 p.m.
On Jun 9, 2009, at 1:21 PM, Shoppa, Tim wrote:
Soakage is always fun. Take a big electrolytic, short
it out and
remove the short, let it sit for a while on the bench, then put a
hi-impedance voltmeter on it. It will not read 0V.
I've run into this! Except one doesn't need a high-impedance
meter for it...I've seen them somehow recover enough charge to
produce a spark when re-shorted. It's very odd.
-Dave
--
Dave McGuire
Port Charlotte, FL
2:26 p.m.
On 9 Jun 2009 at 13:48, Dave McGuire wrote:
I've run into this! Except one doesn't
need a high-impedance
meter for it...I've seen them somehow recover enough charge to
produce a spark when re-shorted. It's very odd.
Anyone who has done a lot of work on CRTs has experienced this.
Disconnect the anode lead, short the anode cap, come back in a few
minutes only to get another spark.
--Chuck
2:34 p.m.
On Jun 9, 2009, at 2:26 PM, Chuck Guzis wrote:
Yes. I used to work on TVs a lot when I was a kid; I'd dig dead
ones out of dumpsters and (sometimes) fix and sell them. I got two
or three good jolts from CRTs before I figured out what was going
on. I never did quite understand how it happened, though...do you
know the physics behind it?
-Dave
--
Dave McGuire
Port Charlotte, FL
I've
run into this! Except one doesn't need a high-impedance
meter for it...I've seen them somehow recover enough charge to
produce a spark when re-shorted. It's very odd.
Anyone who has done a lot of work on CRTs has experienced this.
Disconnect the anode lead, short the anode cap, come back in a few
minutes only to get another spark. 
5:09 p.m.
Dave McGuire wrote:
Yes. I used to work on TVs a lot when I was a kid;
I'd dig dead
ones out of dumpsters and (sometimes) fix and sell them. I got two or
three good jolts from CRTs before I figured out what was going on. I
never did quite understand how it happened, though...do you know the
physics behind it?
I have always thought that there was some residual charge in the
body of
the capacitor. It would take some time for all of it to migrate to the
electrodes after the main charge on the electrodes was first removed. I
assume the time it takes to fully discharge a large capacitor is the
reason we are told to hold a computer's power button down for 15 or 30
sec before opening it up to work on it.
--
Dick Hadsell 203-992-6320 Fax: 203-992-6001
Reply-to: hadsell at blueskystudios.com
Blue Sky Studios http://www.blueskystudios.com
1 American Lane, Greenwich, CT 06831-2560
6:14 p.m.
Richard Hadsell wrote:
I
assume the time it takes to fully discharge a large capacitor is the
reason we are told to hold a computer's power button down for 15 or 30
sec before opening it up to work on it.
See, I don't get the point of that.
Nearly every latching push-on/push-off mains power switch I've seen will hold
in the "on" position if you press it in and hold it while it's in the
'on' state.
There's even less point doing this to a rocker or toggle switch...
The only time I can see any point to it is when you're dealing with a fairly
recent PC -- holding down the power switch on an ATX system forces the power
off. You've still got DC 5V running through the components though (via the 5V
standby power circuit). I wouldn't touch one of these without pulling the IEC
plug out of the back of the power supply (though /technically/ it's a
wire-mounted socket -- sockets generally have holes, plugs have pins).
Of course, it's entirely possible (and highly likely) that I've got the wrong
end of the stick here.
That said, my Standard Operating Procedure of finding the end of the mains
cable and pulling it out of the wall before removing the case screws has
served me well for long enough.
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/
6:37 p.m.
Dave McGuire wrote:
On Jun 9, 2009, at 2:26 PM, Chuck Guzis wrote:
Yes. I used to work on TVs a lot when I was a kid; I'd dig dead
ones out of dumpsters and (sometimes) fix and sell them. I got two
or three good jolts from CRTs before I figured out what was going
on. I never did quite understand how it happened, though...do you
know the physics behind it?
I don't claim to have a full physics understanding of the issue but
I think a rough hand-waving explanation is that with the capacitor charged and
the dielectric under potential stress, some charge creeps into and is held by
the dielectric itself. It takes time for the charge to creep into the
dielectric. Once the potential is removed by shorting/loading the cap, the
dielectric charge will redistribute itself to reflect the new potential state,
i.e. creep back out, but this too takes time. If the discharge path has been
removed the charge will accumulate on the plates.
Search for "dielectric absorption", or look up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
If my understanding is correct, the phenomenom will show up if the dielectric
is a material, as it involves the atoms of the material, but will not if the
dielectric is a vacuum.
I've run into this! Except one doesn't need a high-impedance
meter for it...I've seen them somehow recover enough charge to
produce a spark when re-shorted. It's very odd.
Anyone who has done a lot of work on CRTs has experienced this.
Disconnect the anode lead, short the anode cap, come back in a few
minutes only to get another spark. 
8:40 p.m.
From: chd_1 at nktelco.net
Brent Hilpert wrote:
Hi
There are actually two factors. Dielectric absorption is
one of them. The other is induced charge.
Others have described dielectric absorption.
Induced charge works like this. When the internal part
is discharged, it induces a charge on all the dust and
surfaces that are not covered with conductive paint.
These get a negative charge on them from doing this discharge.
Over time, this outside charge leaks to the conductive
surfaces, reinducing a positive back onto the internal
surface.
The two are factors. I've found that the induced charge
is a major factor. While holding the short, wiping all the
external surfaces reduces the charge that builds by quite
a bit.
Dwight
_________________________________________________________________
Insert movie times and more without leaving Hotmail?.
http://windowslive.com/Tutorial/Hotmail/QuickAdd?ocid=TXT_TAGLM_WL_HM_Tutor…
Search for "dielectric absorption", or
look up "permittivity" in Wikipedia to
get an idea of just how complex capacitors are (!)
Bob Pease called this soakage.
-chuck
10 Jun
10 Jun
12:01 p.m.
On Jun 9, 2009, at 6:37 PM, Brent Hilpert wrote:
I don't claim to have a full physics understanding
of the issue but
I think a rough hand-waving explanation is that with the capacitor
charged and
the dielectric under potential stress, some charge creeps into and
is held by
the dielectric itself. It takes time for the charge to creep into the
dielectric. Once the potential is removed by shorting/loading the
cap, the
dielectric charge will redistribute itself to reflect the new
potential state,
i.e. creep back out, but this too takes time. If the discharge path
has been
removed the charge will accumulate on the plates.
Interesting. And bizarre.
Search for "dielectric absorption", or look
up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
Yes, I'm aware of their complexity...From my childhood days I
thought they were simple, but in recent years I've studied them
pretty closely. I've been dabbling in metrology for a few years now
and have become amazed at the complexity behind the "simple" things
that are the underpinnings of all things electronic. It's
fascinating. Anyone interested in electronics would do well to
investigate this a bit.
-Dave
--
Dave McGuire
Port Charlotte, FL
3:32 p.m.
Dave McGuire wrote:
.. we tend to coast along with 'adequate' models for day to day efforts.
Having an appreciation for the underlying principles helps
when things don't work as expected, amongst other benefits.
--
OT: Too easily distracted from things I should be doing, here are
two ballpark approaches to determining the capacitance of a cloud:
- by cloud and earth as (perfect) plates:
Given a cloud with a base of 2 km diameter, situated 3km above the ground,
(p is permittivity of air), then:
C = pA/d = 8.9e-12 * 3.14*(2000/2)^2 / 3000
= 0.009 uF
- by energy content:
If a lightning strike delivers 30 KWh of energy at 200 million volts, then:
E = 1/2 CV^2
C = 2E / V^2
= 2 * 30,000 Jh/S * 3600 S/h / (2e8 J/C)^2
= 5.4e-9 C^2/J
= 0.005 uF
E & V parameters were just some numbers picked off web sites about lightning.
Interesting they came out with a rough equivalence.
Going by such web sites, there isn't really that much energy in a lightning
strike (it's just very high power as it is released so quickly.)
Search for
"dielectric absorption", or look up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
Yes, I'm aware of their complexity...From my childhood days I
thought they were simple, but in recent years I've studied them
pretty closely. I've been dabbling in metrology for a few years now
and have become amazed at the complexity behind the "simple" things
that are the underpinnings of all things electronic. It's
fascinating. Anyone interested in electronics would do well to
investigate this a bit.
11 Jun
11 Jun
6:18 p.m.
Dave McGuire wrote:
On Jun 9, 2009, at 6:37 PM, Brent Hilpert wrote:
I'm a NIST certified calibration technician. The amount of dorking
around that is done to get around the effects of metal junctions,
including all solder joints, and the effects of temperature and
humidity, is amazing. Getting great accuracy is surprisingly difficult.
Warren
Search for "dielectric absorption", or
look up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
Yes, I'm aware of their complexity...From my childhood days I
thought they were simple, but in recent years I've studied them pretty
closely. I've been dabbling in metrology for a few years now and have
become amazed at the complexity behind the "simple" things that are
the underpinnings of all things electronic. It's fascinating. Anyone
interested in electronics would do well to investigate this a bit.
7:26 p.m.
On Jun 11, 2009, at 6:18 PM, Warren Wolfe wrote:
I'm a NIST certified calibration technician.
Umm, whoa! Can I send you some GR standards and my Thomas one-
ohm?? 8-)
Search for "dielectric absorption", or look
up "permittivity" in
Wikipedia to
get an idea of just how complex capacitors are (!)
Yes, I'm aware of their complexity...From my childhood days I
thought they were simple, but in recent years I've studied them
pretty closely. I've been dabbling in metrology for a few years
now and have become amazed at the complexity behind the "simple"
things that are the underpinnings of all things electronic. It's
fascinating. Anyone interested in electronics would do well to
investigate this a bit. The amount of dorking around that is done to get
around the
effects of metal junctions, including all solder joints, and the
effects of temperature and humidity, is amazing. Getting great
accuracy is surprisingly difficult.
Yes it really is. I fight with thermals, in particular, all the
time. The copper vs. copper oxide thing drives me nuts; I'm about to
buy stock in Caig.
-Dave
--
Dave McGuire
Port Charlotte, FL
12 Jun
12 Jun
7:12 a.m.
Dave McGuire wrote:
On Jun 11, 2009, at 6:18 PM, Warren Wolfe wrote:
Sure. I'll slap my Simpson 260-6P multimeter on them, and see if
they're about right. <Grin> I don't have access to a cal lab at this
point, Dave...
I'm a NIST certified calibration technician.
Umm, whoa! Can I send you some GR standards and my Thomas one-ohm??
8-) The amount of dorking around that is done to get
around the effects
of metal junctions, including all solder joints, and the effects of
temperature and humidity, is amazing. Getting great accuracy is
surprisingly difficult.
Yes it really is. I fight with thermals, in particular, all the
time. The copper vs. copper oxide thing drives me nuts; I'm about to
buy stock in Caig.
People were torn over the use of some of the Caig solvents at the lab.
The buggers won't tell you what it is, which makes people nervous about
using them. I don't know what GR standards you mean, but the Thomas
one-ohm probably cost a couple grand, and they're the LOW end of
standards. People get skittish easily.
Warren
9:49 a.m.
----------------------------------------
Date: Fri, 12 Jun 2009 01:12:56 -1000
From: lists at databasics.us
To:
Subject: Re: bizarre capacitance
Dave McGuire wrote:
Hi
I've always used DC#4 with good results but I'm told that
salt air is a bad mix with silicon grease.
Dwight
_________________________________________________________________
Windows Live? SkyDrive?: Get 25 GB of free online storage.
http://windowslive.com/online/skydrive?ocid=TXT_TAGLM_WL_SD_25GB_062009
On Jun 11, 2009, at 6:18 PM, Warren Wolfe wrote:
Sure. I'll slap my Simpson 260-6P multimeter on them, and see if
they're about right. I don't have access to a cal lab at this
point, Dave...
I'm a NIST certified calibration technician.
Umm, whoa! Can I send you some GR standards and my Thomas one-ohm??
8-) The amount of dorking around that is done to get
around the effects
of metal junctions, including all solder joints, and the effects of
temperature and humidity, is amazing. Getting great accuracy is
surprisingly difficult.
Yes it really is. I fight with thermals, in particular, all the
time. The copper vs. copper oxide thing drives me nuts; I'm about to
buy stock in Caig.
People were torn over the use of some of the Caig solvents at the lab.
The buggers won't tell you what it is, which makes people nervous about
using them. I don't know what GR standards you mean, but the Thomas
one-ohm probably cost a couple grand, and they're the LOW end of
standards. People get skittish easily.
Warren
3:42 p.m.
On Jun 12, 2009, at 7:12 AM, Warren Wolfe wrote:
Sure. I'll slap my Simpson 260-6P multimeter on them, and see if
they're about right. <Grin> I don't have access to a cal lab at
this point, Dave...
Hmm, I apparently missed the word "certified" above, and thought
you worked for NIST. Oh well. :)
I was thinking "Fluke" and typed "GR". My DC standard is a Fluke
732A. I'll have a Josephson standard eventually, but not anytime
soon. ;) The Caig stuff is pretty well regarded by the (few) people
I've talked to, and in the papers I've read. I have had good results
with it here.
-Dave
--
Dave McGuire
Port Charlotte, FL
I'm a NIST certified calibration technician.
Umm, whoa! Can I send you some GR standards and my Thomas one-
ohm?? 8-) The amount
of dorking around that is done to get around the
effects of metal junctions, including all solder joints, and the
effects of temperature and humidity, is amazing. Getting great
accuracy is surprisingly difficult.
Yes it really is. I fight with thermals, in particular, all the
time. The copper vs. copper oxide thing drives me nuts; I'm about
to buy stock in Caig.
People were torn over the use of some of the Caig solvents at the
lab. The buggers won't tell you what it is, which makes people
nervous about using them. I don't know what GR standards you mean,
but the Thomas one-ohm probably cost a couple grand, and they're
the LOW end of standards. People get skittish easily. 
9 Jun
9 Jun
4:38 p.m.
On Tue, 9 Jun 2009, Dave McGuire wrote:
On Jun 9, 2009, at 1:21 PM, Shoppa, Tim wrote:
Isn't that why the Right Thing to do with big capacitors is to store them
shorted?
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
Soakage is always fun. Take a big electrolytic,
short it out and remove the
short, let it sit for a while on the bench, then put a hi-impedance
voltmeter on it. It will not read 0V.
I've run into this! Except one doesn't need a high-impedance meter for
it...I've seen them somehow recover enough charge to produce a spark when
re-shorted. It's very odd. 
5514
days inactive
5517
days old
17 comments
11 participants
participants (11)
-
cclist＠sydex.com -
chd_1＠nktelco.net -
classiccmp＠philpem.me.uk -
dgriffi＠cs.csubak.edu -
dkelvey＠hotmail.com -
hadsell＠blueskystudios.com -
hilpert＠cs.ubc.ca -
lists＠databasics.us -
mcguire＠neurotica.com -
tshoppa＠wmata.com -
wdonzelli＠gmail.com