Ohmmeters and capacitors -- funny readings?

L

larrymoencurly

Guest
I measured a good 4700uF, 50V electrolytic capacitor with some
ohmmeters but got really different readings. With no-name digital and
a Fluke 83, the resistance reading went up to only about 3000 ohms,
but with an analog Radio Shack (50K/volt)it went full scale on x1
through x1K, or up to about 1M-2M, but on the x10K scale it went up to
only about 3000 ohms, just as the digital meters did. My analog meter
uses a 1.5V battery for x1 but a 9V for x10 and up.

Also, why do both of those digital meters sometimes read negative ohms
with 100K and higher resistors that are out of circuit and touched
only with alligator clips?
 
You are not supposed to check caps with an ohm meter! For the small
ceramic or similar caps, you can use an ohm meter to see if it is shorting
or not. If it reads open, you will know that it is not conducting DC, and
this is a good start.

Most digital ohm meters use DC pulse sampling to read resistance. The older
analog type meters use a steady DC from the battery, and therefore will
usually give a reading that decays over time until the cap is charged. The
current source from the meter may not be strong enough to saturate the
larger caps, therefore the meter will settle at a reading of some type.
Using an ohm meter to test capacitors is not a very reliable test.

To test caps for their capacity you should use a dedicated capacitor meter,
or a meter that has a capacitor scale. To read for proper conductance of a
cap during its operation an ESR meter is required. The cap may give a
reading of the proper amount of uF, but its impedance under electrical
stress may not be good. Basically the ESR meter will test the cap to see if
it can handle proper loading for its design. I have seen many caps give a
proper reading for their capacity, but fail the ESR test. This is a common
problem that can fool a technician during the troubleshooting process. The
only other good test is to try a new capacitor in the circuit.

A capacitor is a reactive device, and responds to an AC or pulsed power
source. In practice, the amount of its impedance or reaction can be complex
depending on the capacitor value, the frequency applied, and the structure
(content) of the waveform. In base design, the caps are referenced to a low
distortion sinewave at a chosen frequency to use as a reference. Then the
complex scenarios of design are taken in to consideration from that point
on.

A capacitor is basically not a DC device. It is not a resistor, and should
not be regarded as such, even though that under some circumstances to the
experimenter, it can exhibit some characteristics of such. This is where
many home experimenters get confused with this issue.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"larrymoencurly" <larrymoencurly@my-deja.com> wrote in message
news:755e968a.0401030142.c5337d2@posting.google.com...
I measured a good 4700uF, 50V electrolytic capacitor with some
ohmmeters but got really different readings. With no-name digital and
a Fluke 83, the resistance reading went up to only about 3000 ohms,
but with an analog Radio Shack (50K/volt)it went full scale on x1
through x1K, or up to about 1M-2M, but on the x10K scale it went up to
only about 3000 ohms, just as the digital meters did. My analog meter
uses a 1.5V battery for x1 but a 9V for x10 and up.

Also, why do both of those digital meters sometimes read negative ohms
with 100K and higher resistors that are out of circuit and touched
only with alligator clips?
 
larrymoencurly:
What you described doe not sound strange or funny...... different meters
will act differently from each other when trying to "measure" capacitors....
analog meters will certainly be different because the meter movement damping
differences......
frankly, this kind of testing is only good for determining if a capacitor is
leaky or shorted....
if you are "measuring" a lot of capacitors you would be wiser to purchase
and ESR meter and a DMM or dedicated meter with a "capacity" range.... that
way you could determine if the capacitor has low ESR to function properly in
a circuit and you can also determine the capacity value.... good for
matching or for determining the value of unmarked capacitors.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
-------------------------


"larrymoencurly" <larrymoencurly@my-deja.com> wrote in message
news:755e968a.0401030142.c5337d2@posting.google.com...
I measured a good 4700uF, 50V electrolytic capacitor with some
ohmmeters but got really different readings. With no-name digital and
a Fluke 83, the resistance reading went up to only about 3000 ohms,
but with an analog Radio Shack (50K/volt)it went full scale on x1
through x1K, or up to about 1M-2M, but on the x10K scale it went up to
only about 3000 ohms, just as the digital meters did. My analog meter
uses a 1.5V battery for x1 but a 9V for x10 and up.

Also, why do both of those digital meters sometimes read negative ohms
with 100K and higher resistors that are out of circuit and touched
only with alligator clips?
Click to expand...
 
"larrymoencurly" bravely wrote to "All" (03 Jan 04 01:42:01)
--- on the heady topic of "Ohmmeters and capacitors -- funny readings?"

You need to identify the polarity of the voltage comming out of the test
leads when the analog meter is set to "ohms". This will often be the
reverse of what the leads are usually used to measure. What I think is
happening is that you are applying a reverse voltage to the electro and
it breaks down at a certain voltage thus resulting in a fixed 3,000 ohms
reading. The x1 range applies a much larger test current. If your DMM
are reading negative values on high value resistors, just a guess but
that may be due to stray hum pickup.


la> From: larrymoencurly@my-deja.com (larrymoencurly)

la> I measured a good 4700uF, 50V electrolytic capacitor with some
la> ohmmeters but got really different readings. With no-name digital and
la> a Fluke 83, the resistance reading went up to only about 3000 ohms,
la> but with an analog Radio Shack (50K/volt)it went full scale on x1
la> through x1K, or up to about 1M-2M, but on the x10K scale it went up to
la> only about 3000 ohms, just as the digital meters did. My analog meter
la> uses a 1.5V battery for x1 but a 9V for x10 and up.

la> Also, why do both of those digital meters sometimes read negative ohms
la> with 100K and higher resistors that are out of circuit and touched
la> only with alligator clips?

.... I said, E.T., call OHM; but he resisted.
 
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote:

... I said, E.T., call OHM; but he resisted.
Click to expand...
I wonder watts wrong with him? He should conduct himself better than
that.
 
"Bob Parker" <bobp@bluebottle.com> wrote in message
news:p4hevv06vg6le3dtb2a7ecnjo02gdn1k45@4ax.com...
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote:

... I said, E.T., call OHM; but he resisted.

I wonder watts wrong with him? He should conduct himself better than
that.
Click to expand...
Everybody becomes a smart-ass on the internet...

Leonard Caillouet
 
Happy New Year to you too.

Bob


"Leonard Caillouet" <lcailloNOSPAM@devoynet.com> wrote:
Everybody becomes a smart-ass on the internet...

Leonard Caillouet
Click to expand...
 
Jerry is right about measuring ohms of a cap with a digital meter. Hook it
up to a scope if you have one and look at the waveform. You can check for
leakage using a 9v battery and the ma or ua range of your DMM. A good cap
will read less than 1 ma and down in the micro range if really good.
"larrymoencurly" <larrymoencurly@my-deja.com> wrote in message
news:755e968a.0401030142.c5337d2@posting.google.com...
I measured a good 4700uF, 50V electrolytic capacitor with some
ohmmeters but got really different readings. With no-name digital and
a Fluke 83, the resistance reading went up to only about 3000 ohms,
but with an analog Radio Shack (50K/volt)it went full scale on x1
through x1K, or up to about 1M-2M, but on the x10K scale it went up to
only about 3000 ohms, just as the digital meters did. My analog meter
uses a 1.5V battery for x1 but a 9V for x10 and up.

Also, why do both of those digital meters sometimes read negative ohms
with 100K and higher resistors that are out of circuit and touched
only with alligator clips?
Click to expand...
 
Careful Bob, or you'll get zapped...don't blow a fuse!

John :-#)#

On Sun, 04 Jan 2004 09:40:27 +1100, Bob Parker <bobp@bluebottle.com>
wrote:

"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote:

... I said, E.T., call OHM; but he resisted.

I wonder watts wrong with him? He should conduct himself better than
that.
Click to expand...
(Please post followups or tech enquires to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
 
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