Negative resistance
David Belding
Hello, can someone tell me what the significance
of a negative resistance reading is. I'm using a handheld
digital multimeter and ohming out a module that I'm testing
and at one point read a negative resistance. I believe that
this is some sort of indication of a potential problem but I
don't know what it means. Any help would be appreciated.
Dave
OneStone
David Belding wrote:
It would generally indicate that you have some voltage at these points
which is opposite polarity/greater than the DMM's test voltage.
--
Remove -xxx- from my address to respond
OneStone
OneStone wrote:
Sorry, I forgot to mention (wrongly assumed you'd know) that this
voltage may be the result of charging a capacitor, or other circuit on
the PCB. I assume that you are not doing continuity tests with an
ohmeter and power on the board.
William L. Bahn
I don't know what kind of meter you are using, but my first suspicion is
that you are making measurements on a live circuit. Most Ohmmeters work my
measuring the current (or voltage) that flows through (or is developed
across) the leads in response to a known excitation voltage (or current)
provided solely by the meter itself. If there are live sources in the
circuit being measured then this relationship won't be valid but the meter
will determine it's measurement assuming that it is - and so you can get all
kinds of nonsensical results.
David Belding wrote in message ...
jpop...@rica.net
In article <mGlR.33174$o7.22...@news.internetMCI.com>,
"David Belding" <david....@mci2000NOSPAM.com> wrote:
>
> Hello, can someone tell me what the significance
> of a negative resistance reading is. I'm using a handheld
> digital multimeter and ohming out a module that I'm testing
> and at one point read a negative resistance. I believe that
> this is some sort of indication of a potential problem but I
> don't know what it means. Any help would be appreciated.
>
> Dave
The ohm meter provides a small (usually .1 volt) current limited supply and
measures the current that flows when you connect it to an external resistance.
If the external component includes a voltage that pushes the current
backwards, then the meter will read negative resistance. Sometimes you can
find approximately the resistance in series with the external voltage by
reading the resistance in each direction (swap the leads) with the ohmmeter
scale locked and average the positive and negative result. I say sometimes,
because often, one of the readings will go off scale.
John Popelish
-----== Posted via Deja News, The Leader in Internet Discussion ==-----
http://www.dejanews.com/ Now offering spam-free web-based newsreading
sokos mark
In article <mGlR.33174$o7.22...@news.internetMCI.com>,
David Belding <david....@mci2000NOSPAM.com> wrote:
> Hello, can someone tell me what the significance
>of a negative resistance reading is. I'm using a handheld
>digital multimeter and ohming out a module that I'm testing
>and at one point read a negative resistance. I believe that
>this is some sort of indication of a potential problem but I
>don't know what it means. Any help would be appreciated.
Either you've figured out a way to defy physics, or you are doing
something wrong. You, uh, do have the power to the device off, right?
(don't be offended - this is .basics)
Ohmmeters work by applying a small voltage to the device and measuring
the current. A negative value means that something in the device is also
causing current to flow. Either the device is still getting power from
somewhere, or some energy storage device (a capacitor or inductor)
could be releasing current through the meter.
- Mark Sokos (mso...@gl.umbc.edu) Electrical engineer, computer geek (er,
programmer), no-talent bum musician, and perpetual student
http://www.gl.umbc.edu/~msokos1: alt.comp.hardware.homebuilt FAQ, ISA and
other bus info, and schematics (mostly audio).
Paul Gustafson
Hi David,
You'll get that kind of reading on a circuit that has some voltage in it
somewhere with a dmm. Dont quite know if it is same or opposite polarity
as the source voltage of your meter that causes the negative reading.
Paul G.
David Belding wrote:
> Hello, can someone tell me what the significance
> of a negative resistance reading is. I'm using a handheld
> digital multimeter and ohming out a module that I'm testing
> and at one point read a negative resistance. I believe that
> this is some sort of indication of a potential problem but I
> don't know what it means. Any help would be appreciated.
>
> Dave
David VanHorn
>> Hello, can someone tell me what the significance
>>of a negative resistance reading is. I'm using a handheld
>>digital multimeter and ohming out a module that I'm testing
>>and at one point read a negative resistance. I believe that
>>this is some sort of indication of a potential problem but I
>>don't know what it means. Any help would be appreciated.
There's a voltage coming out of the module that is interfering with the
reading.
Possibly, are you holding both leads in your hands?? That's a no-no!
Mark Kinsler
I nearly went nuts trying to measure the resistance of a soil sample with
a high-grade DC ohmmeter. Had two electrodes extending into the soil
(actually, just wet sand.) Got all kinds of crazy readings, including the
negative ones mentioned. Switched over to the DC voltage range and found
out I'd made a pretty good battery, even though the electrodes were
identical in composition.
I finally measured the resistance with a 100 volt AC source and an
ammeter.
Sheesh.
Mark Kinsler
Sand expert.
--
.............................................................................
Interpretation and instruction in physical science and technology.
Athens, Ohio, USA. http://www.frognet.net/~kinsler
Thomas L DeTogne
After reading the responses, some of them are on the right track. You
are getting a reading of voltage present in your circuit. Assuming that
you turned the power off, there is the possibility of a charged
capacitor still retaining a charge. This can come into play also if you
read a large capacitor with an ohm meter and then reverse the leads.
Alternately, the thing you're reading could generate electricity by
contact with a dissimilar metal (such as the guy measuring the soil). I
have seen a diode cause this when I metered it in direct sunlight. It
was acting like a photocell and producing electricity from the sun.
T-wizard
David Belding wrote:
>
> Hello, can someone tell me what the significance
> of a negative resistance reading is. I'm using a handheld
> digital multimeter and ohming out a module that I'm testing
> and at one point read a negative resistance. I believe that
> this is some sort of indication of a potential problem but I
> don't know what it means. Any help would be appreciated.
>
> Dave