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"zero out" ohms on DMM

L

Lee Carkenord

I have a Micronta DMM by Radio Shack, circa 1985.

When I short the probes together prior to resistance check, it shows
approx. resistance of .8 ohms.

The probes themselves nominally have zero resistance, determined with
another, much more sophisticated DMM.

Is there any kind of internal adjustment to make, to get shorted
probes to read ZERO ohms?

A trim cap or pot? Something to tweak..........?

Any ideas or comments?

Thank you.. Lee Carkenord
 
M

mook johnson

Not worth the trouble. Just subtract .8 ohms from the measurement and
you're there. For most measurement thats close enough. Anything more
precision than that requires a more sophisticated DVM.
 
M

Michael A. Terrell

Lee said:
I have a Micronta DMM by Radio Shack, circa 1985.

When I short the probes together prior to resistance check, it shows
approx. resistance of .8 ohms.

The probes themselves nominally have zero resistance, determined with
another, much more sophisticated DMM.

Is there any kind of internal adjustment to make, to get shorted
probes to read ZERO ohms?

A trim cap or pot? Something to tweak..........?

Any ideas or comments?

Thank you.. Lee Carkenord
Click to expand...

That is about the right resistance for the wire leads, and the
resistance between the two probe surfaces when you touch them together.
Anything you mess with in the meter will throw all the other ranges out
of spec. Just remember to do a little mental math when reading low ohm
resistance.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida
 
R

Roger Hamlett

Lee Carkenord said:
I have a Micronta DMM by Radio Shack, circa 1985.

When I short the probes together prior to resistance check, it shows
approx. resistance of .8 ohms.

The probes themselves nominally have zero resistance, determined with
another, much more sophisticated DMM.
Click to expand...
Seriously, if your 'other much more sophisticated DMM', thinks that the
leads have zero resistance, there is something wrong with _it_. Around
0.4ohm, is typical for a multimeter lead set. The Micronta unit probably has
some extra resistance internally in the sockets, and between these and the
meter sense point.
Is there any kind of internal adjustment to make, to get shorted
probes to read ZERO ohms?

A trim cap or pot? Something to tweak..........?

Any ideas or comments?
Click to expand...
Subtract 0.8ohms from the readings. It is simpler than playing around. The
odds are, that unless the meter is designed to do this (and has the ability
to subtract a value just from the 'ohm' reading), any adjustment, will
result in the opposite error being present in other readings.

Best Wishes
 
J

Jim Yanik

Seriously, if your 'other much more sophisticated DMM', thinks that
the leads have zero resistance, there is something wrong with _it_.
Around 0.4ohm, is typical for a multimeter lead set. The Micronta unit
probably has some extra resistance internally in the sockets, and
between these and the meter sense point.

Subtract 0.8ohms from the readings. It is simpler than playing around.
The odds are, that unless the meter is designed to do this (and has
the ability to subtract a value just from the 'ohm' reading), any
adjustment, will result in the opposite error being present in other
readings.

Best Wishes
Click to expand...

Better DMMs use a 4-terminal measurement mode to eliminate the lead
resistance as a factor. HP14401A is a good example.

I wonder what the guy's DMM zero reads when using a shorting bar instead of
meter leads?
 
M

Michael Black

Roger Hamlett" ([email protected]) said:
Seriously, if your 'other much more sophisticated DMM', thinks that the
leads have zero resistance, there is something wrong with _it_. Around
0.4ohm, is typical for a multimeter lead set. The Micronta unit probably has
some extra resistance internally in the sockets, and between these and the
meter sense point.

Subtract 0.8ohms from the readings. It is simpler than playing around. The
odds are, that unless the meter is designed to do this (and has the ability
to subtract a value just from the 'ohm' reading), any adjustment, will
result in the opposite error being present in other readings.

Best Wishes
Click to expand...
And of course, before we had digital meters, nobody (except those who
were dealing with very small resistances and used special equipment) ever
noticed the resistance of the leads. The scale simply couldn't show that
small a resistance.

Michael
 
J

John Woodgate

I read in sci.electronics.design that Michael Black
) about '"zero out" ohms on DMM', on Thu, 15 Jan 2004:
And of course, before we had digital meters, nobody (except those who
were dealing with very small resistances and used special equipment) ever
noticed the resistance of the leads. The scale simply couldn't show that
small a resistance.
Click to expand...

Meet the shunt ohmmeter. Home constructed multimeters often used a 1
mA/100 ohm meter. OK, put a 1.01 ohm shunt on the meter, so it reads
full-scale with 100 mA. Now connect a battery and a pot and adjust to
get full-scale reading. Now a 0.1 ohm resistor in parallel with the
shunted meter gives a reading of nearly 10% of full-scale. A resistance
of 0.005 ohms can be detected.
 
A

Al

Jim Yanik said:
Better DMMs use a 4-terminal measurement mode to eliminate the lead
resistance as a factor. HP14401A is a good example.

I wonder what the guy's DMM zero reads when using a shorting bar instead of
meter leads?
Click to expand...

Hah, I had an engineer working temporarily in my lab who tried to read
the values of 22 pF caps using 4 ft long leads. Needless to say, he
didn't last long.

Al
 
R

Robert C Monsen

Lee Carkenord said:
I have a Micronta DMM by Radio Shack, circa 1985.

When I short the probes together prior to resistance check, it shows
approx. resistance of .8 ohms.

The probes themselves nominally have zero resistance, determined with
another, much more sophisticated DMM.

Is there any kind of internal adjustment to make, to get shorted
probes to read ZERO ohms?

A trim cap or pot? Something to tweak..........?

Any ideas or comments?

Thank you.. Lee Carkenord
Click to expand...

You should have opted for the analog version; the one I bought in college
(and still appears to work!) has a compensation thumbwheel; touch the probes
together and turn the thumbwheel till it indicates 0...

Regards
Bob Monsen
 
N

normanstrong

Lee Carkenord said:
I have a Micronta DMM by Radio Shack, circa 1985.

When I short the probes together prior to resistance check, it shows
approx. resistance of .8 ohms.

The probes themselves nominally have zero resistance, determined with
another, much more sophisticated DMM.

Is there any kind of internal adjustment to make, to get shorted
probes to read ZERO ohms?

A trim cap or pot? Something to tweak..........?
Click to expand...

What does the meter read in the most sensitive DC Volts range when you
short the probes together? How about the most sensitive DC Ma range?

Norm Strong
 
L

Lee Carkenord

I wonder what the guy's DMM zero reads when using a shorting bar instead of
meter leads?
Click to expand...

It reads .7 ohms with a short fat copper shorting bar placed across the terminals.

Lee Carkenord
 
L

Lee Carkenord

What does the meter read in the most sensitive DC Volts range when you
short the probes together? How about the most sensitive DC Ma range?

Norm Strong
Click to expand...


With switch in DC VOLTS position, it has a constantly changing value
(of millivolts) displayed when leads are shorted together.

With switch in DC MA position, readout says 00.0 whether leads are
shorted together or not shorted together.

Lee Carkenord
 
N

normanstrong

Lee Carkenord said:
With switch in DC VOLTS position, it has a constantly changing value
(of millivolts) displayed when leads are shorted together.

With switch in DC MA position, readout says 00.0 whether leads are
shorted together or not shorted together.
Click to expand...

The fact that it wanders in volts and not in mA suggests that there's
some leakage at the input. I'd open the unit up and clean out around
the input divider and the input to the A/D converter.

One other question. Can you put the instrument in the 1 volt range?
If so, what does that range look like with nothing at the input?

Norm Strong
 
L

Lee Carkenord

One other question. Can you put the instrument in the 1 volt range?
If so, what does that range look like with nothing at the input?

Norm Strong
Click to expand...


Thank you for your response.

Its an economy-type digital multimeter, with auto-ranging. It has
switch positions for AC volts, and for DC volts, but there is no
switch position for "1 volt".

Lee Carkenord
 
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