Author Topic: DMM LEads and Resistance  (Read 435 times)

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Offline DW1961

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DMM LEads and Resistance
« on: July 27, 2020, 02:13:15 am »
If touch the leads together on continuity and they read .002-.005 does that mean the leads are introducing resistance? When I replace them with others, I get 000 all the time. SO I know it must be the leads.
 

Offline MosherIV

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Re: DMM LEads and Resistance
« Reply #1 on: July 27, 2020, 07:42:48 am »
All wires and connectors introduce resistance. It may be very small (milli ohms).

Some DMM deliberatly show 0\$\Omega\$ when less than a certain value, to fool you into thinking the leads are 0\$\Omega\$

Decent DMMs show exactly what they are measuring.
Most of my DMMs show between 0.01\$\Omega\$ and 0.06\$\Omega\$ which is normal.
(fyi I have 15 DMM so I should know)
 

Offline coromonadalix

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Re: DMM LEads and Resistance
« Reply #2 on: July 27, 2020, 10:42:31 am »
On some meters you can remove any introduced leads impedance / resistance with the REL  function, but there is a limit to the value who can be nulled, depend on the meter brand and capability
 

Offline DW1961

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Re: DMM LEads and Resistance
« Reply #3 on: July 27, 2020, 05:59:54 pm »
Thanks guys. Much appreciated.
 

Online graybeard

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Re: DMM LEads and Resistance
« Reply #4 on: July 27, 2020, 10:05:11 pm »
It is a combination of the lead resistance and the contact resistance.  There is a contact resistance between the banana plugs and sockets on the meter as well as the point where you are toughing the leads together.  There is also contact resistance between the leads and what ever you are trying to measure.

The contact resistance increases with surface oxidation.  Sometimes just a few mono layers can screw up low resistance measurements even though the surface looks good to your eyes.

That is why for low resistance measurements is is best to use a Kelvin or 4-point method.   The force leads force a fixed current and the sense leads read the voltage.  Since the voltage leads draw no current the is no effect from thier contact resistance.  However there can (and usually is) some voltage offset due the contact potential.  So long as the current force leads can overcome whatever series resistance there is between them, this method is very reliable for low value ohms measurements.

If I want accurate measurements below a few tens of Ohms I use a Kevin measurement when possible. 
 
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Offline DW1961

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Re: DMM LEads and Resistance
« Reply #5 on: July 27, 2020, 10:39:55 pm »
It is a combination of the lead resistance and the contact resistance.  There is a contact resistance between the banana plugs and sockets on the meter as well as the point where you are toughing the leads together.  There is also contact resistance between the leads and what ever you are trying to measure.

The contact resistance increases with surface oxidation.  Sometimes just a few mono layers can screw up low resistance measurements even though the surface looks good to your eyes.

That is why for low resistance measurements is is best to use a Kelvin or 4-point method.   The force leads force a fixed current and the sense leads read the voltage.  Since the voltage leads draw no current the is no effect from thier contact resistance.  However there can (and usually is) some voltage offset due the contact potential.  So long as the current force leads can overcome whatever series resistance there is between them, this method is very reliable for low value ohms measurements.

If I want accurate measurements below a few tens of Ohms I use a Kevin measurement when possible.

For sure and it was only 001 to 002 so that's really nothing. The same meter read  the same on both leads and the other meter read the same on both leads, one being 000 and one being 002-003 so that's just a diff. in the meters. It's good enough I think. (Unless you're working for NASA.)
 

Online cdev

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Re: DMM LEads and Resistance
« Reply #6 on: July 27, 2020, 11:06:46 pm »
I can think of several RF settings where getting rid of what might seem like unimportant, low levels of resistance and current leakage makes a huge difference. Loop antennas and low noise amplifiers.
"What the large print giveth, the small print taketh away."
 

Offline DW1961

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Re: DMM LEads and Resistance
« Reply #7 on: July 27, 2020, 11:49:18 pm »
I can think of several RF settings where getting rid of what might seem like unimportant, low levels of resistance and current leakage makes a huge difference. Loop antennas and low noise amplifiers.

Yep RF for sure. That's a good point.
 

Offline wizard69

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Re: DMM LEads and Resistance
« Reply #8 on: July 29, 2020, 02:15:09 am »
It is a combination of the lead resistance and the contact resistance.  There is a contact resistance between the banana plugs and sockets on the meter as well as the point where you are toughing the leads together.  There is also contact resistance between the leads and what ever you are trying to measure.

The contact resistance increases with surface oxidation.  Sometimes just a few mono layers can screw up low resistance measurements even though the surface looks good to your eyes.

That is why for low resistance measurements is is best to use a Kelvin or 4-point method.   The force leads force a fixed current and the sense leads read the voltage.  Since the voltage leads draw no current the is no effect from thier contact resistance.  However there can (and usually is) some voltage offset due the contact potential.  So long as the current force leads can overcome whatever series resistance there is between them, this method is very reliable for low value ohms measurements.

If I want accurate measurements below a few tens of Ohms I use a Kevin measurement when possible.

For sure and it was only 001 to 002 so that's really nothing. The same meter read  the same on both leads and the other meter read the same on both leads, one being 000 and one being 002-003 so that's just a diff. in the meters. It's good enough I think. (Unless you're working for NASA.)

There is this idea, floating about, that we all need 8 digit DMM which is highly debatable.    There may be times when every digit is of value but frankly I started out in industry with a 3.5 digit meter and that was more than good enough for what I was doing.   What is important to understand is that everything has resistance unless you are working on super conductors which approach zero ohms.   The other thing is to obtaining an understanding of when all of those DMM, digits might make a difference.
 
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