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DMM Test Lead Resistance
Attorney:
Caveat emptor...
This issue has likely been raised in the past, but thought I would pass along a set of observations made when trying various brands of DMM test leads. Last week, a set of what looked like really nice test leads showed up. The outer quality seemed fine, much like the Fluke branded leads. But upon applying a shorted lead test on my Fluke 45 and Keithley 2100 DMMs, the new leads show 0.31 ohms. Next, I tried my old Fluke 8060A leads. They read 0.15 ohms. Finally, the Unicable leads that came with the Keithley displayed the lowest resistance of 0.08 ohms. The outer diameter of all leads is the same. Lead length is approximately the same. Both DMMs show nearly identical readings (within one count on both DMms). The tests were repeated multiple times to ensure I had maxium tip conduction.
I have not cracked open the new test leads but my guess is that the wire gauge is quite bit smaller than the other sets. As a practical matter for most use, I doubt this would be of concern except when taking high current measurements - something I don't often do with a DMM. In those cases, I use precision 1%, 0.1 ohm series resistors and carefully observe voltage drop across a known value.
Anyone have standard length DMM test leads that measure even higher than mine?
Paul
alm:
Test lead resistance will also introduce an error when measuring low value resistors, like the 0.1 ohm resistor you mention. You can either compensate for this with the relative/null function or use four wire connections (either using a DMM with support for four wire measurements or using a current source and DMM). The relative option is the fastest, but has the disadvantage that lead resistance will often change slightly depending on contact pressure.
Co6aka:
FWIW, a while back I made myself a set of low resistance leads with some of that 12-gauge silicone-insulated super-flexible red-n-black interconnect-wire. (Now THAT's a lotta hyphenated-words!) Still use 'em with meters that don't have 4-wire capability, and I haven't yet lighted up my eyeballs, or let the magic smoke out.
PS- You could prolly ream out the backside (:o I'm in rare form tonight) of the banana plug housings and get the #8 version of that wire in there. Or if not, then the #10 version.
Attorney:
--- Quote from: alm on November 02, 2013, 01:20:34 am ---Test lead resistance will also introduce an error when measuring low value resistors, like the 0.1 ohm resistor you mention. You can either compensate for this with the relative/null function or use four wire connections (either using a DMM with support for four wire measurements or using a current source and DMM). The relative option is the fastest, but has the disadvantage that lead resistance will often change slightly depending on contact pressure.
--- End quote ---
When using the 0.1 ohm resistor for high current measurements, I measure voltage drop across it -- not resistance. Then I compute current. When the input Z of the measuring device is in the megohm range, next to no current flows in the probe leads. With no current, the voltage measurement across the precision resistor is highly accurate. If I know the E drop and if I know the exact resistance, I can accurately solve for current. This is how many industrial control circuits are measure current.
I'm interested in your statement about the shunt when taking high current measurements. If the leads introduce resistance, and as current increases, won't the lead resistance begin affecting current measurement accuracy? I'm referring to a test where the DMM is inserted in series with the circuit for the current measurement.
Paul
robrenz:
alm was referring to when you are measuring the resistance of the 0.1 ohm resistor not when using the resistor as a shunt to measure current. :D
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