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DMM Test Lead Resistance

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Attorney:

--- Quote from: BravoV on November 02, 2013, 02:33:57 am ---Lead resistance is not a significant factor (up to certain limit of course) when the DMM is in voltage mode, because the DMM input impedance is so high (Mega/Giga Ohm) that makes those minuscule resistance at the lead become meaningless.

--- End quote ---

That's why it's often more accurate to use the DMM in voltage mode across a small resistance to measure current.  That way, the DMM in no way affects circuit current, nor the measurement -- apart from it's own DC voltage resolution/accuracy.

Paul

alm:
Yes, lead resistance affects the burden voltage and may indirectly affect the current measurement, as I stated in my second post.

You're apparently just looking for confirmation of whatever point you're trying to make, so I'm out of here.

ModemHead:

--- Quote from: Attorney on November 02, 2013, 12:16:27 am ---Anyone have standard length DMM test leads that measure even higher than mine?

--- End quote ---
For measuring test lead resistance, I made a tip-shorting connector from two 2mm pin jacks and a short length of 12 gauge copper wire.  Plug the leads into a power supply set for 1A current, and measure the millivolt drop across the plugs.

Most of the leads I have around here are various Fluke and Pomona sets.  The venerable old Fluke TL75s generally measure the lowest at around 60 to 70 milli-ohms.  The "premium" TL71s usually measure the highest, anywhere from 100 to 200 milli-ohms, possibly due to the tip alloy.

Pictured are some modular leads I use a lot which are mish-mash of Fluke leads and Pomona probes.  This pair measures about 90 milli-ohms, despite the extra pair of connections.

None of this makes any real difference to me in actual use.  I generally use patch cords and/or clips for current measurements, not probes.  And 4-wire kelvin clips for low-value resistors.

mos6502:
My Fluke 77 has a shunt resistance of 5.545 Ohms on the 300mA range and 0.095 Ohms on the 10A range. A 30 mOhm lead resistance isn't going to affect the circuit in any significant way. Really, the only place where you need a four wire setup is when measuring impedance. And, surprise, impedance (LCR) meters do have four wire capability ( at least the good ones).

Incidentally, a really good way to measure lead resistance is an LCR meter with split banana jacks like the DEREE DE-5000. It will do a four wire measurement and show only the true internal wire resistance.

In the pic, you can see a 110cm long Hirschmann lead that is marked as 1 sqmm. The measured resistance is 19mOhms. Using the formula A=rho*l/R, the wire area calculates to 0.973 mm^2 - only a 2.7% error from the expected value, which is an insanely good result considering all the physical variables.

If you measure both the length and the thickness, this is a also a good way to tell if you're actually dealing with copper or some other copper plated metal.

And one more thing, when using an LCR meter to measure resistance, you also eliminate any thermoelectric effects that will occur at crimp/solder joints in the cable.

Attorney:

--- Quote from: mos6502 on November 02, 2013, 12:57:28 pm ---My Fluke 77 has a shunt resistance of 5.545 Ohms on the 300mA range and 0.095 Ohms on the 10A range. A 30 mOhm lead resistance isn't going to affect the circuit in any significant way. Really, the only place where you need a four wire setup is when measuring impedance. And, surprise, impedance (LCR) meters do have four wire capability ( at least the good ones).

Incidentally, a really good way to measure lead resistance is an LCR meter with split banana jacks like the DEREE DE-5000. It will do a four wire measurement and show only the true internal wire resistance.

In the pic, you can see a 110cm long Hirschmann lead that is marked as 1 sqmm. The measured resistance is 19mOhms. Using the formula A=rho*l/R, the wire area calculates to 0.973 mm^2 - only a 2.7% error from the expected value, which is an insanely good result considering all the physical variables.

If you measure both the length and the thickness, this is a also a good way to tell if you're actually dealing with copper or some other copper plated metal.

And one more thing, when using an LCR meter to measure resistance, you also eliminate any thermoelectric effects that will occur at crimp/solder joints in the cable.

--- End quote ---

Thanks for the suggestion and for the reminder that the DE-5000 has split jacks.  Last week, I placed and order and it should soon arrive.  This will be one of my first tests with the device. 

Paul

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