Electronics > Beginners
DMM vs LCR: a confused beginner on resistance measuring
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ArthurDent:
macboy is correct. Also if you were to measure the D.C. resistance of an inductor with a DMM then you would just be measuring the resistance of the length of the coil of wire used to make that inductor but that DMM reading wouldn't tell you anything about the A.C. characteristics of the inductor. That is why some wire-wound resistors that might be used on A.C. are wound non-inductively to try to cancel out the inductance you'd get by winding the entire coil of wire in one direction. In this drawing the two bottom resistors are wound non-inductively and ones like this would probably agree much closer on both your instruments.

Try a few different resistors and see if you can find some non-inductive ones by the readings you get.
ejeffrey:

--- Quote from: Moriambar on December 10, 2018, 10:43:43 am ---Anyway thanks. Perfectly clear. So 4wire is good for low resistances, otherwise the inaccuracies will be greater than the change brought by the measurement method. Good

--- End quote ---

Yes, for DC resistance, 4 wire measurement is only needed when the lead resistance isn't negligible compared to the DUT.  For LCR meters it is more important it is quite common for lead (or fixture) capacitance or inductance to be large compared to the DUT.
Moriambar:
thanks for the detailed explanation folks
tggzzz:

--- Quote from: mvs on December 10, 2018, 11:59:43 am ---
--- Quote from: Moriambar on December 10, 2018, 10:10:33 am ---Nevertheless the LCR meter shows an extra meaningless digit, and I do not understand why! I mean if the display is 0.9979 or 0.9970 is basically the same since the last digit is not at all accurate ie meaningless.
 
Why is that so? Am I missing something?

--- End quote ---
Resolution and absolute accuracy are different things, so its fully ok that your LCR displays a digit more. Higher resolution is helpful for relative measurements, where only short term stability and linearity matters.

--- End quote ---

Precisely.

A company I worked for made a test set with an accuracy of 0.1dB but a resolution of 0.001dB. The customer didn't care about the 0.1dB, since they had other ways of measuring absolute values. OTOH they did care about the stability and repeatability of the 0.001dB, since they were looking for slow changes in tests that took a week.
David Hess:
At low values of resistance, the DE-5000 is specified to be more accurate and have higher resolution when making an AC measurement.  This may reflect the fact that an AC measurement ignores thermocouple errors which become significant at lower resistances where the sensed voltage is lower for a given excitation current.

So AC measurements go down to 20 ohms with 0.001 ohm resolution while DC measurements are limited to 200 ohms with 0.01 ohm resolution.  AC measurements on the 200 ohm range are specified to be more than twice as accurate as on the DC measurements.
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