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LCR meters and ESR


Some time ago I bought the Agilent U1731B LCR meter. And of course I would like to be able to measure ESR. I thought that measuring the resistance of a capacitor in series mode measurement would yield the ESR of the cap. After reading a post in the BK precision LCR meter review topic I wasn't to sure that my theory added up. So what do you do when you don't know weather something is correct or not, you experiment ;D.


On this page I found a formula that would let me calculate the ESR with the Q and impedance of a cap. So what I did is measure the Q of the cap, the value and the resistance in series resistance measurement mode.

Here follow the results

--- Code: ---Q C F Zc ESR     Measured series resistance
14,4 9,44E-004 120 1,405 0,098 0,096
4,76 3,77E-003 120 0,352 0,074 0,065
7,4 4,04E-003 120 0,328 0,044 0,043
13 6,15E-004 120 2,157 0,166 0,166
11 4,13E-004 120 3,214 0,292 0,302
14,8 8,60E-005 120 15,418 1,042 1,008
9,4 9,14E-005 120 14,514 1,544 1,505
16,8 4,47E-005 120 29,678 1,767 1,746
31,7 4,52E-005 120 29,317 0,925 0,909
78,8 9,70E-008 1000 1640,773 20,822 18,03
172 1,04E-007 1000 1536,245 8,932 9,8
235 9,97E-005 1000 1,596 0,007 2,146

--- End code ---

So as you can see, most of the measurements and calculations match up pretty well, until I get to the Film-caps of 100nF where the calculated ESR is extremely low (high Q) but the measured resistance is quite a lot higher.

So my questions are, Does the measuring of the resistance of a cap in series mode on an LCR meter give you the ESR?
And, of course, Why does the calculated ESR differ so much from the measured resistance once I measure 100nF foil type caps (I measured a few more then are displayed in the table, and they all give about the same result)?

Thanks in advance.

What is the accuracy and precision of that meter for Ohms in that range?

In the 20 Ohms range, the specs are as follows, 1.2%+40 counts at both 120 and 1Khz. This is in parallel mode, series mode is not included in the specifications.

So even in a worst case scenario the 2 Ohm's on the last cap is way out, 1.2% of 0,007 is 0,000084. The maximum value that the meter can display is 19,999 Ohms, so it should read a maximum of 0,047 Ohms. In practice the value is about 50 times higher. I don't expect it to read these very low values accurately, but I do think it is quite strange because it measures 1 Ohm 1% resistors just fine.

I don't think the values would be the same. Series resistance (ohmmeter) would expect a frequency independent load, and may use a very low or DC voltage to probe it, while ESR assumes a cap or inductor, and will seek the proper frequency (~kHz for caps, ~DC for inductors). I am not sure, though, as your meter is made by people much quicker than I.


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