EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Reprobyte on April 28, 2023, 07:26:51 pm
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I have a Keysight U1731C LCR and a few Fluke 80 series 87 and 85, all my Flukes can measure resistors in circuit correctly, but my U1731C is unbelievably far off, like a thousandth of the value or even 1/40Kth of the value or something insane.
Example 39Kohm measuring 34.5Kohm on my Flukes and 1ohm on the Keysight. 680K measures exactly on the Fluke but its 460Ohm on the Keysight.
This is especially on higher wattage resistors 1W 7W 2W
I have some 1/4w resistors that I can test it on and these seem to test ok on both, and I have a 1W 0.33ohm that tests out of circuit.
I am not that well up with the LCR but the difference in behaviour I cant work out, is it the way it measures? AC vs DC and voltage, or is there an issue with the LCR? I would expect to see some what similar results? Anyone who knows what I have done wrong I would appreciate the advice, it just seems weird that the Flukes can all do it.
Thanks
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The two meters will use different measurement techniques but the bottom line is that you can't measure resistors in circuit. There can be capacitors and all sorts of other things around.
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Hi,
I am not that well up with the LCR but the difference in behaviour I cant work out, is it the way it measures?
Probably yes (without knowing the circuit you´ve measured into), UC1731 measures impedance, only the U1733C have a DCR range, which means it could measure resistance the way your fluke do.
Have you check what the keysight shows when you separate the resistor from the circuit ?
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LCR meter most likely measures resistances with AC voltage, so capacitors in the circuit can impact measurements which may be fine when measured at DC.
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Thank you for the knowledge sharing guys, that actually makes a ton of sense. Now I think of it the capacitors didn’t allow the DC test through, but the AC was being eaten up by those capacitors in circuit. I appreciate the help very much.
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@wraper,Yep, therefore he could measure only 1Ohm on a 39kOhm resistor (neg. reactance= cap).
@Reprobyte: Measured value should change when you change the measure frequency of the keysight.
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I did a small experiment to confirm why my LCR works the way it does and thought this would be useful for someone like myself to see the difference between a test using AC vs DC on the component in a circuit.
When the LCR is in resistor mode. The 0.33Ohm resistor measures correctly, (image 1) but in a small circuit with the 2200uf capacitor it lowers the Ohms. (image 2)
The DMM in resistor mode. The same circuit does not lower the Ohms reading from the resistor. (image 3)
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When the LCR is in resistor mode. The 0.33Ohm resistor measures correctly, (image 1) but in a small circuit with the 2200uf capacitor it lowers the Ohms.
This makes sense, when only the resistor is measured, it´s "parasitic" capacitance and inductance won´t take a greater effect ( respectively it´s parasitic inductance is dominating, see the phase angle in the upper right corner of the display, it´s not 0°).
With the "big" 2200µF cap, the capacitive reactance is dominating, see also the phase angle (negative).
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Put your LCR meter in DCR mode instead: Press and hold the Freq down button for 2 secs. Exit the mode the same way.
Ooops never mind... That's for the 33 version.
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see also the phase angle (negative).
Thanks I will check this angle while testing to get more understanding, I see it is neutral almost on the single resistor