Home brew inductance meter is best? - Page 1
Forum section: electronics / testgear

author: Faringdon
author_rank: Super Contributor
timestamp: January 26, 2024, 07:48:21 pm
content: Hi,Today i measured the single coil inductance of a common mode choke (PE-53914NL) with the Tenma72-960 LCR meter.The coil read 30mH on the 120Hz settingIt measured 40mH on the 1kHz setting.The datasheet says its 13.2mH.The leakage inductance, with this LCR meter, was 70uH on 1kHz setting..and 100uH on 120Hz setting.A nominal 1mH inductor measured 1.1mH , so sometimes it can be quite accurate.When i press to calibrate it it only asks me to short the terminals, -unfortunately does not ask me to open them.Anyway, LCR meters , wherever you go, are invariably poor. They are often damaged since people measure capacitors with a few 100V's on them, and that breaksthe LCR meter for good. But it often seems to work, so you persevere with it, and waste your time.Do you agree, the attached, (LTspice and jpeg) simple, home-brew 555 timer based device is by far the best way to measure inductances?...it simply allows you to put it into a resonant circuit with aknown capacitor, and then you read off the resonant frequency on a scope.Its cheap, so if you break it, just make another one.Why is it not more used?PE-53914NL common mode chokehttps://www.mouser.co.uk/datasheet/2/447/SPM2007_62-2904079.pdfTenma 72-960 LCR meterhttps://manualzz.com/doc/6845245/72-960-tenma-vom-meter

author: Kean
author_rank: Unknown
timestamp: January 26, 2024, 09:29:27 pm
content: The inductance or impedance of a CMC is usually specified as measured with the windings connected in parallel. The leakage inductance would be measured on one winding, with the other winding shorted. Many manufacturers will show this measurement setup in their datasheets. I am surprised that Pulse doesn't.So measuring the inductance of a single winding will give you a reading of roughly double the specified value per the parallel inductance formula. Also note the datasheet value will be for minimum inductance, and there isn't generally a tolerance specified. So you'd expect a somewhat higher value, but that may depend on your LCR meter and test frequency.I am surprised that you don't know this when you've worked in the field of power supply design for years. I just design embedded systems and I am aware of it.

author: Martin72
author_rank: Super Contributor
timestamp: January 26, 2024, 10:02:07 pm
content: Hi,QuoteThe frequency does not set itself, you have to "drive through" it to find the resonance and then you can calculate the inductance with known C.We do this with a generator and amplifier, drive through the generator to determine the highest voltage or lowest current.

author: Martin72
author_rank: Super Contributor
timestamp: January 26, 2024, 10:36:36 pm
content: Here a nice solution with the 555 IC:

author: Faringdon
author_rank: Super Contributor
timestamp: January 27, 2024, 01:52:45 pm
content: QuoteThanks, when i get a common mode choke back from manufacture, i measure the inductance of each coil with the other one OPEN.I also measure the " total path" inductance, which is effectively the leakage inductance.....its the path that "normal electricity" takes through the cmc....Ayk, its taken by putting the coils in series and measuring from one end to the other........it gives , ayk, a low inductance value pertinent to what will be seen by "normal" electricity in the circuit.Measuring each coil individually is valid because that is the inductance seen by a common mode emission which obviously only passes through one of the coils, and not the other one.I also do a V/I resistance measurement (ie, not with a simple ohm meter DMM) .QuoteVery interested to see the doc which insists that a common mode choke must be spec'd as to the inductance of both of its coils in parallel?Ayk, CMC's are not generally used with their coils in parallel.QuoteThanks, and i am sure that you appreciate, that if two coils with N turns are on the same core, and are phased the same, then there parallel connection inductance is the same as each of their individual coil inductance.Its best to measure each coil individually....because then you know how equalised the inductances are. If you take a parallel measurement, then the coils may not have the same number of turns and you wouldnt know.....and the field cancellation may not then occur so well.