Strange behavior of cored coil - Hameg HM8118 - Page 1
Forum section: electronics / beginners

author: Folxs
author_rank: Contributor
timestamp: April 25, 2015, 04:20:55 pm
content: Hi, I got a really big problem!Here are my measurements:And page of LCR Meter:http://www.hameg.com/h009_hm8118.0.htmlAnd my question is: why on 20kHz and above, inductance is with " - "? I know, that should be still with " + ".Sorry about my english, but I think - that everybody will understand me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2015, 04:31:02 pm
content: What does the coil look like?

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2015, 05:37:52 pm
content: This low-Q coil apparently is in parallel resonance with its own capacitance and the parallel capacitance of your test set-up somewhere around 15 kHz. Above parallel resonance, the circuit looks capacitive (which is shown as negative inductance on your instrument). The capacitance required to resonate 2 mH at 15 kHz is roughly 50 nF, which sounds high.What kind of coil is this that the Q factor is so low?

author: Folxs
author_rank: Contributor
timestamp: April 25, 2015, 05:41:07 pm
content: It looks like a transformer - You can take out core. My measurments was with the core.It is laboratory coil (I'm student and it was on my lesson).I don't know why Q is so low, maybe because it is very old and with removable core?

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2015, 05:44:00 pm
content: What do you measure on the coil itself with core removed?

author: Fraser
author_rank: Super Contributor
timestamp: April 25, 2015, 06:23:30 pm
content: Is this coil part of a learning experience designed by your tutor ? Maybe they wish to demonstrate the effects of self resonance ? Has it been 'rigged'. By this I mean, have your tutors fitted it with some parallel capacitance to see if you can work out what is happening.This is the sort of test that my Maritime College tutors gave us back in the 1980's to see if we could think outside the box, and solve unusual component behaviour. They also liked giving us chips to use that had been deliberately damaged on one pin to see how long it took us to recognise a fault in an apparently new IC ! They could be real bas*ards to us sometimes, but we learnt a great deal from such tests.Self resonance is a VERY important factor in selecting inductors, especially when working at RF frequencies. Choosing an inappropriate inductor can really ruin your day as all manner of weirdness can occur in the asociated circuit.Just food for thoughtAurora

author: Folxs
author_rank: Contributor
timestamp: April 25, 2015, 08:19:52 pm
content: Quote from: Aurora on April 25, 2015, 06:23:30 pmHe didn't design it, but yes - He do that to demonstrate us "something" and we have to figure out why that happend. Now I know - thank all of You!(Later I will make photos of that coil and put here my measurments without core)

author: TimFox
author_rank: Super Contributor
timestamp: April 26, 2015, 01:20:32 pm
content: Please measure the coil without core. As a learning experience, it is possible that the core was built to allow eddy currents, which would reduce the inductance of the coil. Normal cores are built with electrically-insulated laminations, or using a non-conductive material such as ferrite, to avoid current flow in the coil. If you insert a solid brass core into a coil, there is a substantial reduction of inductance due to the "shorted-turn" effect.

author: Folxs
author_rank: Contributor
timestamp: April 27, 2015, 03:37:39 pm
content: This is how it looks like and measurements without core:

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2015, 09:16:42 pm
content: I see that the 2.13 mH in your first post was a typo: must have been 2.13 H at 20 Hz. The core is not very strong, only increases the inductance from 0.45 H to 2.13 H. Using the correct value of 2.13 H means that the 15 kHz parallel resonance resulted from about 50 pF parallel capacitance, which is reasonable.