Curious as to the cause of unexpected inductor behaviour - Page 1
Forum section: electronics / projects

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 20, 2022, 09:24:20 pm
content: I have a LCR circuit driven at 100khz by a squarish wave(Green trace in scope screenshot is the input signal) using a coil that is approximately 20uH @10kHz.I installed it and fully potted it in epoxy adhesive and on the scope at ambient (20C) it would sit in the position indicated by the blue trace (simulated in the screenshot). About 400 to 500mV p-pWhen hot air was passed over it (breathing on it or hair drier) the amplitude would decrease and also it would move further to the left of the other traces down to about 200-300mV p-p (simulated in the other screenshot)The coil was intended to be a reference and to limit exposure to ambient temperature changes I potted it in pla plastic.After doing that the result is (at both ambient and heated) the purple trace in the screenshots. 700mV p-p and a phase shift to the right.WHY???My expectation was that after applying the PLA that it would just sit at the location it was at ambient temperature and would be more resistant to temperature change. I even expected that it might heat up in it's little plastic cage and actually drop in amplitude.But what it did was the complete opposite of what I expected.Why did the amplitude increase and the phase shift move it to the right? or Presuming the position after potting it in PLA is where it was meant to be why was it lower and to the left when potted in just epoxy at ambient and also even lower again when heated?Unfortunately I didn't get screenshots prior to adding the PLA. And I wasn't going to try and remove it.The coil underneath looks like this. about 20uH @10kHzcoil.jpgApologies. The attachment functionality doesn't seem to work.Installed and potted in plasticcoil_installedScreenshot showing green drive signal. Purple final (potted in PLA) and Simulated ambient position (blue)CoilatAmbientScreenshot showing green drive signal. Purple final (potted in PLA) and Simulated heated position (blue)CoilWhenHeated

author: RoGeorge
author_rank: Super Contributor
timestamp: July 20, 2022, 09:51:21 pm
content: How do you know it's the coil's fault, and not caused by some other components that got heated together with the coil?

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 20, 2022, 09:57:34 pm
content: Because just the heat from my finger from touching the coil would cause the amplitude to drop and the phase to shift. And the yellow trace in that screen shot would have moved too.And everything else is down circuit anyway so it doesn't matter if other components were impacted.Edit: and after coating it in plastic I can hit it with a hairdrier and nothing moves.

author: moffy
author_rank: Super Contributor
timestamp: July 21, 2022, 04:21:52 am
content: Without the circuit for reference it is not possible to draw any meaningfull conclusions. Could you please include a schematic as well as the measurement points for the oscilloscope traces?

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 21, 2022, 06:22:23 am
content: Quote from: moffy on July 21, 2022, 04:21:52 amAttached. Hopefully you can understand it.

author: moffy
author_rank: Super Contributor
timestamp: July 21, 2022, 07:07:40 am
content: Sarcasm aside, you will get a shift to the right and an increase in amplitude as you aproach the resonance of your LC.Included is a simple LTSpice file simulation, perhaps that might help.

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 21, 2022, 07:36:07 am
content: Quote from: moffy on July 21, 2022, 07:07:40 amI feel the sarcasm is justified particularly given that you have made it quite clear that you never read the initial post.It's pretty poor form on your behalf because now i'm going to look like the bad guy for calling you out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2022, 08:06:54 am
content: That's not a reference, it's a ferrite bead.Use a gapped core or air inductor. No worries.Tim

author: moffy
author_rank: Super Contributor
timestamp: July 21, 2022, 08:10:49 am
content: Quote from: m12lrpv on July 21, 2022, 07:36:07 amFYI, I did read the initial post.

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 21, 2022, 08:23:36 am
content: Quote from: T3sl4co1l on July 21, 2022, 08:06:54 amYeah. Nah. Again someone who hasn't read the initial post.Gapped core maybe but an air inductor... hell no due to the EMI.

author: m12lrpv
author_rank: Regular Contributor
timestamp: July 21, 2022, 08:29:52 am
content: My theory is that due to some high frequency physical resonance the inductance was effectively lowered.The adhesive epoxy has enough flexibility to allow physical movement. i.e. transformers hum = actual physical movement.Adding the PLA plastic stopped that physical movement allowing it to reach a fixed inductance free from external physical interference.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 21, 2022, 03:57:54 pm
content: I assume OPs who don't provide a schematic up are just idly texting on the bog.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2022, 08:57:29 pm
content: Quote from: m12lrpv on July 21, 2022, 08:23:36 amNot read what?Quote from: m12lrpv on July 20, 2022, 09:24:20 pmAnd this is the coil, right?(Also FYI, the [ Attachment Invalid Or Does Not Exist ] thing is broked... gotta embed images the old fashioned way again, go figure.)If that's not reading, I don't know, it takes two to communicate, maybe you can't write?For sake of information, the blue thing here,is 18uH air core. Maybe too much DCR to test at 100kHz or lower (I forget the spec offhand, it's a Coilcraft 1812 chip, whatever series), but it won't need to be much larger to get adequate inductance down there. With known stability, no massive tempcos from ungapped ferrites.Tim

author: eugene
author_rank: Frequent Contributor
timestamp: July 22, 2022, 03:33:23 pm
content: Quote from: m12lrpv on July 20, 2022, 09:24:20 pmI am both surprised and not surprised. I'm surprised because, like you, I can't think of any reason why epoxy or PLA would have high enough permeability to affect inductance at all, never mind have a tempco of permeability. I'm not surprised because inductors are the most ill behaved, non ideal components ever. They do all sorts of things that I can't explain.I'm with T3sl4co1l. If you want a stable inductance, then start with a core that has a very low tempco (like air.) Make sure there is nothing nearby that will affect the inductance (no copper on any layers underneath) and then do NOT cover it in goop.FWIW, the scope shots suggest that the circuit has a resonant frequency just a little above the drive frequency, perhaps around 105kHz. I don't see how that could be with the given values of L=20uH and C=47nF. You'd need to more than double one or the other (double the product L*C) to get down close to where your circuit seems to be.

author: Bud
author_rank: Super Contributor
timestamp: July 22, 2022, 03:41:23 pm
content: It is a poor choice to use a ferrite core inductor as a reference, because of ferrite temperature coefficient.At minimum try a different core with a lower tempco.