About inductors and their inner resistance? - Page 1
Forum section: electronics / beginners

author: MrZwing
author_rank: Regular Contributor
timestamp: March 14, 2017, 07:42:18 am
content: Hii have a question about inductors and their inner resistances.this is a hypothetical scenario so don't panic over the values as "real" or "impossible" its the theory i'm after.let us say i have a precision filter circuit that needs a .82 mH inductor and the schematic specifies that it "must have" or the schematic is based on a internal resistance of .37 OHM.at home i have some wire so i can make a .82 mH but the internal resistance is lower than the schematic needs. here is my question.can i put a resistor in series to make up for the resistance loss? or do i need the internal resistance to be exactly that, internal?/MrZ

author: Ian.M
author_rank: Super Contributor
timestamp: March 14, 2017, 08:06:50 am
content: It depends on the frequency and the other parasitics e.g. turn to turn and/or layer to layer capacitance. At low frequencies lumped models for the parasitics are valid and external resistance can be treated as if it was internal. At high frequencies approaching the self-resonance with its parasitics, internal resistance will lower the Q of that resonance but external resistance will not. As one can't make a general assumption, give us specific details of the filter circuit and the desired frequency response, and we can assess whether it clearly falls in either limiting case or if its somewhere in between, requiring detailed modelling or ecperimental testing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2017, 09:28:51 am
content: Lossy inductors tend to cause droopy filters: that is, if you designed a 1dB Chebyshev, but built it with too much loss, you'll find it's more like -3dB, or even worse, at the cutoff frequency (where it was still supposed to be within the 1dB band). The result will be less peaky, but may not be so soft that it ends up looking like a Butterworth.The usual solution is to tweak the poles for a naturally over-peaked response, so the losses flatten it back out.Very precise inductors or capacitors are unusual, anyway; it's more likely that, in the process of tuning the filter, you end up adjusting it towards a compensated response anyway.As for inductors, better or worse: you can always make a good one worse by adding DCR, but you can't make a bad one better by removing resistance! If you're adjusting it anyway, to enough precision that loss matters, you'll /probably/ have enough adjustment range to resolve a small difference in loss.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: March 14, 2017, 09:32:15 am
content: Quote from: MrZwing on March 14, 2017, 07:42:18 amyes. no.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 14, 2017, 09:37:59 am
content: it its withing hearing frequency. i have a few audiophile filter circuits and though i might save a bit of audiophoolery by winding my own coils. but the schematic is kin of specific with the internal resistance and the coil.

author: danadak
author_rank: Super Contributor
timestamp: March 14, 2017, 10:39:38 am
content: You can save space and cost by using OpAmp active filter, or a DSP solution.Solutions are for for small signal case, if doing speaker crossovers you wouldfollow by a power stage.http://www.ti.com/lit/an/sboa093a/sboa093a.pdfhttp://kvetakov.net/~mira/literatura/Electronic%20Filter%20Design%20Handbook%204th%20Ed.pdfDSP using PSOC -https://onedrive.live.com/?authkey=%21APhaNy1IR0H7Hxo&cid=F4A7FEFB2582895D&id=F4A7FEFB2582895D%21952&parId=root&o=OneUpRegards, Dana.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 14, 2017, 01:33:47 pm
content: yeah but i do want to use coils and make the filter analouge non IC. that is why i want to know how to make my own coils. and if it turns out to be to complicated ill just buy them and try to find them from "non-rip off" prices.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2017, 01:49:48 pm
content: For audio frequencies it is OK to add an external resistor.It is only for RF frequencies close to the coils self resonance it makes a difference where the resistance is, but even than if there is enough external capacitance the difference will not be that large. Also in the RF range there is more than just the DCR that sets the Q. So it is unusual to call for a specific DCR, more like requiring a certain Q.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 15, 2017, 02:50:29 pm
content: Sounds great, have done some inductor calculations and with the option to add a resistor to balance the inductor resistance i will save much hazzle and no need to buy spools at audiophoolery prices by making my own,have an LCR meter(DER EE 5000) to make my job easier.

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 16, 2017, 01:16:37 pm
content: I finally put together , tested, and calculated some low and high pass RC and RL last week. It was fun. And from the basic level model equations, the DC resistance of the inductor is very very very insignificant compared to the size of the typical resistor someone might use. Its like parts/million difference in the equations, and even more irrelevant in real world simple uses. So it made no difference to the impedance.IDK enough to know when it starts to become a factor. I was studying a book on inductors in ICs and the equations they use to model inductors are a lot more detailed/complicated