Non-inductive resistors  - Page 1
Forum section: electronics / beginners

author: jeffjmr
author_rank: Regular Contributor
timestamp: November 01, 2021, 11:19:47 pm
content: Question:Are non-inductive, wire wound resistors so called because when in an AC circuit they don’t generate inductance, or are less susceptible to inductance from outside sources, or both?Thanks,Jeff

author: Benta
author_rank: Super Contributor
timestamp: November 01, 2021, 11:25:35 pm
content: They are called that, Because they do not act as inductors (standard WW resistors do, due to their construction)

author: bob91343
author_rank: Super Contributor
timestamp: November 02, 2021, 04:36:00 am
content: All wire wound resistors exhibit inductance and capacitance and self resonance. There is a myriad of winding techniques that help correct for that but none of them goes too high in frequency, say a few hundred kHz.Regardless of that, they are not characterized with regard to the possibility of magntic coupling, but just for self inductance.

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 02, 2021, 05:29:31 am
content: If you had a really long wire wound with 0 inductance, you could make a high frequency delay line with it, using the electrical propagation delay.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2021, 08:04:35 am
content: The main part is the self inductiance.Low self inductance also comes with little sensitivity to an external field. The other way around is not true : one can high self inductance and still low coupling to a homogenous external field, e.g. with a toroid.edit:Some types of non inductive winding can increase the parasitic capacitance. So instead of getting inductive, the resistor will go capacitive relatively early.

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 02, 2021, 08:49:39 am
content: Quote from: Kleinstein on November 02, 2021, 08:04:35 amIf you send pulsed dc, down a wire, thats not the same as reversing polarity, so parasitic capacitance would be significantly lowered down the "resistor" itself.

author: CaptDon
author_rank: Super Contributor
timestamp: November 02, 2021, 03:24:17 pm
content: We use several non-inductive resistors on our locomotives. They are mostly rated at 100 watts or above in our case. You can spot them easily, first they may say 10R N.I. which is 10 ohms non-inductive. Also the winding chris-crosses itself in the shape of an 'X'. They will ALWAYS show some amount of inductance, and will ALWAYS have some stray winding capacitance and as such will have both a parallel resonant frequency which will be definite with a good 'Q' factor and other minor resonances which may give the appearance of series or parallel components.

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2021, 03:31:11 pm
content: In this context, "inductance" means "self-inductance", which is the normalLin the defining equationV= -LdI/dtfor the voltageVinduced by a change in the currentI.An interesting note: when measuring higher value standard wire-wound resistors, around 10 k, 3 to 5 W, for audio purposes, I found that by the top of the audio range the parasitic reactance had shifted from (series) inductance to (parallel) capacitance.

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2021, 03:33:06 pm
content: Quote from: Capernicus on November 02, 2021, 05:29:31 amAnd how would that work? Delay in a transmission line results due to series inductance and shunt capacitance per unit length.I don't understand the phrase "really long wire wound with 0 inductance": please clarify.An example of a transmission line with substantial delay is a long straight wire a finite distance above a conductive plane, such as a traditional telegraph/telephone line. If you look up that case in a textbook, it has a calculatable inductance and capacitance per unit length: their ratio determines the characteristic impedance and their product determines the transmission velocity.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2021, 05:09:21 pm
content: A non inductive resistorhttps://en.wikipedia.org/wiki/M%C3%B6bius_resistor

author: bob91343
author_rank: Super Contributor
timestamp: November 02, 2021, 05:31:17 pm
content: I am not convinced that the moebius resistor has no inductance. Obviously the current traverses a volume and thus generates a field. There is capacitance also.

author: magic
author_rank: Super Contributor
timestamp: November 02, 2021, 05:52:58 pm
content: Current splits in half and takes a circular path in two opposite directions. The fields mostly cancel out.You could use a loop of twisted pair instead. Then you even get to choose whether to wire the two conductors in parallel or in series.

author: Vtile
author_rank: Super Contributor
timestamp: November 02, 2021, 06:16:31 pm
content: Quote from: Capernicus on November 02, 2021, 05:29:31 amSuch thing is called reel of optical fiber.

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 03, 2021, 07:40:54 pm
content: Quote from: TimFox on November 02, 2021, 03:33:06 pmMaybe u can do it with picofarad capacitors, and you keep shifting back the phase 90 degrees capacitor to capacitor. so maybe its a big long line of capacitor manifesting in minute wire disconnections. if u keep the farads down, it could be really high frequency, then if you delay back 4* the amount of bits you supposedly have in the wire, maybe it works. i dunno, but it would be like a bucketbrigade. (maybe not so good) that you end up needing even maybe x caps per bit...

author: TimFox
author_rank: Super Contributor
timestamp: November 03, 2021, 07:42:58 pm
content: Yes, one can make a finite-element delay line with a long string of small inductors and small capacitors, withmuch less than 90 degphase shift from capacitor to capacitor, but the inductors are required to make this a transmission line. Fortunately, you can't make a finite wire without a finite inductance."Bucket brigades" require switches between capacitors, where the switch controls are carefully sequenced to transfer the charge down the line.

author: jeffjmr
author_rank: Regular Contributor
timestamp: November 04, 2021, 12:44:51 pm
content: Thanks for all the feedback. It is clear I came to the right place with this question.I’ll be more specific; a friend and I both have stereo speakers with built in subwoofers and plate amps. The speakers are out of production so we both took opportunities to buy working spare amps. Nearly all components are still available but not some of the switches or the transformer.The issue is 5 of the 6 amps we own between the two of us exhibit 60hz hum, and a bit of 120hz as well. I have replaced filter caps, rectifiers, scoped the rails and signal paths, replaced opamps, voltage regulators, you name it. I can not determine why one of our amps is silent, and the other 5 hum regardless of attempts to mitigate.The power section has one 1 watt resistor and two 5 watt resistors mounted adjacent to each other. In all the amps, they get so hot they scorch the PCB despite having ceramic standoffs. Having run out of ideas to solve the hum, I am wondering if it might be useful to replace the resistors with non-inductive types. They are fed ~38vdc, but still see some ripple and noise and they are not physically far from the rectifier, caps and transformer. Measured ripple is very similar among all the amps which is what has me thinking the hum is inductively introduced as opposed to dirty power.Relevant section of schematic attached. Resistors in question are R82, 84 and 85.Thoughts?Jeff

author: Manul
author_rank: Super Contributor
timestamp: November 04, 2021, 04:44:17 pm
content: Quote from: jeffjmr on November 04, 2021, 12:44:51 pmNo. That would be insane to think that 60Hz couples inductively into these resistors and creates any measurable ripple on the power rails. In high impedance small signal circuits it is possible to some extent. In this power supply circuit - forget about it.How much ripple there is? Maybe there are ground issues? Something causes a circuit to have really bad PSRR?