Wire-Wound Resistors - Inductance a Problem? - Page 1
Forum section: electronics / projects

author: ThunderCat
author_rank: Contributor
timestamp: August 25, 2017, 06:28:25 am
content: Hi, new member here. I'm working on a power supply for an audio project. I found some super low-noise wire-wound resistors and want to use them in the circuit. Trouble is I don't know if the inductance of the resistors will cause a problem. I know just enough to be dangerous.In general is it OK to use wire-wound resistors in applications like power supplies? Or are they likely to cause trouble the likes of which I can't currently fathom?Thanks kindly; great forum!Mike

author: ejeffrey
author_rank: Super Contributor
timestamp: August 25, 2017, 06:53:02 am
content: What are you using them for? Wire wound resistors are normally for high power or high precision since they are easy to trim and can have low long term drift. Noise will be the same for a wire wound or a metal film resistor.As to whether the inductance will cause a problem, that depends on the application -- do you have a schematic or at least a description of your project?

author: Circlotron
author_rank: Super Contributor
timestamp: August 25, 2017, 07:15:10 am
content: If they are placed in the source lead of a mosfet they can cause problems with oscillation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2017, 07:16:32 am
content: A resistor, single layer helix, nichrome, has a cutoff frequency in the fractional MHz.Audio? No problem whatsoever!Tim

author: daqq
author_rank: Super Contributor
timestamp: August 25, 2017, 09:20:06 am
content: Wire wound resistors can have an inductance some times in the uH range, but for audio it should begenerallyOK.That said, I don't think you will get any particularly better noise performance from a wire wound than from, say a metal film resistor. Could you post the specific type of the resistor?

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2017, 10:51:30 am
content: Quote from: ThunderCat on August 25, 2017, 06:28:25 amIt isn't just the actual frequency which is important, which will be DC in a linear PSU, but the gain and bandwidth of the error amplifier in the power supply. Please post a schematic.

author: drussell
author_rank: Super Contributor
timestamp: August 25, 2017, 03:39:32 pm
content: They also make non-inductive wirewounds which are wound back and forth instead of in a spiral so it cancels out most of the inductance, if you do end up needing to reduce the inductance.Depends on the application, the resistor value, etc. as to whether you need 'em...Quote from: T3sl4co1l on August 25, 2017, 07:16:32 amReally? If you try to pad down a tweeter with most regular wirewound resistors, there is a noticeable change in the character of the very high end, it loses the "sparkle."All depends on the resistor value and construction...

author: cdev
author_rank: Super Contributor
timestamp: August 25, 2017, 09:50:23 pm
content: This is quite unlikely to be an issue at audio frequencies but it is once you get up into the multi MHz range.The only situation where I could see issues would be in a sensitive preamp or somewhere where the "coil" might act as a pickup (of hum). Also, any wire carrying current will create a magnetic field which is magnified by coiling it. Any changing magnetic field will induce a small current in an other wire nearby.Generally if you offset coils at 90 degrees from one another interaction between them is minimized. Also, some materials block magnetic fields.If you have any kind of strong magnetic field, any extraneous wire (or coil, especially) in any high gain amplifier (like a phono preamp) tends to pick it up and amplify it.Grounding and shielded wires are your friends there.Quote from: ThunderCat onTodayat 00:28:25super low-noise wire-wound resistorsand want to use them in the circuit.Trouble is I don't know if the inductance of the resistors will cause a problem. I know just enough to be dangerous.In general is it OK to use wire-wound resistors in applications like power supplies? Or are they likely to cause trouble the likes of which I can't currently fathom?Thanks kindly; great forum!Mike

author: DaJMasta
author_rank: Super Contributor
timestamp: August 25, 2017, 11:30:42 pm
content: The issue is placement, there are plenty of places in a power supply or other where a bunch of inductance is no problem at all, but there are instances when the inductance can cause issues - and with many wirewounds, it's not specified.Generally, in the same series of resistors, lower values will be less inductive, but if they switch over to a different gauge/kind of wire they're made from, this is certainly not a hard-rule. You can be basically guaranteed that they'll be more inductive than a different kind of resistor of the same dimensions, but if you happen to be using a value that only uses a few turns of its resistance wire, it may not be much more. I think I've seen three digits of uH, but I doubt they'll get really high.I think you'd run into more troubles from them in signal circuits, where the inductance can also induce distortion instead of just worrying about building in oscillation, and you probably don't have to worry about this in a PSU.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2017, 11:57:44 pm
content: The pattern is this:If you make a coil with more turns, but it's more length of wire, then both the resistance and inductance are higher. L goes as N^2 (for everything else the same).R goes as N for wire length, but you can't fit the same wire -- it has to be thinner. For the same winding factor (e.g., an 80% fill -- diameter is 80% of the winding pitch), wire diameter goes as 1/N, so wire cross sectional area goes as 1/N^2. So, R goes as N^3!That means a high value resistor (say, 100k+) should be a better resistor than a small value (100s ohms, or even single strip ("zero" turns) current sense resistors). This won't actually end up the case, because capacitance dominates very quickly, for higher value resistors (> 1k?).There is such a thing as a material time constant. A bulk material has resistivity and inductivity. Inductance arises as a property of space itself. Thus we can divide these, and get the bulk L/R time constant. The time constant of a finished item will vary depending on geometry, but will be within a modest factor of this value.This justifies what I said, earlier: the cutoff frequency (1 / (2*pi*time constant)) is generally in the low to fractional MHz for nichrome resistors.For copper, it's in the 10kHz range, which is one reason why PCB traces make such terrible current sense resistors!Tim

author: ThunderCat
author_rank: Contributor
timestamp: August 27, 2017, 07:35:47 pm
content: Thanks kindly for the many responses. Here's a schematic.Thanks again for any thoughts.Mike

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2017, 08:01:14 pm
content: Yeah, it's not going to matter.The resistor noise isn't going to matter either, by maybe four orders of magnitude. Three terminal regulators aren't renowned for their noise performance. If a 78xx is good enough for the circuit, then that's that.Tim

author: ThunderCat
author_rank: Contributor
timestamp: August 27, 2017, 09:22:29 pm
content: Quote from: T3sl4co1l on August 27, 2017, 08:01:14 pmThanks for the insights Tim!