When to twist wires together and use ferrites ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 21, 2019, 08:20:24 pm
content: For instance, transformer winding wires, should I be twisting them together some ?In an ideal world, would 2 or more wires going to about the same place, be twisted together ? I don't remember magnetic's enough behind this stuff. I know that magnetic fields have gauge equations and special relativity.I learned to fold a wire in 1/2 before coiling it and NOT just making a big inductor. (folding a wire in 1/2 the other way, now that would be MOST impressive)I've watched how differential cables like USB work, but can't remember, I know they have to be twisted.Sometimes scrapping printers, I've seen sensor wires, twisted together.Some printers/digital tech in general, have lots of ferrites, even on ribbon cable.What about in audio and RF in general

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2019, 10:39:56 pm
content: Read up on "image currents". You always want currents to be paired, so that the net current flow outside of the cable is zero.So, you might twist the leads of a transformer primary together, or the secondary. But not one from each, because the currents will in general be different.Conversely, a common mode choke has the two windings twisted together (or wound in the same direction), because the currents will be equal and opposite (a CMC is specifically a 1:1 transformer).Note that a transformer is a multi-winding inductor not intended to store energy (i.e., it transforms voltage and current, rather than storing inductive energy). (We still use terms like "flyback transformers" instead of "multi-winding inductors", by convention rather than by purpose.)Anywhere the currents mostly cancel, is a likely candidate for a ferrite bead. This at least reduces the likelihood that the ferrite will be saturated due to current flow; but whether the ferrite will do anything at all (or improve things, even -- things can get worse, too) is quite another matter.Tim

author: pwlps
author_rank: Frequent Contributor
timestamp: June 22, 2019, 06:47:31 pm
content: Quote from: lordvader88 on June 21, 2019, 08:20:24 pmYou definitely don't need gauge theory and special relativity to understand twisted pairs and transformersTwo examples :1) twisting wires will reduce the magnetic flux between them and so the EMF pickup, this is why low-level signal cables should use twisted pairs2) with controlled twisting we can have well defined inductance/capacitance per unit length, making a transmission line with a well defined characteristic impedance. This is how USB and Ethernet cables are made.Usually you don't twist wires in a transformer - unless it is a transmission-line transformer (RF balun)QuoteFerrites around a digital (USB etc.) cables work like ferrites in a 1:1 balun, their role is to reduce common mode currents.

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 23, 2019, 05:45:06 pm
content: "image currents", ok never heard of that thanks.I barely know any equations for EM theory, I need to learn more math 1st.WHat about sometimes ferrites are on 60Hz mains input, long time ago I read it was to help keep the electrons more evenly spread through out the wire(s). Assuming they are not talking of skin effect, since it;s 60Hz, is it more for stranded wire ? I've seen it on different types of electronics. Maybe it's just a HF choke, or to smooth the inrush currents or something.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2019, 03:24:54 am
content: It's for HF, typically to dampen resonances at harmonics of the cable length.Beads on cables do almost nothing in terms of substantive filtering (a few dB), but by bringing down peaks, you can go from a rather nasty say 10 or 20dB tall, narrow peak, you get a few dB tall, broad peak. This applies to radio emissions and susceptibility, not the intended signals inside the cable.Tim

author: autotel
author_rank: Contributor
timestamp: June 26, 2019, 08:16:03 am
content: I'm a newbie myself to electronics, so I hope that any mistake said here is rectified.What I know about twisting cables and ferrite rings in cabling has a lot to do with the so-called "parasitic inductance".By adding a ferrite to the cable, it changes the inductance changing behaviour in the frequency domain in some way, probably used to remove noise/remove capacitance effects from the cable. I don't understand exactly how.I saw that twisted pairs are used in CAN networks, and it helps achieving higher transmission frequencies in long cables. In a CAN network, each line containing information is paired with another line that contains the reverse logic of the same information. These pairs are twisted, as to remove electromagnetic noise coming in and out from the line. I have never used CAN networks myself, but read about it at some point.http://www.ti.com/lit/an/sloa101b/sloa101b.pdf

author: pwlps
author_rank: Frequent Contributor
timestamp: June 26, 2019, 11:41:29 am
content: Quote from: autotel on June 26, 2019, 08:16:03 amThe role of a ferrite around a cable is relatively simple to understand: since it is difficult to pass a net current through it because of large inductance, the ferrite will enforce the ground return via the same cable (so that the total net current through ferrite is zero) rather than some outer ground path.