| Electronics > Beginners |
| When to twist wires together and use ferrites ? |
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| lordvader88:
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 |
| T3sl4co1l:
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 |
| pwlps:
--- Quote from: lordvader88 on June 21, 2019, 08:20:24 pm ---I don't remember magnetic's enough behind this stuff. I know that magnetic fields have gauge equations and special relativity. --- End quote --- You definitely don't need gauge theory and special relativity to understand twisted pairs and transformers :) Two 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 pairs 2) 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) --- Quote ---Some printers/digital tech in general, have lots of ferrites, even on ribbon cable. --- End quote --- Ferrites around a digital (USB etc.) cables work like ferrites in a 1:1 balun, their role is to reduce common mode currents. |
| lordvader88:
"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. |
| T3sl4co1l:
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 |
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