(Electronics) Cylinder in computer cable - what is it?

March 26, 2008 12:15 PM

So, on lap top power cables, and USB cables, what is that cylindrical object near the end? Kind of like if the cable was a snake, the snake had gotten hungry and just swallowed a plump, unfortunate hamster. (Or a thirsty, unfortunate snake had swallowed a can of soda.) What is it? What's it called? And what does it do? - Digest electrons?
posted by coffeefilter to computers & internet (17 comments total)

It's a choke.
posted by GuyZero at 12:16 PM on March 26

Ferrite choke -- prevents electromagnetic interference. (preview: and it's GuyZero by a nose!)
posted by LordSludge at 12:17 PM on March 26

Heh. Now to explain... it cuts out high-frequency noise from being transmitted by the cable. A cable is not unlike an antenna for high frequency signals.
posted by GuyZero at 12:20 PM on March 26

I think it is a repeater.
posted by Blazecock Pileon at 12:23 PM on March 26

Though I may be wrong.
posted by Blazecock Pileon at 12:23 PM on March 26

It's *nearly* always a choke. But very occasionally that cylinder will be a spying device (a hardware keylogger).

See this page (scroll down about halfway to the BEFORE and AFTER pics)
posted by Tapioca at 12:23 PM on March 26 [1 favorite]

You're not the only one curious about this. I always thought it was for the integrity of the cable (you know, so the connector isn't harmed when you yank on the cord).
posted by spikeleemajortomdickandharryconnickjrmints at 12:24 PM on March 26

I actually had to install chokes on about 127 phone cables inside a car wash controller because of interference from the dryer motors.
posted by notsnot at 12:34 PM on March 26

I was wondering this three years ago...
posted by Faint of Butt at 1:29 PM on March 26

They don't so much prevent interference as choke it off. The cable becomes even more lossy at high frequency, dissipating the energy of unwanted signals. It is the kind of extra measure that you don't need or notice 99% of the time, so I'm really curious how effective notsnot's efforts were :P
posted by Chuckles at 1:57 PM on March 26

You will notice it on USB cables connecting to peripherals like printers and scanners that need control sleep cycling based on data from the USB cable.

As others have mentioned, the idea is that coiling the wire around anything (and even nothing, simply coiling it) increases resistance to high frequency signals. AC signals travel on the outside of a conductor. The higher the frequency, the shallower the depth into the conductor they go. By winding the cable into a coil, the proximity of one winding to another severely limits AC current flow (see proximity effect), while allowing lower frequency or DC current to pass. Wrapping the coil around a magnet reinforces the DC currents, while further weakening the high frequency AC.

The high frequency interference that chokes choke may cause connected peripherals from ever powering up (or coming out of sleep), so using a USB without a choke in an EM noisy environment will cause the connected peripheral to operate poorly.
posted by Pastabagel at 2:38 PM on March 26

Actually, the lumps are solid ferrite beads, not coiled chokes. They are just a solid doughnut surrounding the wires in the cable. They function as lossy inductors that suppress the common mode noise present on cables. The noise from all of the high frequency switching circuits in the device leak onto the cables and out of the box. The unwanted noise gets onto the cable which radiates it like an antenna. The ferrite bead converts the noise to heat, although much too little heat to detect.

Ferrite beads on external cables are often a cheap fix for poor design of the device to allow passing FCC and CE tests for electromagnetic interference.
posted by JackFlash at 3:03 PM on March 26 [1 favorite]

That's a great point JackFlash. I'm sure it is much more about interference radiated by the cables, rather than interference picked up by the cables. You need to attenuate the radiated noise that extra 2dB at 200MHz, to bring down a little peak just inside the 28dBuV region (or whatever, based on the standard you are trying to meet)... And then everybody uses cables without ferrites in the field, just like everybody turns spread spectrum off in their BIOS..
posted by Chuckles at 4:26 PM on March 26

(and that "attenuate the radiated noise that an extra..." link is just included for the pictures. I didn't read the article..)
posted by Chuckles at 4:29 PM on March 26

I don't know why anyone would turn off spread spectrum in their BIOS. There is no point in polluting the spectrum with unnecessary peak noise. If spread spectrum clocks were really a problem then the system would cease to function. The idea that it degrades performance is a hobbyist myth. It would just stop working at all. It is possible that it might slightly affect overclockers, but the mount of modulation, 0.5% is tiny compared to overclockers trying to achieve 10% or even 50% higher frequencies. The amount of spread spectrum modulation is small and the frequency of modulation is typically about 30KHz which is well withing the capability of any downstream PLL detector to track without error. Spread spectrum does not introduce any clock to clock jitter.
posted by JackFlash at 5:30 PM on March 26

Pastabagel: Skin and proximity effects are unimportant here. The proximity between the turns in a coil doesn't choke high frequency currents as much as it enables them by forming capacitive shortcuts.

Wrapping the coil around a magnet doesn't reinforce DC currents, and you really don't wrap the coil around a magnet at all, but around a core with high magnetic permeability; something that could be magnetized but isn't. The coil partly magnetizes the core when it passes a current, storing some energy in the magnetic field. A high permeability core means the magnetic field is stronger for a given current.

In a low-loss coil, like one with an air core, the magnetic energy is returned to the circuit again but at a later time, which impedes fast changes in the current. Such a coil is an inductor, and even though it doesn't burn up the energy from high-frequency currents, it forms a block that may redirect them through an easier path to ground, e. g. a capacitor.

In a lossy coil, like one with a ferrite core at high frequencies (or a loudspeaker, previously discussed here), the magnetic energy is not returned to the circuit but gets wasted rearranging the magnetic field, which makes the coil act as a resistance at high frequencies.

There are exotic filters that utilize the skin effect (e.g. cryogenic powder filters, which can't be bought and are a mess to make...), but ferrite chokes don't work that way.
posted by springload at 8:30 PM on March 26

Thank you all!

(As for the prior ask/answer, it didn't turn up in my search (both metafilter and google), probably because I used the terms "cylinder" and "bulb," and not "tube" which was in the prior. Must improve my search skills.)
posted by coffeefilter at 12:01 AM on March 27