Why do they put those ferrite things on computer monitor cables?

Why do they put those ferrite things on computer monitor cables? I'm referring to those round cylinders molded onto the cables.

Thanks

As a 'choke' to convert 'conducted emissions' or electronic noise into harmless heat.

See the chart on the top right corner of page 3. Just by adding series inductance, the conducted emissions from a switching power supply are quieted down below the 'compliance line'. (An additional cap on the filter improves things a bit more.)

--Winston

They act as a common-mode choke- like putting a little single-turn inductor in series with each line of the cable, but using the same core. So the inductance impedes net current through the cable, but not differential currents between conductors in the cable.

They put them there so their product can pass EMC tests.

Shouldn't both sides of the cable get a ferrite choke? I often see cables with just one choke and this never seemed kosher to me.

Maybe, maybe not. If the cable has two close-spaced conductors, like AC hot and neutral, that carry antiparallel current, THAT current (differential current) is innocuous; it doesn't cause radiation. It's only the common-mode current (where the two currents sum to a nonzero quantity) that need be choked off.

The single magnetic core around both conductors also acts as a transformer, and forces the two currents to be equal in the high frequency limit, which makes the occasional accidental ground connection unlikely to start acting as an antenna. Just because one end of a wire is grounded, doesn't mean the wire isn't an antenna.

just one choke and this never seemed

Both share the same core so both sides get a choke, yes?

--Winston

The radiation impedance of six feet of cable isn't very low, so the incremental benefit of the second is much smaller.

Imagine the wire as a piece of antenna. If it's 1/4 wave long from the driving source, it acts as a whip antenna, when the far end is "open circuit" (read, sufficiently high impedance).

If you have a wire grounded at both ends, it can resonate at 1/2 wave, basically an inverse dipole (which is grounded in the middle and has antinodes at the open ends).

The quality of the ground matters; a computer monitor would have to be very large, physically, to make a good ground against a 6' quarter or half wave resonance. Since it therefore does not make a great ground, it develops some voltage on its exterior, which means it serves, in part, as the antenna as well, and so on to anything connected to it.

The quality of the wire is most important at the nodes, where current is highest. For any arbitrary frequency at any arbitrary position, this is difficult to define -- the nodes could be distributed anywhere along the wire. However, nodes are almost guaranteed at the ends (where the monitor or whatever serves as something of a ground plane, even if a poor one), so increasing the impedance there with a ferrite bead does an excellent job at dampening wideband resonances, and thus propagation of noise.

Damping at the antinodes is also possible, but less likely to help, because you need a ground to work against, and dielectric loss rather than core loss.

Tim

Proves you lack in electronics knowledge.

The ferrite is a single turn inductor, and the location on the wire(s) it is an inductor for does not matter, and there does not need to be "one at each end".

You obviously know how to use the word choke, but also obviously have a limited grasp of what it means in electronics, much less the effect it has.

Have you EVER seen one where the idiot put one at both ends? EVER?

Here ya go, needle dick:

"Chokes may be needed at both ends of cables longer than about ?/8. "

My reason in posting this is because I got a LCD monitor at a local auction and it came without the cables. The power cable is just a common computer cord. But the data cable is what I needed to buy. Most monitors have the data cable permanently wired right into the monitor. This one dont. The data cable has a male plug on both ends and plugs into computer and monitor. Same plug on both ends. The cable I ordered has the ferrite on BOTH ends. That's what lead to this posting.

I will say that every monitor I've ever seen with the permanent data cable only has one ferrite and always near the computer end. But the cable I ordered has two.

Thanks for all replies!

as it's the same plug both ends they need a ferrite at the other end also incase it's installed backwards :)

most flat panel monitors have detachable video cables, especially now that most do both VGA and DVI connections.

Stupid audiophool graphtard. Zero emissions data. all simple "series resistance" readings. Big deal.

Hahaha! $10.00 for a 6 foot long printer cable because it has micro-thin gold plated connectors and DUAL ferrites? BULLSHIT!

Almost as bad as "Monster Cable". I'll bet you defend their stupidity as well, eh?