# What is this device on the cord for my AC charger, and do I need it? **URL:** \ **Category:** Factual Questions\ **Created:** [April 3, 2016, 5:06pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095 "2016-04-03T17:06:14Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [April 3, 2016, 5:06pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/1 "2016-04-03T17:06:14Z")
I have an AC wall charger for an JBL Bluetooth speaker. It has this[cylindrical thing inline in the cord](http://seiglefamily.com/data/thingonplug.jpg), similar to what you see on some cords that I believe is supposed to reduce RF interference (required by FCC maybe?). But I haven’t seen this on any other wall charger. Is that what this thing is? Or is it something else that is required for the charger to work properly? My son broke the wire right at the point where the plug comes out of this thing. I could patch the wire if I can remove the device. ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [April 3, 2016, 5:57pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/2 "2016-04-03T17:57:35Z")
> [@CookingWithGas](#): > > …cylindrical thing inline in the cord…that I believe is supposed to reduce RF interference (required by FCC maybe?). But I haven’t seen this on any other wall charger. I think you’re right - it appears to be a ferrous “toroid” in a cosmetic cover. If it’s as heavy as a chunk of iron you can be sure of it. They’re used on some cables to reduce radiated EMF interference and the wires likely go straight through. I would probe the wires to confirm the “straight though” connection and eliminate it in a repair. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [April 3, 2016, 7:37pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/3 "2016-04-03T19:37:13Z")
I thought the purpose of those things was to reduce the RF signals being picked up by the cord. That is, it helps make sure the device being powered has a clean power source. It isn’t absolutely necessary, but it reduces the effects of RF has on the powered device. ---
**Author:** ![core](https://avatars.discourse-cdn.com/v4/letter/c/d26b3c/32.png) [@core](https://boards.straightdope.com/u/core)\ **Post date:** [April 3, 2016, 7:42pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/4 "2016-04-03T19:42:46Z")
> [@excavating\_for\_a\_mind](#): > > I thought the purpose of those things was to reduce the RF signals being picked up by the cord. That reminds me: What does it mean when the sticker on the device says “This device must accept harmful interference, including interference which causes undesired operation”? In my mind that means the device can’t try to eliminate the interference, but that makes little sense. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [April 3, 2016, 7:55pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/5 "2016-04-03T19:55:59Z")
> [@core](#): > > That reminds me: What does it mean when the sticker on the device says “This device must accept harmful interference, including interference which causes undesired operation”? In my mind that means the device can’t try to eliminate the interference, but that makes little sense. I always thought that statement meant that, when exposed to “harmful interference” it would not make the device dangerous. That is, for example, undesired interference will not cause the device to catch fire. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [April 3, 2016, 7:58pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/6 "2016-04-03T19:58:05Z")
> [@core](#): > > That reminds me: What does it mean when the sticker on the device says “This device must accept harmful interference, including interference which causes undesired operation”? In my mind that means the device can’t try to eliminate the interference, but that makes little sense. It makes more sense if you think of it as meaning that it bars attempts to unscramble a cable signal. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [April 3, 2016, 8:01pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/7 "2016-04-03T20:01:27Z")
Explained here. Not all that necessary unless you have electronic devices nearby that are fairly sensitive to possible RF interference. Instrumentation or RF tuners might be one example. > **[Ferrite bead](https://en.wikipedia.org/wiki/Ferrite_bead)** > > A ferrite bead – also called a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite choke – is a type of choke that suppresses high-frequency electronic noise in electronic circuits. > Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices. > Ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to a device. A conductive cable acts as an antenna – if the device produces radio-... > [@](#): > > Ferrite beads prevent interference in two directions: from a device or to a device.[1] A conductive cable acts as an antenna – if the device produces radio frequency energy, this can be transmitted through the cable, which acts as an unintentional radiator. In this case the bead is required for regulatory compliance, to reduce EMI. Conversely, if there are other sources of EMI, such as household appliances, the bead prevents the cable from acting as an antenna and receiving interference from these other devices. This is particularly common on data cables and on medical equipment.[1] > > Large ferrite beads are commonly seen on external cabling. Various smaller ferrite beads are used internally in circuits—on conductors or around the pins of small circuit-board components, such as transistors, connectors and integrated circuits. > > Ferrite beads are used as inductors to form a passive low-pass filter. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in an impedance for high-frequency signals, attenuating high frequency EMI/RFI electronic noise. The energy is either reflected back up the cable, or dissipated as low level heat. Only in extreme cases is the heat noticeable. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [April 3, 2016, 9:30pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/8 "2016-04-03T21:30:35Z")
> [@core](#): > > That reminds me: What does it mean when the sticker on the device says “This device must accept harmful interference, including interference which causes undesired operation”? In my mind that means the device can’t try to eliminate the interference, but that makes little sense. This means that if your microwave oven, or other household device causes this device to not work, or work incorrectly, you have no recourse to the manufacturer. Many devices are built in accordance with FCC Part 15. So if your neighboring Ham radio station causes the attached device to glow red and emit sparks, don’t go knocking on his door. ---
**Author:** ![core](https://avatars.discourse-cdn.com/v4/letter/c/d26b3c/32.png) [@core](https://boards.straightdope.com/u/core)\ **Post date:** [April 3, 2016, 9:36pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/9 "2016-04-03T21:36:21Z")
> [@GaryM](#): > > So if your neighboring Ham radio station causes the attached device to glow red and emit sparks, don’t go knocking on his door. I hold an Amateur Extra license, and IMHO going and knocking on his door is precisely what you should do. A nice operator will assist the neighbor with resolving the interference, even if that means installing filters on the customers equipment and paying for them if necessary. ---
**Author:** ![Doug\_K](https://avatars.discourse-cdn.com/v4/letter/d/b5ac83/32.png) [@Doug\_K](https://boards.straightdope.com/u/Doug_K)\ **Post date:** [April 5, 2016, 1:16am UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/10 "2016-04-05T01:16:39Z")
> [@core](#): > > That reminds me: What does it mean when the sticker on the device says “This device must accept harmful interference, including interference which causes undesired operation”? In my mind that means the device can’t try to eliminate the interference, but that makes little sense. Especially since it’s always coupled with “This device must not cause harmful interference.” Why must my device accept harmful interference from a device that must not cause harmful interference? ---
**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\ **Post date:** [April 5, 2016, 12:02pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/11 "2016-04-05T12:02:08Z")
> [@Doug K.](#): > > Why must my device accept harmful interference from a device that must not cause harmful interference? Even if all the devices in your home meet FCC radiated emissions limits there will still be electrical noise events that could be picked up by a victim device, e.g. operating light switches, fridge compressor turning off, car driving past with unsuppressed ignition leads etc. > [@CookingWithGas](#): > > I could patch the wire if I can remove the device. You can remove and re-fit this ferrite choke by chopping off the moulded strain relief ends, pulling the cable out, and then enlarging the bore with a drill bit or round file before re-inserting the cable and reattaching the plug. The wire would be more prone to breaking in the same way again now there are no strain relief sections, but this can be improved by fitting a length of heatshrink sleeving to reinforce the cable where it goes through the choke. Alternatively you could buy a snap-on common mode choke to replace the original. I’ve got these things scattered over the house on devices that have been giving me noise issues, mostly CFL light bulbs and things with electric motors. ---
**Author:** ![Bert\_Nobbins](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@Bert\_Nobbins](https://boards.straightdope.com/u/Bert_Nobbins)\ **Post date:** [April 5, 2016, 3:40pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/12 "2016-04-05T15:40:34Z")
> [@excavating\_for\_a\_mind](#): > > I thought the purpose of those things was to reduce the RF signals being picked up by the cord. That is, it helps make sure the device being powered has a clean power source. It isn’t absolutely necessary, but it reduces the effects of RF has on the powered device. In almost every case the ferrite is there to cut down radiation **from** the power cable. It will almost certainly be connected to a switch-mode power supply at one end, or both, and these emit RF. Electro Magnetic Compatibility (EMC) is at best an approximate business, and I have sometimes suspected that they are added because it’s the only place where EMC precautions are visible from outside “Hey look, we tried!” Against this hypothesis is the fact that the ferrites are heavy and relatively expensive. I don’t know the real answer, despite having spent some 10 years in the EMC business. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [April 5, 2016, 10:44pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/13 "2016-04-05T22:44:28Z")
> [@core](#): > > I hold an Amateur Extra license, and IMHO going and knocking on his door is precisely what you should do. A nice operator will assist the neighbor with resolving the interference, even if that means installing filters on the customers equipment and paying for them if necessary. As a fellow Ham, I agree. I was trying, poorly I see now, to make the assertion that the neighbor has no legal requirement to help or cease operation. I can see that I failed. ---
**Author:** ![joema](https://avatars.discourse-cdn.com/v4/letter/j/a9adbd/32.png) [@joema](https://boards.straightdope.com/u/joema)\ **Post date:** [April 5, 2016, 11:42pm UTC](https://boards.straightdope.com/t/what-is-this-device-on-the-cord-for-my-ac-charger-and-do-i-need-it/751095/14 "2016-04-05T23:42:08Z")
> [@CookingWithGas](#): > > …I believe is supposed to reduce RF interference… This video explains and demonstrates the RFI suppression features of ferrite beads: [![](https://img.youtube.com/vi/81C4IfONt3o/hqdefault.jpg "#84: Basics of Ferrite Beads: Filters, EMI Suppression, Parasitic oscillation suppression / Tutorial") ](https://www.youtube.com/watch?v=81C4IfONt3o)