This small device came in the box with my new laser printer. According to the instructions provided with it, I’m supposed to clip it to the printer’s power cable. What in the sweet holy hell is it? I’ve never seen something like this before.
It’s a ferrite core.
The printer manufacture obviously found out that their device emits too much RFI, so they are having their users do a “field fix” for them.
ETA wikipedia link.
You’ve seen something like it every day, I think. There’s one on every computer monitor cable that I’ve ever seen.
So that’s what those things are for!
OK, yeah, but I’ve never seen one on a power cable, and never one that wasn’t built-in to the cable in question. Seems like sourcing power cables with ferrite cores on them would be cheaper than just dumping a core into the box afterwards, but what do I know. I guess they didn’t discover their power problems until later.
Thanks for the replies.
Fixed.
ETA: Any length of wire can act as an antenna. They likely found their power cables tend to pick up noise from something. So by putting a Ferrite choke on the power cable, close to the printer, helps to eliminate whatever interference it was picking up.
Doubtful.
The input to switching power supplies is pretty RFI-insensitive. However, they can generate plenty of harmonics. It’s possible that the printer was malfunctioning due to RFI interference, but I think it’s unlikely.
I agree with beowulff - switching power supplies are renown for generating RFI and conducted emissions, not being sensitive to it. The manufacturer likely either found out late into the design that they needed a filter, decided to ship into a country with stricter emissions rules than originally designed for, or had a problem with a cable supplier and had to use a quick fix. All that could have happened after a quantity of printers and cables were made overseas, and they were stuck with inventory to fix.
Correct me if I’m wrong friedo, but that power cable is just a standard 3 prong computer power cable, is it not? The power supply for that printer is internal, right?
Putting a choke on the input side of a power supply is because the power supply is picking up something, either from the line or from outside interferences. The problem could be as simple as the printer has a nice 60hz buzz without it, or generating visible scan lines in the display, etc. But enough that people in certain situations complained about it and it was easier to just put a band-aid on it.
Haven’t you ever had a monitor that “squealed” at certain resolutions and was fixed by simply changing the power cord?
Now if that cord with the Ferrite clamped on is actually the output side of a power brick, that is more what you two are thinking. The power supply is generating the interference and the choke is there to reduce it from getting to the printer.
That choke isn’t going to have any effect at 60Hz - not sure how it would prevent a 60Hz buzz in a printer. It likely doesn’t have much effect below, say 10MHz.
This is not the only, or even most common reason for putting a choke here. Clamp-on chokes are used extensively for meeting conducted and radiated emissions limits, due to switcher power supply noise. With a choke clamped around all three power line conductors, you have a common-mode choke.
Unless you’re designing a radio receiver, or something with a wide-band analog input, common mode chokes are much more frequently used to prevent noise from getting out rather than getting in. The conducted and radiated emissions limits are pretty strict in some places (e.g. the EU), and it’s easy for switcher designs to fail without extra filtering, even when designing for those limits up front.
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Sorry, yes my mistake.
The noise being suppressed isn’t generated by the power supply itself since the choke is BEFORE it. If it WAS then putting a choke on the incoming line would do NOTHING. The power supply is picking up, or amplifying, or converting, or doing something to the signal that is coming IN from the AC.
The choke is a linear passive attenuator, and works in either direction. In some sense, it’s like a frequency-dependent resistor, presenting a high impedance at high frequencies looking into it from either direction, and separating both sides at high frequencies (and leaving things unchanged at lower frequencies, where the actual 60Hz power transfer happens). Most of the time, they’re added to keep noise from leaving the power supply, not coming in.
Check out the function of T1 in figure 4 of this article: Low-Cost Conducted Emissions Filtering in Switched-Mode Power Supplies. A clamp-on choke is essentially doing the same thing.
It’s also easily possible for high-frequency conducted noise to end up as radiated noise, if the power leads (outside the device itself) are long enough. Common-mode chokes are handy for that too.
No, sorry. Ferrite cores on the lines side are designed to prevent the harmonics generated by the device from getting out into the common power system. Switching supplies, as stated above, do generate some nasty high frequency waveforms which get imposed on the AC source lines, and will travel to other devices potentially causing interference or other problems up to and including excessive heating.
Chokes on SIGNAL leads, like your mouse/keyboard cable are there to reduce external interference from playing havoc with the attached device and connected port, but they also function to reduce high frequency emissions from them as well.
In my experience, ferrite cores are added to cords like that to meet FCC emissions regulations. If there were an actual problem with the device malfunctioning due to interference they would have redesigned it and fixed it. Nobody wants to have a product with a reputation for not being reliable.
Meeting the FCC requirements is a legal thing. If shipping the product with a ferrite core makes it pass the FCC’s specs, then it’s a cheap and easy fix. If the customer doesn’t bother to put the ferrite core on the power line, the manufacturer doesn’t care. It’s not the manufacturer’s legal responsibility at that point, and the product will work just fine without it. The FCC won’t care either, unless the neighbor calls to complain that something is interfering with something that they own.