Help with Common Mode choke - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: November 27, 2015, 06:34:51 pm
content: So right now I am working on two power supplys. The first one is one is an old magnetically regulated 10 amp 13.8v supply, which is fairly noisy. I planned to quite the main rail down as much as possible and then add two adjustable regulators. Im not doing anything fancy just want something I can use for electroplating or quickly testing something.So I have all these common mode chokes I got from electronics gold-mine in a surprise box, I have really never had a need to use one until now. Problem is these guys dont have any data sheets I can find, thank you surplus. So im hoping some one can tell me buy eyeballing if these chokes are gonna blow up at at 150 watts?The second filter is one I got off tindie, I am building a super RTL dongle.. Ive added a TCXO with a an LO band pass filter for stability and am working on an up converter along with a 3ghz down converter. But i am definitely cleaning up all the USB crap and power to the board I have replaced the cheap LDO, along with switcher with a TI TPS dual package 3.3/1.2v LDO. As far as power USB power lines would I be better off filtering with the bel choke, or the common mode filter from tindie? Since I dont know how to isolate hi speed usb im also looking in to some filtering solutions on the data lines, due to how USB works a common mode filter cant be used on the data lines. I have been playing with beads and looking at chips other SDR's use on there USB data lines.

author: szhighstar
author_rank: Contributor
timestamp: December 06, 2015, 08:53:03 am
content: I mayber can help you, please advie your exact application and necessary information, e.g. dimension requested, SMD or pin through configuration/ rated current/ inductance value/assembly dimension, pin distance, we can design for you.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2015, 04:49:07 pm
content: Have you verified that the signals you intend to filter, are in fact having a problem?I think rather than address the direct electrical consequences of filtering (impedances, amplitudes, internal and external interference, noise and sensitivity, etc...), the underlying question is more philosophical.Throwing components around randomly tends to have one of three effects:1. It unequivocally gets worse.2. Nothing happens, or the circuit has enough margin that you don't notice the difference.3. It gets better.If you don't have a means to measure what's going on, and you don't have understanding of how the circuit and signals are working, then, what you are doing is little more than superstition, and like superstition, can easily do more harm than good!The absolute necessity is being able to positively measure the effect of an action, and justify it. If you don't have a number, it's meaningless, just superstition.Many engineers spend their entire careers throwing in extra bypass caps and filter chokes, and many manage to avoid #1. But the lack of being wrong, does not prove them right, either. Lack of #1 does not imply #3. One must also eliminate #2 to prove that.When taking the time to analyze the circuit, almost all such cases actually fit into #2, and when the action is due to an arbitrary rule ("put bypass caps on everything") without regard for the actual circuit in question, it tends to fall randomly to either side (sometimes better, sometimes worse!).One must always be on the lookout for unfalsifiable claims, or claims that have not been subjected to falsification. Without proof, it's just superstition -- or religion, or metaphysics, whatever!In electronics, and a few other scientific fields, we are uniquely privileged, that absolutely nothing* within the domain of our field, need be such a thing. There are exactly two barriers to preventing #2, and they are large: a. engineers are lazy fucks,and b. the analysis takes time and understanding, and can't always be justified in the larger scope of a project.(*Well, almost absolute. One can construct a computer algebra system (CAS) which is capable of proving P = /P, and therefore the whole house of cards falls apart. But if we make exception for the likes of Godel, then this remains true. And by "CAS", I really mean, any system which has this self-referential property, digital or otherwise. After all, digital is a subset of analog, so it's not a separate, deliberate thing by any means, but a potential property of resistors, capacitors, transistors and such, suitably arranged.)So be very careful about what you propose. If you haven't even identified a problem in the first place, at best you're wasting your time thinking about it. If you throw those parts in randomly, it might get better, worse, something else entirely might break instead -- there are many considerations at work, in any random circuit. It's not by any means impossible to work with those considerations -- but it does take many years of study and practice to identify them, and this is what separates the Engineer from the Technician.Tim