Purpose of mains EMI filter ripped out of plasma? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 09, 2016, 07:01:44 pm
content: So I scrapper an OLD 50 inch plasma from about 2000, lots of cool stuff in it, even some through hole parts! Anyways I was curious about this filter I hooked it up to a scope and saw that it really reduces ripple/spikes between ground and neutral. I was wondering if this would be usefull at all on a linear PSU, and if not what application would you use this in? Yes it will give cleaner power to the transformer but how does this effect anything after the rectification caps?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 09, 2016, 08:10:28 pm
content: Its a more or less normal mains filter. So you find such filters or a similar circuit at the input of most switched mode supplies - they usually need it to keep high frequency signals away from the grid.In a linear power supply there is not much high frequency coming out (at least it should not). Here the purpose would be more like reducing HF noise from the grid coming in. So it can make some sense to use such a filter if used in a very noisy environment. The mains transformer will reduce much of the noise anyway, but not perfectly.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 09, 2016, 08:49:05 pm
content: Yeah, I don't see any other point in adding it to a linear supply unless you are creating some crazy super-low-noise stuff (like some 24-bit ADC precision instrumentation thing) and want to filter out high-frequency crap coming from the mains. But normally, the transformer, capacitors and the linear regulator filter very well to begin with.With switch-mode, it's a different story; you really need the filter so that you don't pollute the mains with the switching noise; it would then couple everywhere.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: April 09, 2016, 11:05:33 pm
content: it could make sense depending on the kind of transformer they use for the linear supply.if the transformer has a high winding to winding capacitance then it will pass the HF, which the linear regulator will not be able to deal with, plus common mode noise, so it kind of makes sense to put one on a linear supply.look at a linear regulators PSRR to see why. you need creative active circuits or actual inductors/ferrite to deal with the HF stuff. will a linear regulator do much to 30MHz noise? nope. is the gigantic capacitor post rectifier particularly suitable to shunt the HF? not really.the filters stack, so it will only tend to improve things *unless you create a resonator*its a good idea for lab equipment that is used to develop things, as complex interactions can develop, the more test equipment you plug into something the more weird shit can happen. the "purer" your equipment is the better the experiment.does it make sense to put it on a power supply meant to power a specific isolated thing in a known environment? not really. in a B2 bomber? yes.keep in mind, you can do alot better if your building something. take one apart and look at it. these units are designed to basically be a off the shelf part you can stick into something while you are in the EM compliance laboratory being charged <5000$ a day so it passes. if you are building your own equipment (which comes out to being really fucking expensive if you factor design time into it) you might as well design better filters and learn more. the parts in that thing are probobly selected by price only.if you ever work for a company you will see the additude people have towards compliance lol. most companies would probobly want to attack the place that makes the EM regulations with an angry mob. nothing makes you want to pull your hair out more then when you have a working piece of equipment that you cannot sell because of some outlandish graphs and legal regulations.is it important on a macroscopic scale? yes. does anyone want to deal with it? no.hence, EMI specific shit = built the absolute cheapest so people don't storm the bastilleif your posting on this forum then you probobly care about keeping the airwaves and your circuits clean.. but that is NOT the industry view.also, it can be a reliability issue, especially since its likely built cheap. you could be creating a "short" for noise that would just not go into the circuit in the first place if it has a high input impedance (i.e. there is AC current flowing through your filter capacitors that might not have went into your transformer, since that filter is a reactive load.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 10, 2016, 01:15:50 am
content: Hmmm so basically this thing could be pretty useful for RF or cleaning up wall power when working with high resolution ADCs, that's actually pretty useful to me. What about packing it in to an enclose and putting it between your scope or something like that?Sounds like the little box is more about keeping noise off the grid lines rather than cleaning up the signal going in to your device/project, it was mentioned one could build better filter than these off the shelf ones. Any recommended reading on this? It would be nice to acually understand the filter and how to test exactly how much of an improvement it is making. Is this what is known as a power conditioner?Also since this is just a multi pole RLC, could one use this a DC line feeding the the reference pin of an ADC to clean up any of the unwanted crap? Why this filter over a common mode choke?

author: sarepairman2
author_rank: Frequent Contributor
timestamp: April 10, 2016, 04:47:45 am
content: http://educypedia.karadimov.info/library/doc191_CMFiltDesign.pdfa big part of doing this is understanding X and Y capacitors. you need to use them to make a safe line filter. and discharge caps.http://electronics.stackexchange.com/questions/22039/why-would-a-power-supply-unit-with-insulating-case-and-galvanic-separation-needhttp://www.murata.com/~/media/webrenewal/products/emc/emifil/case/household/k_3.ashxhttp://powerblog.vicorpower.com/2013/06/what-are-y-capacitors/http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdfall it is is a common mode filter and a differential mode filter in series, designed with power components and intrinsically safe components. some thought and research needs to go into making it, as you won't have the performance you expect from X and Y capacitors (absolutely necessary for saftey) ,and large chokes.just a common mode choke = first order filterif you add capacitors and stuff you increase order.

author: 51mmz0rz
author_rank: Newbie
timestamp: April 12, 2016, 11:16:16 pm
content: I bet they failed conducted emissions and whacked a filter in there. Like others have said, it's about keeping noise from getting back onto the grid.

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2016, 12:07:24 pm
content: No, it would have been in there by design to ensure that it meets conducted emissions, you need to allow for it in the mechanical design.The clip-on ferrite beads that you sometimes find on appliance mains leads are the ones introduced to scrape a conducted emissions re-test.