Question about emi filter - Page 1
Forum section: electronics / projects

author: teslinius
author_rank: Regular Contributor
timestamp: May 30, 2022, 10:10:00 pm
content: I have a low powered device (max. 10W), that is powered by a 50Hz transformer, so it doesn't make a lot of emi. So i was wondering if it's a good idea to use an emi filter despite that, or is just wrapping a few turns of live and neutral around a ferrite core enough, it's a measuring device with a resution of 1mA/1mV. In the attachment there is schematic of the emi filter, is 1uF ok for the x2 caps, since i didn't find much info about the component values.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 31, 2022, 01:51:40 am
content: What is the device?How noisy is it?How is it connected to other things (connectors/cables)?What noise level do you need to meet? (EMI standard and level, or just being well-behaved in your immediate environment to some hand-waving level)?Without these points, adding one is nothing more than a feel-good exercise. You can fool yourself, and that's fine... But as yet, electronics don't feel, and will not be so easily fooled by such exercises.Tim

author: teslinius
author_rank: Regular Contributor
timestamp: May 31, 2022, 05:52:13 pm
content: Electronic loadI used 10 w 50Hz transformer, linear regulators and they power a display, mcu and some other chips, so not that noisy, the case is also covered with aluminium foil for shielding.It has a few connectors approx. 20cm long to the other board.This is just a hobby project so i don't need to satisfy any requirements, just interested about emi filters and if i should use them. How do you even test if the emi filter is working, HV ac probe on the oscilloscope and measure before and after it to see if any noise reduced? or with spectrum analyzer?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2022, 02:39:42 am
content: External connections. Internal connections either don't matter because they're within shielding (but I'm guessing the shielding isn't complete, so would matter on about the same scale as the next thing), or because there's not much RF between them, or at high enough frequency. Noise going between power input and load connection would be the the most likely offender.Probably the most common case novices will encounter EMI in, is ground loop, say when breadboarding. When you see some spiky high-frequency signal at essentially any point in the circuit, that's probably what's going on. The proof is when you see it even with scope probe clipped to its ground. What you're measuring is the voltage drop across the ground lead itself, the 15cm or so of plain wire. Which has significant inductive reactance at say 10s of MHz up, hence the fairly small 10s to 100s of mV, and peaky waveform you can see.Common mode, most basically, means there's AC voltage on the circuit ground, that's not on the scope ground -- or more exactly, a difference between them, because there are no absolute voltages, after all -- hence the voltage drop across any wires between them. With well shielded (e.g. coax) cable, the voltage drops across the outside, while the signal within the cable is preserved (as the difference between core and shield at each end). With uncoupled lines (like the probe GND and tip), the voltage drops aren't balanced or coupled and so the probe reads a difference.(The alternative is coupled and balanced lines: twisted pair for example. In that case, the CM current flows equally between both wires, so has the same voltage drop, and the differential mode stays clean.)As you might guess, a scope will still show something about this. For low noise or EMC purposes, a scope might not be sensitive enough, or have the right kind of response; but to be able to see anything, absolutely, it's something.As for sources, typically it'll be the same sort of thing -- poorly coupled signals and grounds, carrying something noisy, like the fast switching edges from MCU comms, or the peaky voltages or currents within a switching supply. Loose wires (unshielded harnesses) between boards, with few ground wires among them, are a typical case.Tim

author: Andreas
author_rank: Super Contributor
timestamp: June 01, 2022, 06:48:48 am
content: Quote from: teslinius on May 30, 2022, 10:10:00 pmHello,there are some general rules:X-Caps have to be at least factor 10 above the Y Caps. (symmetry reasons).The value of the Y-Caps is limited due to security reasons depending on device category.A 1uF foil cap has its lowest impedance (best filtering) around 1 MHz. (above 1 MHz it gets more and more inductive).Radio interferance voltage is measured up to 30 MHz in EMI-Lab.So if you have EMI noise above 1 MHz it might be better to reduce the X-Value of the capacitor for optimization.with best regardsAndreas

author: Terry Bites
author_rank: Super Contributor
timestamp: June 01, 2022, 12:42:59 pm
content: I've used "yellow" ring cores and they work well. I've had this kind of filter resonate after being hit by current spikes. A low value reistor in series with the second cap will redcuce ringing in your fliter by lowering the overall Q.