| Electronics > Projects, Designs, and Technical Stuff |
| Help wanted suppressing conducted mode EMI |
| (1/1) |
| rhb:
I've discovered that I still have an EMI problem from my 4 ft LED T-8 lamps. I installed a $3 Chinese EMI filter and that made a huge improvement. But I've discovered I still have a lot of noise in the 45-120 KHz region. I reinstalled my 1/8" galvanized metal screen and that had no effect, so it seems pretty clear it is conducted mode EMI being radiated by the house wiring. I'm beginning to think very seriously about getting up in the attic and rewiring my test bench area with BX cable. The attached screen shots show the spectrum with the lights on and then off using 256 sweep averaging. The scope probe is just laid out on a wooden shelf in a straight line from the Instek (which is also quite noisy and in need of shielding the SMPS) to a point about the middle of the room along the wall. I need to design a filter to suppress this, but I'm a bit unsure how to go about that. I tried designing a stop band filter with Elsie, but that convinced me I didn't know what I was doing. So I thought I'd ask for some advice. Designing a filter for the AC line is enough different from past filters to feel the need to do more research. IIRC the total current draw of the two lamps is about 1/2 A, though I'll measure it before actually building and installing an additional filter. I'm also going to test just adding a 2nd of the $3 filters if sweeping one on the Instek suggests it will be effective. FWIW The spectrum displays were made with an Instek MSO2240EA. I discovered I could start the MDO SA demo on my MSO, turn off the AWG and use the SA. It's quite good and with the MSO on 1 mV/div much more sensitive than my 8560A. It also goes to DC while the 8560A only goes to 9 KHz. With a bare 1X probe tip without a ground it's a very sensitive E field detector, and equally good H field detector with a small loop. And by adjusting the input vertical you can desensitize it to make exactly pinpointing a noise source on a PCB very easy. Sadly, GW marketing insists this is for the MDO line only and don't understand how important it is as a tool for embedded system designers which is the target market for the MSO line. So I keep using 1.32. Once I mentioned how much I liked it, they locked the MSO out of the demo in subsequent releases. |
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