How to decide the Cutoff frequency and Characteristic impedance  - Page 1
Forum section: electronics / beginners

author: XaeroDemon
author_rank: Contributor
timestamp: August 01, 2016, 05:45:05 am
content: I am designing a LC filter to filter out DC power from 6s(22.1 nominal voltage) lipo battery.I know the how to calculate Cutoff frequency and Characteristic impedance but I don't know what they should be. Should the be lower as possible.If the Cutoff frequency is low then lot of high frequency will be filter so I guess its good?.I am totally lost and characteristic impedance.lower impedance is better right?.Thank you very much in advance.

author: singapol
author_rank: Frequent Contributor
timestamp: August 01, 2016, 08:02:12 am
content: I am also lost too.I learnt in school a battery is a DC or direct current storage device and viewed with an oscillocsope it's a straight horizontal line. As far as I remember filtering only applies to AC or alternating current circuits. For example in Sri Lanka I think your country follow the british electrical system meaning ac power at 50hz. and so after dc rectification you get 100hz. ripple and that you want to filter although some of the ripple is reduced by the main filter caps. Yes in this case you want to have LC filter. You can find online calculators for this and many more electrical /electronic ones. Or if you are building a battery charger using switchmode circuit. Apart from 100hz. ripple there is also high frequency noise going to the megahertz region.www.designers-guide.org/design/bypassing.pdfwww.ti.com/lit/an/scaa048/scaa048.pdfwww.ti.com/lit/an/snva538/snva538.pdfwww.illinoiscapacitor.com/pdf/papers/filtering.pdfwww.changpuak.ch/.../AN1707-POWER_SUPPLY_DESIGN.pdfhttps://www.ampbooks.com/mobile/amplifier.../LC-ripple-filter

author: danadak
author_rank: Super Contributor
timestamp: August 01, 2016, 10:29:57 am
content: The cutoff frequency and input/output Z of a filter design are dependenton your requirements. Lower or higher values are a function of what youare trying to accomplish.In many instances the filter is designed to freq domain criteria, componentsizing, magnetic energy requirements, then the I/O Z levels are dealt with,eg. their loading effects, or incorporated in the design loading effects as partof the design.Its a very broad topic with many fascinating twists and turns, design approaches.Regards, Dana.

author: XaeroDemon
author_rank: Contributor
timestamp: August 01, 2016, 01:50:49 pm
content: Thanks for the info @singapol I get your point bcus in most cases battaries are stable but the in this case I am using LIPO in a quad that requires sudden burst of I which results spikes that needs to be filtered

author: StillTrying
author_rank: Super Contributor
timestamp: August 01, 2016, 03:22:38 pm
content: There's so many wrong and right places to put the filter(s), that you might as well show us a block wiring diagram of the devices connected to the lipo.

author: XaeroDemon
author_rank: Contributor
timestamp: August 02, 2016, 10:02:15 am
content: here u go

author: StillTrying
author_rank: Super Contributor
timestamp: August 02, 2016, 03:18:30 pm
content: At first guess I think it should go here(red&blue) to protect the AP, GPS and RX from the ESC switching noise.If the gimbal's servos are powered through the LC filter it will have to carry 4-5 amps safely.