How to use ferrite beads - Page 1
Forum section: electronics / projects

author: Yaro
author_rank: Contributor
timestamp: October 29, 2016, 09:40:30 am
content: Hi all,I see a lot of schematics and boards that use ferrite beads to clean power rails from unwanted noises from power source. In particular, talking about digital electronics, I see that microcontrollers and particular sensitive sensors have a ferrite bead on VR output or, sometimes, before connect device to common ground (more often when it's an analog ground that need to be connected to common ground).In the case I want to add a FB to improve my custom circuit reliability, which values I should look and how can I understand them to choose the correct or aproximative FB? Is there common values that can be always good?Thank you!

author: jonovid
author_rank: Super Contributor
timestamp: October 29, 2016, 11:37:52 am
content: component that suppresses high frequency noise in your circuits. them spikes on your scope.I experiment with wrapping wire around them ferrite rings, using a AM radio as a tester for high frequency noise. as a hobbyist my ferrites are salvaged from used PC & RF electronics . if the noise drops then their working if not try add more looping of the wire . the better . use a size that lets you do this. anything that looks like an antenna to RF will radiate RF noise. output & input wires. the flat ferrites are for ribbon cable. DC motors with brushes are bad for RF noise. others may have a better Answers them me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2016, 02:45:34 pm
content: They're almost never applied correctly.Few people understand the electrical characteristics of ferrite beads. Few manufacturers even bother to provide data. (However, Laird has DC bias curves on all their ferrite chip products!) Perhaps people think there's nothing to worry about?Most appnotes suggest dotting them around liberally. That can beactively harmful.You wouldn't use a resistor on a power supply, or a humongous capacitor on a fast signal. (A ferrite bead likely shouldn't be used on either!)The best application of ferrite beads (dampening cable resonances) is also one of the most obscure. Any average prototype will never see an EMC testing lab. Most engineers haven't even stepped inside one!Tim

author: Neilm
author_rank: Super Contributor
timestamp: October 29, 2016, 05:12:50 pm
content: Quote from: Yaro on October 29, 2016, 09:40:30 amThose are two things I would avoid to get good EM performance. Splitting the 0V supply is pretty much a bad idea in all cases (unless isolation is required). Putting ferrites in the 0V will more than likely cause all sorts of resonant issues. Products that have them there were probably tested and found to be non-compliant. Careful choice of ferrite and application may mean that a ferrite in the 0V may have moved the frequency and fixed the issue.

author: Yaro
author_rank: Contributor
timestamp: October 30, 2016, 10:20:46 am
content: Thank you for your answers. I'm using stepper and brushed motors with my custom control PCBs. But my last board had a very huge problem with a brushed motor that makes crash my microcontroller when start spinning. I've found that was an interference problem becouse a bad ground layout. Since this kind of motors are very noisy on control boards when sharing same power supply, after a PCB remake, I wanted to improve reliability adding a FB as a very common advice when using brushed motors. What I can't find are values that I must look for. I can of course choose current rating, but how I can choose resistance and frequency parameters?I know at which frequency I toggle my MOSFET, and aproximative speed of the motor, should I use MOSFET frequency as noise frequency?

author: _Wim_
author_rank: Super Contributor
timestamp: October 30, 2016, 10:56:27 am
content: If your microcontroller crashes, be sure to check if it has no floating pins (use pull ups or tie to ground everywhere). Be sure to check if the master clear pin has a pull up to vdd. Are you sure the problems comes via the power supply of the micro? Trie and power it from a seperate source and check if the problem remains

author: JPortici
author_rank: Super Contributor
timestamp: October 30, 2016, 11:12:54 am
content: Quote from: _Wim_ on October 30, 2016, 10:56:27 amand a capacitor from MCLR to ground to suppress the spikes. always overlooked, often essential.

author: wraper
author_rank: Unknown
timestamp: October 30, 2016, 12:43:10 pm
content: Quote from: JPortici on October 30, 2016, 11:12:54 amGenerally will cause more problems than good with modern MCUs. Better to just using a strong pull-up.

author: bktemp
author_rank: Super Contributor
timestamp: October 30, 2016, 01:06:49 pm
content: Quote from: Yaro on October 30, 2016, 10:20:46 amAt the switching frequency of motor drivers ferrite beads won't help much. They are most effective in the MHz to GHz range.But often there is ringing due to parasic wiring inductance, parasitic capacitance and reverse recovery effects in diodes. The frequency is typically much higher than the switching frequency, therefore ferrite beads can help.One often overlooked cause of crashing microcontrollers is the clock source: The large metal can of a crystal acts as an antenna and injects glitches into the clock. That's why the crystal package is sometimes soldered to GND. Also the traces between crystal and clock should be as short as possible and embedded into the GND plane.For the Reset/MCLR pin I prefer a small capacitor (1-10nF) and a 1-10k pullup. The low capacitance does not delay much, but filters high frequency noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2016, 01:17:47 pm
content: Quote from: bktemp on October 30, 2016, 01:06:49 pmIn a current project, I'm working with a stepper controller that generates 20ns edges at the output. At 48V and 10A!Ferrite beads do a surprising amount there, though they saturate too easily because of the high current.Quote from: Yaro on October 30, 2016, 10:20:46 amGreat! Fantastic! We have an offending source, a susceptible target, and a connecting cable.Now we need to look at the circuits, look at the cables, and look at the PCB layouts, to see where the offending currents and voltages are going.We can also probe the circuit, to prove that the interfering signal is present, and driving enough amplitude into the target to cause problems. Or subject the circuit to an external source of interference, to see what amplitude causes it to fail.Once we know "where" and "how much", we can determine which wires need ferrite beads, and what types to use.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: October 30, 2016, 01:38:40 pm
content: Quote from: T3sl4co1l on October 30, 2016, 01:17:47 pmThere, fixed that for ya!

author: Cerebus
author_rank: Super Contributor
timestamp: October 30, 2016, 01:42:55 pm
content: Quote from: T3sl4co1l on October 29, 2016, 02:45:34 pmmuRata are unusual in providing reasonably useful SPICE models for every single one of their range; although the printed data sheets merely hit the 'adequate' point.

author: janoc
author_rank: Super Contributor
timestamp: October 30, 2016, 02:43:48 pm
content: Quote from: _Wim_ on October 30, 2016, 10:56:27 amActually powering the motor from a separate supply than any other electronics was always *the* solution for avoiding this kind of issues.And then putting a capacitor across the motor pins and an a choke in each lead.

author: _Wim_
author_rank: Super Contributor
timestamp: October 30, 2016, 03:01:40 pm
content: Quote from: janoc on October 30, 2016, 02:43:48 pmI agree, but my suggestion was more to find out what was causing the micro to reset. By (temporary) supplying it with a clean source, he could check if the problem was really coming true the power rails. But your advice is indeed a good one, using seperate supplies for power and control...

author: rstofer
author_rank: Super Contributor
timestamp: October 30, 2016, 03:09:05 pm
content: Separate power supplies for the uC and motors is a given. Opto isolation of the uC outputs is another possibility. For the steppers, I would put the driver over with the motor power and isolate just the step and direction lines. CNC machines almost always use opto isolation between the parallel port (remember those?) and the drives.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2016, 04:54:09 pm
content: Quote from: Cerebus on October 30, 2016, 01:42:55 pmNot a nonlinear model, though.The nonlinearity is the point I'm trying to emphasize.When is a ferrite bead not a ferrite bead? When you run it at rated DC amps and it's little more than a pricey bit of wire!Tim

author: juanfermed
author_rank: Regular Contributor
timestamp: October 30, 2016, 06:25:54 pm
content: Look athttp://www.sigcon.com/Pubs/edn/ferritebeads.htm,http://www.sigcon.com/Pubs/edn/SMTFerrites.htm