Hi - I was recently reccomended to use a ferrite bead to get rid of noise on a very noisy supply line. I have never used a ferrite bead so am unfamiliar with their usage. The supply line is a 6VDC line and it is supplying a GPS module that consumes approximately 70ma. The noise is caused by 18 motors running on the same 6V line. I don't care about the motors getting a noisy supply but I've been told I need to keep noise under
100mV on the GPS supply. Can anybody tell me how to choose a ferrite bead for this application? I don't have any measurements for just how noisy this
6V line is as the board is still being designed! So what that in mind, I'd like to error very far on the side of caution.
Another approach is to put your gps supply through a diode and reservoir cap. The cap function is obvious, half the noise is eliminated by the cap staying at 5.4v when the noise is down-going (cant think of a better word!) and positive going noise has to face the diode resistance then the cap.
"Anthony Fremont" wrote in news:mCUwf.243$ snipped-for-privacy@tornado.texas.rr.com:
Oh I should have bypass caps on the motors? I thought bypass caps were only for things that were sensitive to large loads. Any idea how large of a bypass cap I should use? The motors peak at about 200ma at 6VDC. Thanks,
Ferrite beads are used to block RF frequencies. The manufacturers usually supply a graph of effective impedance versus frequency and the limits of DC that will saturate the bead.
But I am worried that your motors are making lots of noise well below the frequencies that are effectively attenuated by a ferrite bead (or bead on a lead). I think you need a wound inductor on the order of a millihenry. You will still need high frequency bypassing where the motor current is drawn from the 6 volt supply and also a fairly large capacitor after the inductor that has a low impedance at high frequencies.
Did you see my last post in your LDO thread that gave specific part suggestions?
At the points where each motor drive circuit makes connections to the
6 volt bus and ground, you should bypass across the supply with a capacitor that acts as a local supply to keep the current pulses to the motor from having to wander around the supply and ground buses.
For the motor current you mention, I might use something like a Panasonic series FC, 470 uF @ 10 volts, 0.117 ohm E.S.R., part number EEU-FC1A471:
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A 200mA current step will produce only a 20 millivolt step on step on the bus in the short term. That sort of step will be fairly easy for the series inductor between that and the GPS supply line to suppress.
I'm not sure how much they will help you though, since you're not really talking about RF frequencies, but they may help tame the spikes. Make sure you have bypass caps on your motor terminals and use some bypass and filter caps near the GPS module.
About .001 uF (1 nf) disk ceramics, right across the motor terminals. With a high enough voltage rating, of course. It keeps the commutator hash from going back to the supply lines. Maybe one from each motor terminal to frame, as well.
I don't know much about ferrite beads, but I'd say get the biggest ones you can find.
Ideally you'd put caps from each motor terminal to the case of the motor. If this is just not possible, at least put one accross the power leads right on the motor terminals themselves. Try .1uF ceramic caps.
Here's some decent advice, but be carefull about heating the motor case to solder to it.
Rich Grise wrote in news: snipped-for-privacy@example.net:
3 1nf ceramics per motor should be possible. They should be able to be squeezed in. Will definitely be a pain to install - but no part of this project has been easy - so why let this part be? Argh
John Popelish wrote in news:mJOdnYMJNa-j3FneRVn- snipped-for-privacy@adelphia.com:
Hi John - those are some big caps! I haven't finished work on the boards that will connect to the motors - I'll try to fit those on, but not sure if I'll be able to. Definitely might have to go smaller. Do there exist smaller (footprint-wise) capacitors that would work as well? Price is not a major concern.
John Popelish wrote in news:yNydnQKL_vv snipped-for-privacy@adelphia.com:
Hmm - I don't expect RF frequencies on the line. I expectnoise more from the combination of a DC/DC switcher (in the khz range I believe) and from the motors (I'm guessing 50 hz range - but I could be wrong). Are there other types of beads besides ferrites? I was told by the maker of the GPS module that a 'bead' would be beneficial.
If I am reading your post correctly - you're suggesting a 1mh wound inductor connected to the +6v line. Bypass cap(s) would be attached to the other end of the inductor and ground. The input to the GPS module would be then connected to the capacitors. For the capacitors I'm thinking maybe a 10uf tantalum and a .1uf ceramic (as I already have a large number of both on the board - why not add a couple more?). For the
1mh inductor - I really don't have any idea how to choose such a part. Could you reccomend one that is as small as possible?
I missed your post in the LDO thread (I have now seen it though). Google groups has been acting up and never mentioned it to me. I have since switched back to my trusty ol newsgroup server and will no longer be missing posts. After reading everybody's posts here I think I will actually make an attempt to just smooth the 6V line and forget about using an LDO. Would the inductor you mentioned in that thread (CDRH127/LDNP-102MC) be a good fit for this application?
The bigger the cap, the lower the frequencies remaining in the bumps left by motor operation. I am just guessing how well you need to filter the 6 volt supply. You may get by with just removing the higher frequency components of the bumps. In that case, a low E.S.R. multilayer ceramic surface mount capacitor may be enough. This might be as small as an 0805 surface mount cap like a X5R 10 uF, 10 volt unit, like Panasonic ECJ-2FB1A106K. I would avoid the Y5V and F types, even though they are available in higher capacitance per volume. Their E.S.R is higher.
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Do you have a scope to look at how well various options work?
A lot depends on how well you lay out the circuit board. When you get to that point, I would be happy to discus principles and criticize your artwork.
There are beads of various dimensions, and some with multiple passes through the ferrite, for an effect that is like several beads in series. Look up part 28C0236 on Digikey. But they are still intended to attenuate only 10s of megahertz.
That's right.
Are you using surface mount parts or through hole?
It took 2 days for it to show up.
I think it would. It is fairly small for the energy stored. You may be able to find a slightly smaller part in through hole in an axial would part (like a resistor with wire wound around it), but they spray field out all over the place, where the part I suggested keeps almost all its field inside.
No, actually, I haven't. I know cores can be had in practically any arbitrary size - at what point does it cease to be a "bead" and start to become a "core"? :-)
And everybody knows I'm too lazy to do what looks like somebody else's homework - I even try to duck my own. ;-) But, here's where I'd start:
In addition to the other suggestions, if at all possible, split the two 6V busses right at the power supply - that way the power supply filter will help keep the motor noise out of the GPS. Also, split their return lines the same way.
A ferrite bead is just a little torus of ferrite that strings on the power lead just like stringing beads on a string. It adds a little inductance, and presents a high impedance to higher frequency noise.
But with only 70 mA, you might even use an ordinary RF choke - it might make it easier to assemble, but only you can determine that. :-)
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