Choosing a ferrite bead

[Mike Noone]

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.

Thanks,

-Mike

 

[Anthony Fremont]

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.

Here's some decent information about using them:

http://www.antennex.com/shack/Dec99/beads.htm

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.

 

[Mike Noone]

Here's some decent information about using them:

http://www.antennex.com/shack/Dec99/beads.htm

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.

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,

-Mike

 

[[email protected]]

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,

-Mike

make sure theyre ceramic plate, not wound types.

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.


NT

 

[Clark]

First I would use .047 - .0 uF caps across the motors and then power the
Gps unit through a choke with a 47 - 100 uF cap across the Gps unit.

 

[John Popelish]

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.

Thanks,

-Mike

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?

 

[Anthony Fremont]

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,

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.
http://www.superdroidrobots.com/product_info/motor_wiring.htm

Heres some more stuff on bypass caps in general.
http://www.seattlerobotics.org/encoder/jun97/basics.html

 

[John Popelish]

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,

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:
http://www.panasonic.com/industrial/components/pdf/ABA0000CE22.pdf

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.

 

[Rich Grise]

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,

-Mike

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.

Good Luck!
Rich

 

[John Popelish]

Rich Grise wrote:
(snip)

I don't know much about ferrite beads, but I'd say get the biggest
ones you can find.

Have you checked, lately, how big you can get them?

 

[Michael Noone]

Rich Grise wrote:
(snip)

Have you checked, lately, how big you can get them?

I have.

Big.

Very big.

-Mike

 

[Michael Noone]

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.

Good Luck!
Rich

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

-Mike

 

[Michael Noone]

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:
http://www.panasonic.com/industrial/components/pdf/ABA0000CE22.pdf

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.

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.

-Mike

 

[Michael Noone]

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?

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?

Thanks,

-Mike

 

[John Popelish]

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.

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.
http://dkc3.digikey.com/PDF/T061/1165.pdf

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.

 

[John Popelish]

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).

We agree on this.

Are
there other types of beads besides ferrites? I was told by the maker of
the GPS module that a 'bead' would be beneficial.

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.

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.

That's right.

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?

Are you using surface mount parts or through hole?

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.

It took 2 days for it to show up.

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?

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.

 

[Rich Grise]

Rich Grise wrote:
(snip)

Have you checked, lately, how big you can get them?

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:
http://www.google.com/search?q=ferrite-beads

Thanks,
Rich

 

[Rich Grise]

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.

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.

Good Luck!
Rich

 

[martin griffith]

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.

Good Luck!
Rich

how about
http://uk.farnell.com/jsp/endecaSea...amSearch=true&st=parametricSelection&x=27&y=8

ugh, wrap probs, or something like this
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=248290&N=401


martin

 

[Paul Mathews]

Add this to your toolbox: Provided that you have adequate bulk bypass
capacitance near the GPS module, you can add series resistance in the
6V supply line to the GPS. This can be VERY effective at quite low
frequencies. To decide how much resistance to use, determine a voltage
drop budget for filtering. Suppose you decide that you can spare 200
millivolts. That means that you can add 200mV/70mA = ~3 Ohms. The
combination of 3 Ohms with your bypass capacitors can provide a lot of
filtering, without resorting to an inductor. The power rating
dissipation of the resistor is P = I^2*R = 15 mW, so you can use a
really small part. IF the gps module needs a long surge of current at
some point, the resistance can be a problem, and you'll have to use an
inductor instead.
]I agree with the other posters who suggested that filtering spikes at
the motors is your first priority.
Paul Mathews