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PICList Thread
'How to use this EMI filter.'
1999\01\08@183948 by Ricardo Ponte G

       Hi!!! out there:

       I need support of you on this:

       The last week some good friend of mine give me an "EMI filter".  He get
from Digikey.  Its internal connection is in "T":

       In the both sides two bobbins.

       in the middle a capacitor.

       How can I connect it ????   How can you explain me the function this
internal connection ????.

       Can I use it to avoid a PIC reset (caused for some kind of noise from th
AC lines) in the operation of my   design???

       Thanks for your help, bye.

1999\01\08@190437 by dave vanhorn

>        The last week some good friend of mine give me an "EMI filter".  He
>get it
>from Digikey.  Its internal connection is in "T":
>        In the both sides two bobbins.
>        in the middle a capacitor.

This sounds like one that I use a lot of, from Murata or Panasonic (I buy
them from Digi-Key)
They are used to take noise off of a power supply or signal line. You can't
use them on high speed signals, but anything up to say 100kHz should be
fine.  They are much superior to bypass caps.

The center lead is ground, and there is an internal capacitor to the signal
line. The ferrites on the outside look like a resistor to RF signals, but
allow large DC currents to flow without any voltage drop. With a good
ground, you will easily achieve 50dB or more noise supression. They offer
these in several frequency ranges, I usually pick the one with a sharp dip
at the processor's third harmonic.

For example, I'm in the habit of using one on my CPU, (which is always a
noisy part) Locate the filter at the CPU ground pin, and take VCC to one
side, and out the other side to the CPU VCC pin. Ground the center pin to
the uP GND.

I also use them on the output of switching power supplies, and in the motor
supply leads of stepper circuits.
They won't solve every problem, but they will solve a lot of them :)

1999\01\09@150841 by Mark Willis

dave vanhorn wrote:
{Quote hidden}

 These also come in versions designed to be put on the AC power lines
(Though those aren't always "T" style as is the one you describe.
Usually two inductors - one in each AC line {hot/neutral}, then
capacitors from each to ground, often 3 caps to connect all three lines
together.)  They work pretty well at 60 Hz (Provided they're designed
for that!) as well, to block RF etc. from outside the project and RF
from radiating from tha AC power line outbound from the project (These
are quite commonly used there.)  Use yours as it's designed to be used
though (If it's optimized for 12 Volt or less use, don't use it on 220
VAC, for example!)  <G>  Mentioning this as I'm not sure how well a
T-filter designed for 117VAC would help on a Vcc connection, I'm not an
RF expert by any stretch of my imagination (And that's a pretty flexible
imagination <G>)


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