Ferrite barrels

Discussion in 'General Electronics Chat' started by mik3ca, Mar 8, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    I bought some special ferrite barrels. It looks exactly like a resistor except the center is just a cylinder of ferrite, that has a charcoalish colour.

    I think I can use these to suppress EMI or RFI noise.

    So far, a PI filter containing a transformer's primary and secondary windings in series as an inductor, and the two capacitors being 1000uF seems to remove some of the noise, and my wall plug cord's location seems to have less of an effect on the radio.

    I think adding ferrite barrels can benefit me. I'm thinking I should add them in series with the power supply, so that +ve must go through one barrel, and -ve must go through the other barrel. These barrels have 0 ohms resistance at DC.

    Please let me know how I should use the barrels. Thanks.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Just like everyone else - thread your wires through the beads. This would be useful if you think the RF from your regen radio is causing the power supply to lose regulation.
  3. Audioguru

    New Member

    Dec 20, 2007
    1000uF electrolytic capacitors are useless at 100MHz. You need to use 0.01uF ceramic disc capacitors with very short leads. They have hardly any inductance.

    Your super-regen radio has an AM detector. Almost all interference is AM. An FM radio has an fM detector and a limiting IF amplifier that does not pickup AM interference.
  4. SgtWookie


    Jul 17, 2007
    Beenthere - Mike's talking about small ferrite cylinders that already have a wire through the center hole. They actually do look a great deal like he describes them; as a typical carbon resistor, except they are a uniform grey in color, with a tinned wire lead through the middle.

    Audioguru - I agree with your assertions. However, Mike's "wall wart" power supply may not be well regulated or may have a bad cap/rectifier; so a 1000uF cap would certainly go a long way towards removing that kind of low-frequency noise.

    In addition to your 0.01uF ceramic disc recommendation (located VERY close to the components affected) a 1uF tantalum, poly or ceramic cap in the vicinity would also help to eliminate noise.
  5. Audioguru

    New Member

    Dec 20, 2007
    Hi Sgt Wookie,
    Mike mentioned EMI and RFI interference in this thread. He says moving the power supply's cord changes the amount of interference.
    He asked about hum on another thread.