R, C, and L circuits

Discussion in 'Homework Help' started by bujak18, Mar 11, 2009.

  1. bujak18

    Thread Starter Active Member

    Nov 12, 2008
    i dont understand this question i cant find any information about it anywhere can you help?

    Why are RC filters used more often than RL filters at audio frequencies?
  2. wr8y

    Active Member

    Sep 16, 2008
    I'll give you a hint - how big would the inductors be if you made filters out of inductors and capacitors for audio?

    (By big, I mean inductance value as well as physical size.)
  3. bujak18

    Thread Starter Active Member

    Nov 12, 2008
    about twice to three times bigger i think
  4. KL7AJ

    Senior Member

    Nov 4, 2008
    Capacitors are less subject to stray magnetic fields than inductors. They tend to be smaller, as well. In some applications, they're preferable, however, especially with regard to phase response. Large values of capacitance usually have parasitic inductance AND resistance, that makes them act like transmission lines...not good for transient response!

  5. bujak18

    Thread Starter Active Member

    Nov 12, 2008
    thank you i don'y know how to do this stuff that well. i am a hands on person i learn better that way
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    To most hobbyists, Inductors seem to have nearly magical qualities. Until recently (past decade), coils of wire were rarely seen outside of a radio or transformer. No DMMs had a built in "inductance" range, like nearly all do for capacitance, which made inductor value and function even more mystic. Measuring voltage (caps) is easier than measuring current (coils), for people that like demonstrations.

    Inductors are becoming more popular as operating frequencies increase, voltages decrease while currents increase, culminating with the switch mode power supply. Ferrite technology has also advanced so usable sizes of small to mid value do not saturate as easily as they did a few decades ago. Toroids help with the interference issues, but they are far more expensive compared to a similar value straight coil.

    Useless filter example: You need an impedance of 10kΩ @ 50kHz, in a series filter circuit where a 5k resistor is already present. To achieve that impedance, you would need one of two items:
    A) 5k Resistor in series with a 633pF cap
    B) 5k Resistor in series with a 16mH Inductor

    With "Homemade components":

    A 633pF capacitor could be 20 1 inch square single sided circuit boards stacked on top of one another, total height, around 1/2". 10 of the boards wold be tied to one side of the cap, 10 for the other.

    A 16mH Inductor would be 450 feet of 30 gauge wire, wound 1,410 times around a 4 inch long thin coffee straw, resulting in a final diameter of over an inch, with a final weight of just over 2 ounces.

    Using "Real"/Industrial components: a non surface mount 633pF cap would be smaller than your fingernail, even for a high voltage one. The inductor would still be a 1.5" Diameter toroid about 1/2" thick.

    When entering the audio range, inductors with a high Q (quality) and current ability are physically huge.

    Otherwise, it's a matter of preference where filters are concerned and phase/interference isn't an issue. :)