Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rectification and fluctuating DC

  1. Sep 5, 2007 #1
    Does rectification of AC into DC with the use of diodes always create a fluctuating DC wave? Are there methods of rectification which create a steady DC current or do you always have to smooth the current out afterwards?

    I'm having a hard time grasping the concept of half-wave and full-wave rectification. Are there any advantages to half-wave rectification or is it only used because it's a simpler method of rectification?

    Wikipedia has good diagrams of half-wave and full-wave rectification circuits.
    Last edited: Sep 5, 2007
  2. jcsd
  3. Sep 5, 2007 #2


    User Avatar

    Staff: Mentor

    The key to understanding rectification is to undersand the behavior of a diode. Have you read through this wikipedia page?


    Let us know if you have questions about how a diode works, and that will help you understand the rectifiers that you are asking about.
  4. Sep 5, 2007 #3
    I've read about most common electronic components on wikipedia but I still don't fully understand diodes and when it comes to transistors I'm completely lost.

    The only thing I really understand about diodes is that they are the electronic equivalent of a one way valve in a water pipe system. I have no idea what the purpose of diodes are in a DC current. Before I get into semiconductor diodes and LED's I want to learn the purpose of regular diodes in AC and DC circuits.

    I can see why an AC current flowing through a diode would create a half wave rectification. I haven't tried to understand full wave rectification circuits yet. Would a fluctuating DC current perform as well as a steady DC current in a basic circuit like a flash light? I'd imagine a half wave DC current would make the light flicker since the voltage constantly goes on and off but would a full-wave DC current power the light bulb the exact same way a steady non fluctuating current would?
  5. Sep 5, 2007 #4


    User Avatar

    lessee AC corrent alternates direction. it goes back and forth.

    DC current is in only one direction. does not go back and forth.

    might a one-way valve be useful in coaxing something that alternates direction into something that flows in only one direction?
  6. Sep 5, 2007 #5


    User Avatar

    Staff: Mentor

    The one-way valve is a reasonable analogy. Basically you get no back-current through the diode.

    If the DC current is fluctuation, but never going in reverse, then the diode stays forward biased, and the only effect is that you get about a 0.7V forward drop across it.

    You can drive a light with an AC or fluctuating DC current, and as long as the fluctuations are at a frequency above the "flicker fusion frequency" of the human eye (around 70-80Hz, I think for most people), then no flicker will be perceived.
  7. Sep 5, 2007 #6


    User Avatar
    Science Advisor
    Homework Helper

    Berkeman overlooked that a filament in a flashlight lightbulb is a slowly responding load. It won't flicker using halfwave rectified voltage at 50 or 60Hz, because the supply pulses more rapidly than the filament has time to heat up and cool down. It is also a simple way to make a dimming switch. Using a fullwave supply for bright light and halfwave supply for dimmer light.

    To further help you understand rectifier circuits, here are some illustrations
  8. Sep 5, 2007 #7


    User Avatar

    Staff: Mentor

    Yes, good point. Thanks for pointing that out, Ouabache.
  9. Sep 8, 2007 #8
    There is a circuit configuration called Fullwave bridge filter circuit. A capacitor is used between the output and load. This reduces the ripple and smooths out the current.

    You should look into this.
  10. Sep 8, 2007 #9


    User Avatar

    Staff: Mentor

    Does one have a link.

    I found this - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

    I think there will always be some ripple. In addition to pooface's recommendation, there are system using multiple AC phases in which each is separately rectivity, and one can always increase the AC frequency to high frequencies. One does not have to be constrained to 50/60 Hz if the generator is isolated/alone.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Rectification and fluctuating DC