Rectifier filtering

Discussion in 'Homework Help' started by snarf, Jun 26, 2008.

  1. snarf

    Thread Starter Active Member

    Jun 7, 2007
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    Hello everyone,

    I would like to add a question to this topic. I know that the bridge current and voltage rating has to be relevant to the load and use of the DC supply. I also know that the capacitor Voltage value needs to be more or less twice the output Voltage of the transformer.

    1. How does one determine the capacitor capacity value? Is this related to the maximum load conditions? Is there a calculation to find the right value.

    2. Which is the best filtering circuit and how does one decide which filtering circuit to use?

    3. If using the capacitor - choke - capacitor filter how can one determine the choke value?

    Thanks.
     
  2. snarf

    Thread Starter Active Member

    Jun 7, 2007
    56
    0
    PS

    Sorry omitted something in question 3.

    3. If using the capacitor - choke - capacitor filter how can one determine the choke and capacitors value?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    When you have a new question, always post it in a new thread. We discourage tacking your question onto an existing thread, however similar the topic may be.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    A rule of thumb I use for sizing power supply filter caps is C=1/(2pi(f)(0.1*∆E/I))
    Where f = rectified frequency (will be double the line frequency for a full bridge) I = current drawn by the load, and ∆E = acceptable ripple.
     
  5. snarf

    Thread Starter Active Member

    Jun 7, 2007
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    My apologies I didn’t know.
     
  6. snarf

    Thread Starter Active Member

    Jun 7, 2007
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    0

    Thanks for reply.

    Therefore assuming that the supply is 50Hz, f (rectified frequency) will be equal to 100, right?
    If the current drawn by the load is not constant because the load is variable does one take the maximum current drawn?
    How does one determine the acceptable ripple?
    Is the ripple value measured peak to peak of the ripple?

    Assuming an AC supply of 12V 50Hz using a full bridge and ripple of 0.5VDC with a 1ADC load would the following working be correct?

    C=1/(2*3.14159265(50*2)(0.1*0.5/1))
    C=31.8268mF

    Kindly also reply to questions 2 and 3.

    2. Which is the best filtering circuit and how does one decide which filtering circuit to use?
    3. If using the capacitor - choke - capacitor filter how can one determine the choke and capacitors value?
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    100Hz is indeed correct for a 50Hz input to a full wave rectifier. Half wave rectification would of course have been 50Hz, same as the input.

    Use the value for your expected maximum current draw. I like to multiply by 1.2, just to be thorough.

    The degree of ripple considered "acceptable" will vary widely based on application. An lamp can accept a lot more ripple than a music system. A variable speed polisher motor can accept a lot more ripple than a medical monitor.

    And yes, ∆E is p-p in this instance.


    Regarding the last two questions:
    1) Please define the word "best" as it is used in this context. Do you need the least expensive? Easiest to build? Prettiest color?
    2) Good question! I look forward to finding out the answer myself! Perhaps one of these will help: http://www.educypedia.be/electronics/electroniccalculatorsfilter.htm
     
  8. snarf

    Thread Starter Active Member

    Jun 7, 2007
    56
    0



    Thanks for your reply and link which has many details with some insight on the topic.

    With "best" I mean how does one determine the most appropriate filtering circuit for a particular application?
     
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