Circuit Analysis Help - Need to know how to pick components

Discussion in 'Homework Help' started by Erin_2006, Sep 18, 2006.

  1. Erin_2006

    Thread Starter New Member

    Sep 18, 2006
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    0
    Hi All,

    I'm a newbie here. This seems to be a very helpful site though :)

    I found this circuit on microchip's website. (Sorry for the obvious questions but I just can't seem to figure this out =(

    Here is the link: http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en025234

    I want to build this circuit but before I do, I want to understand it. Particuarly, I'm concentrating on the voltage regulator circuit to bring AC voltage to 5V to power the MCU. (.25A Fuse, R2, R3, D1, D2, C2). However, they post an explanation but it's very diluted.

    I just need to be able to understand the calculations so that I can configure the circuit to how I want it. But I'm not sure how they make the calculations to choose the particular values of each component. Any help would certainly be appreciated. :)

    Thank you in advance!
    Erin
     
  2. Papabravo

    Expert

    Feb 24, 2006
    9,912
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    It is an incredibly unsafe circuit to be fooling around with. Are you sure you want to risk death and disfigurement?

    The zener diode limits the voltage across itself to about 5 Volts plus or minus maybe 10%.

    The resistors limit the current

    The other diode prevents conduction on the negative half cycle

    The capacitor holds it's charge until the next positive cycle
     
  3. Erin_2006

    Thread Starter New Member

    Sep 18, 2006
    5
    0
    Thanks for the fast response.

    My follow-up question would be:

    1. What will make it a safer circuit? (or maybe...why would microchip post something so dangerous?)
    2. My main goal is to basically make a dimmer circuit. And I needed to start somewhere. But before I did that, I wanted to see if I can power my MCU with the wall power. Do you guys think I should go safe and build two circuits? One that powers the MCU and then use an output from the MCU to turn the triac on (which is part of a different circuit?)

    I'm kinda scared to even take on this project now.....
     
  4. Papabravo

    Expert

    Feb 24, 2006
    9,912
    1,724
    What will make it "safer" is transformer isolation. I can't tell you why microchip would publish such a circuit except to promote the use of their parts. Did you notice the very prominent warning at the beginning of the application note?

    Let me refresh your memory. Yellow triangle, lightning bolt with arrowhead. Ring a bell -- so to speak.

    I quote from that initial paragraph

    Only persons experienced in electrical manufacture should use this document to complete the system
    described. FAILURE TO FOLLOW PROPER SAFETY PRECAUTIONS COULD RESULT IN PERMANENT
    INJURY OR DEATH DUE TO ELECTRIC SHOCK HAZARDS. To avoid risk of injury from electric shock, do not
    build, or use, the system described in this document without implementing proper safety measures.

    What is it about this paragraph that you missed or did not understand?
     
  5. Erin_2006

    Thread Starter New Member

    Sep 18, 2006
    5
    0
    Thanks for the help and tip!
     
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