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  1. #1
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    Discrete power Supply Circuit Working

    Hi,

    I need a transformer-less power supply circuit to power my PIC microcontroller. I came across the following circuit from here
    http://ww1.microchip.com/downloads/e...tes/40171a.pdf

    Click image for larger version. 

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    It converts 120V AC to 5V DC (or that's what I understand). I can make out that zener is used as regulator but still cannot get a clear picture

    Can some one please explain how this circuit works and how I can modify it to convert 220V AC to 5V DC. Also how to calculate the current it can source?

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  2. #2
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    Re: Discrete power Supply Circuit Working

    Hi,

    do a search for "transformerless supply".

    In short:
    The key is: C3 forms a capacitive impedance for current limiting. Xc = 1 / ( 2 x Pi x f x C)
    If you have about twice the input voltage, then you need about twice the impedance for the same current.
    --> Choose half of the initial capacitor value.

    But you don´t talk about
    * mains frequency (see formula above "f" )
    * load current

    ****
    Maybe it´s easier for you that C3 is replaced by a resistor with the value "Xc".
    You won´t use the resistor in reality, because it will generate a lot of heat.
    The benefit of the capacitor is that it doesn´t generate heat. It doesn´t waste true power.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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  3. #3
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    Re: Discrete power Supply Circuit Working

    The capacitive drop method is an inexpensive supply which gives a few mA at low voltage. It is non-isolated, so if it fails it may expose your expensive equipment to 220 VA, directly or indirectly.

    To find capacitive impedance here is the standard formula. Use it to get an approximate figure.

    XC = 1 / (2 Pi f C)

    Your schematic has 0.1 uF.
    At 50 Hz it might provide one or two mA.



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