# Discrete power Supply Circuit Working

1. ## 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

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. ## 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.

* mains frequency (see formula above "f" )

****
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

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3. ## 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)

At 50 Hz it might provide one or two mA.

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