# Understanding a variable 220v AC to DC Power Supply

Can someone just please explain how and why the diodes D3 D2 and D4 are used? Why D3 is single while D2 and D4 are in series? How do we calculate the capacitance of capacitor required in a circuit like this?

Please, be a little descriptve I am a newbie.

The circuit has too many errors even to serve as a bad example! 100pF polarized capacitors shown with wrong polarity, cap in series with DC input to a regulator, confusing schematic....

Why does it keep getting resurected? (and I'm sure the OP is long gone)

Capacitance of the smoothing capacitors should be between 2000uF-3000uF for 1 Ampere. 100pF capacitors are originally just fine - they are used for preventing unwanted oscillations/noise.

What is with diodes, I don't know. And also that variable DC voltage output you have there should be regulated with resistor which is also not included in your schematic.

Capacitors which prevent oscillations on the output are also needed on the variable stage of the power supply.

About capacitors, C1 (smoothing capacitor) used to filter out twice the input frequency generated from D1 (full wave rectifier). Generally calculated based on how much ripple you would want to allow. Rule of thumb less than 10% of output voltage is preferred. This article tell you how to calculate.

Other capacitors C2 - C6 serves same purpose to decouple AC and provide stable DC output. Those values are often suggested in the datasheet.

For 78XX series IC product datasheet says the following,

The diode simply blocks DC Voltage. Diode D3 serves no purpose as even without it conducts only in forward direction. D2 and D4 in series treated as single diode as it doesn't enhance or affect any electrical properties except when forward biased will drop twice compared to single diode. Again in this schematic it serves no purpose at that location.

• Could you please build me a more comprehensive circuit schematic? One that eradicates any useless uses and serves same functions – Ihtesham Ullah Mar 25 '17 at 18:15