SMPS circuit

Thread Starter


Joined Nov 19, 2010
what a great thread
i spent the whole last week googling and reading books abut smps trying to understand it.
my project is an smps 220v AC input and 5v ,5A DC output(if it worked well i will try to inhance it to give 10A).
the schematic in on page 7 is very useful but i could not understand all the circuit
can any one please explain the operation of the circuit
i want to modify it to accept 220V input



Joined Jul 9, 2011
With 220VAC your DC will also be higher.

You will need to increase the voltage ratings of the 220uF capacitor, the input rectifier and maybe the transistor (I didn't look up his datasheet).

The 56k resistor can be increased, (it's wattage too) , the 3842 requires only 1mA startup current.

The transformers specs will probably need to be modified too.



Joined Jan 24, 2011
Hello Moby

I opened your topic wondering if it may have some useful info and it turns out you were hijacking mine. I don't mind though.

I am still going through the info I need for this project as it is by far the most complex I have undertaken to date and there is a lot to understand. I myself have not found a schematic that I can simply directly copy so modifications must be made particularly due to having 220-240 input rms voltage which is about 320v once rectified.

If you look at page 4 it shows what each part does.

Keep us updated, particularly in how you design your transformer. Make sure that all the parts you use are rated higher than needed.

I would also imagine that although the 220uF capacitor needs to be higher voltage it can have lower capacitance as the energy they stored goes up experimentally with voltage. (someone else should confirm or deny this though)

Thread Starter


Joined Nov 19, 2010
SgtWookie,BrainFog :sorry this is my first time to use forume ,i read in rules that i should use search first, i thought the tow threads were similar
praondevou :I really do aprichiate Your information and it was really helpful to me,thanks

BrainFog :
Keep us updated, particularly in how you design your transformer.
sure,i found a book called "Power Supply Cookbook" by marty brown ,i started reading it today it has a step by step practical information,it has a section about Design of the Magnetic Elements on page 37.
hope it will be useful.
i will put any updates in this thread