5v Wall Charger

[mscott03]

I'm trying to create this for a home project but have a few concerns. I simply want to take the 110v from a wall socket and take it down to 5v for a cell phone charger and understand so far:

110v > Stepdown transformer (12v) > Rectifier > Cap for stabilization > 7805 Regulator > +5V...

Only thing is, I've opened up a plug in wall charger and there is a lot more to it than that! Not to mention, the transformer inside these are tiny, and I can't seem to understand what is going on in these wall chargers, it seems a lot more in depth than the instruction I've got so far..

Is it that simple to make? And will it be very small?

[tom66]

Re: 5v Wall Charger

The first question is, why? A wall charger is a cheap, inexpensive device. If you are doing this to learn, then there are much better and safer projects to attempt...

The majority of wall chargers use switch mode power supplies to reduce the cost of that transformer. A smaller transformer can be used by using a higher frequency than 50/60Hz mains would otherwise allow. However, the component complexity is much greater and reliability isn't usually as good -- but it's a cost compromise.

[mariushm]

Re: 5v Wall Charger

What you plan to do is a linear power supply, which sounds good the way you explain it.

Only you wouldn't need a 12v stepdown transformer, the 7805 only needs about 8V to output stable 5v. So a 9V transformer would do, it would be cheaper.

And if you want to save even more money on transformer, you could opt for a 6v transformer and use a linear regulator that needs much less voltage drop to output smooth voltage.

7805 needs 1.5-2v to output 1-1.5A, but a chip like MIC2940 only needs about 0.5v above 5v to output 5v at up to 1.25A:

http://uk.farnell.com/micrel-semicon...-0v/dp/1556712


Wall chargers are switching power supplies. They can use smaller transformers because they work differently compared to power supplies.

They use a chip to switch on and off the input voltage at a very high frequency (50k+ times a second) sending pulses of electricity through the transformer and then smooth out the output using the capacitors.

This high frequency allows smaller transformers - your linear design would run at the main AC frequency ... 50-60Hz, so it has to make up for the lower frequency by using bigger transformer (with more metal and magnetic field).

Nowadays, you'd have a hard time making one of those wall chargers cheaper than they're available commercially, they buy components in big volume while your components would be more expensive.

If you're really interested into learning about switching power supplies, you'd have to read about it.. There's application notes for various switching power supply controllers which explain how they work and the datasheets for various controllers.

For example look up the Viper series of controllers: https://cdn.badcaps-static.com/pdfs/...1394dd41e8.pdf

Or the TNY chips : https://cdn.badcaps-static.com/pdfs/...4b65260377.pdf

They're highly integrated chips which use a small transformer to output 5v or whatever you want.

And if you're interested in general about switching power supplies, there's good books which explain how to make one, such as these :

http://www.amazon.com/Switching-Powe...er+supply+book
http://www.amazon.com/Switching-Powe...er+supply+book