how about converting and stepping the 120 ac volt down to 5 dc volt. how can i do it

 

Hello,

You just hyjacked an existing thread.
http://forum.allaboutcircuits.com/showthread.php?t=34307
I moved it here so others can help you too.

Take a look in the links of this page for some info on powersupplies:
http://www.educypedia.be/electronics/circuitssupplies.htm

Greetings,
Bertus

 

For starters, you need a suitable transformer. If you are in the USA, you can get a transformer from your local Radio Shack store. You'll also need a power cord and project enclosure box.

The transformer will need to have a suitable voltage and current rating. You should use a 1A fuse between mains power and the transformer. You can add a switch after the fuse, if you would like.

Then you will need to rectify the AC into rippled DC using diodes or a rectifier bridge.

Rather than purchasing a transformer and cord, you could use a "wall wart" plug supply. You probably have some of these around the house already. They generally output a rippled DC voltage.

You will need (a) large filter capacitor(s) to remove most of the ripple from the DC.

Then you will need some sort of voltage regulator. A 7805 is an easy-to-use 5v regulator IC. It has three terminals; IN, OUT, and GND. You will need a heat sink for it.

You will also need an 0.33uF cap on the input of the 7805 to ground, and a 0.1uF cap from the output to ground. If you do not include these last two capacitors, the regulator might oscillate at high frequency.

 

Hi,
The basic idea is:
120vac ----> fuse ---> transformer ---> rectifier(bridge rectifier or full wave) ---> bulk capacitor (1000uF) ---> decoupling cap (0.1uF) ---> regulator (7805) ---> bulk capacitor (220uF) ----> decoupling cap (0.1uF) ---> load

Take a look at these for reference:
http://www.aaroncake.net/circuits/supply6.asp
http://www.epanorama.net/circuits/vga2tv/psu_5v.html
http://www.electronics-project-design.com/DCPowerSupply.html
http://en.wikipedia.org/wiki/Diode_bridge
http://www.play-hookey.com/ac_theory/ps_rectifiers.html
http://web.njit.edu/~mlh3/procedures.pdf

Hope this helps.
Tahmid.

 

This schematic:
http://www.aaroncake.net/circuits/supply6.asp
has a problem, as it looks like the hot wire is fused, but he has the switch on the neutral line. That is not good.

The neutral line should not be switched.