Power supply How to convert AC to DC (12v ac to 5v dc)

You need a 5v Dc but you have 12v Ac. Now what do you have to do.

The simplest circuit will be using a voltage regulator like LM 7805 These regulators are very common.
It is simple to build: connect the 12V wire at the left most terminal of the IC,

Now I'm trying to use a wall transformer to step the voltage down to 12VAC

You need a Voltage regulator IC
Example:

5v regulator IC number is 7805

9v regulator IC number is 7809

It usually brings on 3 pins

Pin No 1 is Voltage input

(it's maybe 12 v battery )

pin no 3 output voltage

pin no 3 common Ground

while looking at the inscription and with the pins down. Connect the 5V output for USB port at the right most pin. Connect the ground of your 12V supply and of your USB to the middle pin or to the heatsink. You may also add two capacitors like:

now

1st step is diode bridge and ripple capacitor.

The ripple voltage equation is:

V r i p p l e = I 2 f C

Here;

I = Load Current (1A)
f= AC Frequency(60hz)
C= Filter Capacitor (?Uf)

1000 µF at this voltage isn't terribly big. Are you limited by size or something?

To completely get rid of the ripple and produce 5 V, you need to add a voltage regulator after the capacitor.

12 V_RMS = 17 V_Peak, which, minus the two diode drops, is the peak DC voltage you'll see at the output of the rectifiers: 17 - 1.1 - 1.1 = 14.8 V. So there's no threat of exceeding the input limits of the regulator (35 V input).

If the ripple is 8.3 V, then the DC voltage will be varying from 6.5 V to 15 V. This is just barely high enough to feed into the regulator without dropping out of regulation, since the 7805 has about 1.5 V dropout at 1 A (depending on temperature). So yes, you should use a slightly higher capacitor (or multiple capacitors in parallel, if space is an issue).