# Full wave rectifier - smoothing capacitor value?

#### drabina

Joined Oct 24, 2008
13
I am building a simple full wave rectifier with smoothing capacitor. I am using the following transformer to drop the input voltage:

in: 120V 65W
out: 12V 4170mA AC

The rectifier bridge is 50V 25A (I also have 400V 8A). The power supply is going to be used for 18W guitar amplifier based on this schematic:

schematic

Since I am still a noob when it comes to electronics, I have have no idea what value the smoothing capacitor should have.

Anybody can help me out?

Thanks.

#### jimkeith

Joined Oct 26, 2011
540
C = 0.7 *i /ΔE /F = 0.024F or 24,000uf

Assuming 1V peak to peak ripple and 120hZ ripple freq

#### drabina

Joined Oct 24, 2008
13
Thanks for replies. So if my math is correct, according to Jony130's equation I am getting:

C = (2A * 1/120Hz) / 1V = 0.016F

Is that correct? The amp draws 2A max to produce 18W.

#### jimkeith

Joined Oct 26, 2011
540
Actually, the capacitor may be reduced by the 0.7 factor--jony130's method assumes instant charging--in my method, it is assumed that the transformer supplies the current for 30% of the cycle and the capacitor supplying current for the remaining 70%.

#### drabina

Joined Oct 24, 2008
13

#### Jony130

Joined Feb 17, 2009
5,241
But you don't need such big capacitor, simply buy 6800uF.

#### jimkeith

Joined Oct 26, 2011
540
In agreement with Jony130, the value is not all that critical--I would add that it does no harm to go larger as well.