# Full wave rectifier - smoothing capacitor value?

Discussion in 'General Electronics Chat' started by drabina, Mar 6, 2012.

1. ### drabina Thread Starter Member

Oct 24, 2008
13
0
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.

2. ### jimkeith Active Member

Oct 26, 2011
539
99
C = 0.7 *i /ΔE /F = 0.024F or 24,000uf

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

Feb 17, 2009
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4. ### drabina Thread Starter Member

Oct 24, 2008
13
0
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.

5. ### jimkeith Active Member

Oct 26, 2011
539
99
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%.

6. ### drabina Thread Starter Member

Oct 24, 2008
13
0
Thanks again for the replies. I know now how to calculate the smoothing capacitor value! The problem is that the price of 24,000uF cap is somewhere close to \$35. I may be better reusing one of my switching power supplies that I have in my project box.

7. ### kubeek AAC Fanatic!

Sep 20, 2005
4,691
806
You don´t need a 24000uF cap, you can use for example 5 4700uF caps in parallel. For 35V they should cost like \$5 total.

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
But you don't need such big capacitor, simply buy 6800uF.

9. ### jimkeith Active Member

Oct 26, 2011
539
99
In agreement with Jony130, the value is not all that critical--I would add that it does no harm to go larger as well.

10. ### drabina Thread Starter Member

Oct 24, 2008
13
0
I have few 2200uF caps and maybe one 3300uF. I can probably add up to over 10,000uF with what I have in my parts bin.