# Smoothing capacitors

#### floomdoggle

Joined Sep 1, 2008
217
Hi all,
I am making windmill generators. With the AC generators, after the bridge rectifier, is there a formula, or rule of thumb, on the size of the smoothing capacitor? I am using anywhere from 1.5 volts to 48 volts. I have lots of recovered capacitors from old vcrs and betamaxes. And 1 or more?
Dan

#### Metalfan1185

Joined Sep 12, 2008
160
There may be, if there is i dont know what it is...

Iwould try a few different values in paralell and use an oscilloscope to display the ripple, and then add/remove caps until you are rid of the ripple the best you can.

#### Audioguru

Joined Dec 20, 2007
11,251
For a 60Hz mains frequency and a full-wave rectifier bridge I use a graph that shows the peak-to-peak voltage of the ripple at various currents with various capacitor values. There is a formula on the graph for you to convert to a 50Hz mains frequency.

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#### SgtWookie

Joined Jul 17, 2007
22,201
Hi all,
I am making windmill generators. With the AC generators, after the bridge rectifier, is there a formula, or rule of thumb, on the size of the smoothing capacitor? I am using anywhere from 1.5 volts to 48 volts. I have lots of recovered capacitors from old vcrs and betamaxes. And 1 or more?
Dan
Dan,
Those old caps, if electrolytic, are more than likely no good anymore. They're electrolytic. Electrolytic caps can sometimes be re-conditioned over several days using current limiting resistors, but the process is tedious at best. Electrolytic caps should be derated (rule of thumb) by 50% of their voltage rating; eg: if a cap is rated at 20v, it should not be used in a circuit that may have more than 10v on it.

If you want to try to recondition some of your salvaged caps, try charging them using a 20k-30k resistor to their rated voltage, over a period of time.

Salvaged capacitors aren't a good choice for storing energy, IMHO. Batteries are far better. Lead-acid are expensive. Nickle-iron batteries were widely used in the 1930's-1940's, and were quite inexpensive - although they didn't retain a charge like the lead-acid batteries did. Still, the nickle-iron batteries had an extremely long life cycle by comparison.

#### Wendy

Joined Mar 24, 2008
21,839
For a 60Hz mains frequency and a full-wave rectifier bridge I use a graph that shows the peak-to-peak voltage of the ripple at various currents with various capacitor values. There is a formula on the graph for you to convert to a 50Hz mains frequency.
Interesting chart, been looking for something like that myself. What is the source, if I might ask?

#### Audioguru

Joined Dec 20, 2007
11,251
I don't know where my chart came from.

#### Ron H

Joined Apr 14, 2005
7,012
Interesting chart, been looking for something like that myself. What is the source, if I might ask?
See Fig. 7 here. Looks like someone (AG?) added a line for 32000uF.

#### floomdoggle

Joined Sep 1, 2008
217
Thanks for the responses. Sarge, the caps will be used to smooth rectified AC, will they work for that?
Audioguru, I don't understand how the chart works. I'll be using 200rpm as my full charging speed.
Dan

#### thingmaker3

Joined May 16, 2005
5,073
(Revolutions per second) times (pairs of poles) equals frequency.