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# A Rectifier with a Filter Capacitor

## A Rectifier with a Filter Capacitor

(OP)
I am working on connecting a bridge rectifier and a capacitor to my AC 3 phase generator to get a DC power with small voltage ripple. But, I don't know how to choose a correct capacitor.

Is there anyone can share some information about how to choose a right capacitor to me?

My generator is going to produce about 100W (8V~60V).

Thank you very much.

### RE: A Rectifier with a Filter Capacitor

We need a little bit more info Peter, Since you mention 8V to 60V, are these the phase-to-phase peak voltage levels from your generator due to various rotational speeds? I'll make some assumptions.

For calculating ripple, a good approximation can be made assuming that rectification only occurs in pulses at the top of the sine peaks (terrible power factor) and decays between the sine peaks. For example, if it's a 6krpm generator with 1 pole-pair, you'd have ~1.6ms between rectification. If you want <1V ripple at 1.7A out of your 60V, the capacitor would be: C = 1.7A*1.6ms/1V = 2,700uF.

Darrell Hambley P.E.
SENTEK Engineering, LLC

### RE: A Rectifier with a Filter Capacitor

Evox-Rifa published an excellent applications guide a few years ago. Kemet don't seem to have it listed since they took over, but there are a few copies out there, for example http://materias.fi.uba.ar/6648/archivos/RIFA_elect.... It's all useful information but page 9 is probably the most helpful for your specifc question.

### RE: A Rectifier with a Filter Capacitor

(OP)
Yes, the power and voltage will change due to different RPM.

Our generator is 3 phase 12 coils and 16 magnets. How to calculate C in this case. Do I use the parameters of maximum RPM to calculate the C?

and how about the voltage of capacitor? how to choose the right value? If the maximum output voltage of my generator is 60V, can I just choose 60V capacitor?

Thank you all!!!!

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