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# how to minimize the voltage ripples after the rectifier?

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#### JackyH

##### Newbie level 6
in order to convert the AC supply to DC supply for the internal circuit, normally a rectifier is needed. But the output of the rectifier can not be a constant and always has the ripples.

Are there any methodes to minimize the ripples after the rectifier?

Or is it possible to obtain a constant voltage after the rectifier? for example, to use a voltage regulator?

After the rectifier you need a capacitor, mostly an polarized capacitor, make the 100 or 120Hz (2x 50 / 2x60Hz) ripple smaller. After that you can use a linear voltage regulator, resistor/zener, switchmode regulator etc to make the ripple small enough for your application.

The value of the pol. capacitor depends mainly of the current you need for your application.

### JackyH

Points: 2
thanks.
when my circuit needs 5mA for the operation, which capacitance value should I take?

That depends of the maximum voltage ripple your application accepts.

For a bridge rectifier the formula is ~:

Uripple = 1.5 * (Io / C) (Io in mA, C in µF)

So let's say your maximum ripple is 20mV -> C = 0.38µF -> E6 range -> 470nF

remark: your DC-output voltage will be higher because of the pol-cap.

~?2*Ueff(ac) -> 9V ac -> ~12.6V! (-2* voltage diode drop ~0.6V)

### JackyH

Points: 2
As far as ripple rejection is concerned there is no better alternative as voltage regulators - on average they offer at least 60dB ripple attenuation ..
One can only dream of similar result while using passive components such as smoothing capacitors, RC or LC filters, not to mention another important feature, namely constant output voltage vs. connected loads ..

Regards,
IanP

### JackyH

Points: 2
are there any more detailed information or reference regarding the design of voltage regulator for this application?

thanks

IanP said:
As far as ripple rejection is concerned there is no better alternative as voltage regulators - on average they offer at least 60dB ripple attenuation ..
One can only dream of similar result while using passive components such as smoothing capacitors, RC or LC filters, not to mention another important feature, namely constant output voltage vs. connected loads ..

Regards,
IanP

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