# Capacitor sizing for no-Amp load - Bridge Rectifier

#### jagermini

Joined Dec 19, 2017
3
Hi folks!

I've got an alternator providing 3 phase voltage at 12.4V AC. I'm putting one of these phases through a full wave rectifier diode bridge, which brings it down to around 11V DC or thereabouts.
This is then going through a voltage divider, and into an Arduino to measure voltage.
I can't find any formula for capacitor size calculation to smooth out the ripple of the regulated voltage, that doesn't include current. Since I'm sending it into an Arduino, I believe there is no current draw.

Is there another formula to calculate this, without needing a current value?

Or can I just stick in a 100uF capacitor and expect it to do a reasonable job??

#### ericgibbs

Joined Jan 29, 2010
16,421
hi jag,
Welcome to AAC.
What value of resistors are you using for the divider.?
E

#### jagermini

Joined Dec 19, 2017
3
Hi Eric!

I'm using a 3.3kOhm and 9.1kOhm to bring the 11V DC down to around 3 Volts for the Arduino.

Those figures are approximate as I don't have my notes with me, but it's as indicated, or very close.

#### ericgibbs

Joined Jan 29, 2010
16,421
hi j,
With 100uF and a low 50Hz this is the ripple level.
E

Update: @jagermini
I guess you know that when your 12.4Vac is full wave rectified and smoothed the Vcap = (12.4v *1.4) - 2*Vdiode voltage drop = ~16Vdc, so the 9k1 resistor value will have to be increased to ensure the Arduino input is still 3Vdc.? Change the 9k1 to 15k

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#### crutschow

Joined Mar 14, 2008
30,822
I can't find any formula for capacitor size calculation to smooth out the ripple of the regulated voltage, that doesn't include current.
That would be the current through the voltage divider.

#### jagermini

Joined Dec 19, 2017
3
Ok, cheers guys for the responses. Yep, the current through the voltage divider, that is a fair point, had not occurred to me......!

I'll have a look at those links, thanking ye!

#### ebp

Joined Feb 8, 2018
2,332
What frequency range do you expect from the alternator?