# What is the formula used for AC to DC using a full-bridge rectifier?

#### Austin Jasss

Joined Jul 13, 2017
5
I am trying to have 120v AC to 12v Dc, but I can't find a formula to get it.

#### crutschow

Joined Mar 14, 2008
29,465
A full bridge rectifier DC output with a capacitive filter will be about 1.4 * Vrms - (2 * Vd) where Vd is the forward drop of one diode in the bridge.
So for 12Vdc out and a silicon junction-diode bridge you will need a transformer with 120Vac in and ≈10Vac out.

#### MrAl

Joined Jun 17, 2014
8,861
I am trying to have 120v AC to 12v Dc, but I can't find a formula to get it.

Hi,

I think you need to be a little more specific with this.

What kind of formula are you looking for?
Also, do you intend to use filtering too, and if so, what kind of filtering (no filtering or capacitive or capacitive+inductive, etc.)?

#### Austin Jasss

Joined Jul 13, 2017
5

Thank you for the help. the circuit is what I want to power with 120v AC.

Joined Jul 18, 2013
24,973
Wall Wart.
Max.

#### Janis59

Joined Aug 21, 2017
1,383
Formulas is dependant on (1) do You have a capacitor after rectifier or not, (2) do You have a sin signal or probably meandric (1,11 surplus factor), and does Your load is small enough to not bleed the capacitor effect. If those all questions are ok, then standard 220/380 Volt and 50 Hz grid generates 311V in one phase (4 diodes) network and 535V in three phase (6 diodes) network. The voltage loss on diodes (Uf) here is taken in account (without it is 314 and 538). If have an another standard, calculate proportionally.

#### wayneh

Joined Sep 9, 2010
17,163
Thank you for the help. the circuit is what I want to power with 120v AC.
The answer from @crutschow in #2 is what you need for that circuit. But don't shoot for the maximum voltage the circuit can tolerate. That might risk destroying it. It might be better to shoot for 6-9V, to give yourself some room.

#### crutschow

Joined Mar 14, 2008
29,465
But don't shoot for the maximum voltage the circuit can tolerate. That might risk destroying it. It might be better to shoot for 6-9V, to give yourself some room.
Yes, for that circuit the supply voltage should be no greater than the reverse Vbe rating of the transistors you use.
For the 2N9014 that's 5V.

#### gerty

Joined Aug 30, 2007
1,302
If it's a formula for converting ac to dc using a transformer and rectifiers, try this

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