# Full Wave Rectifier with Variable Load

#### sn369

Joined Nov 19, 2015
3
Hello,

First off I'm a complete novice when it comes to electronics but I'll try and provide as much information as I can! I'm trying to convert an AC signal to a DC. I believe the best method is to use a full wave rectifier with a capacitor. The AC input can have a variable voltage (though with a max of 1V) and the load is variable too (unknown). How can I calculate what diodes and capacitor I require with an unknown DC load?

Thanks for the help!

sn369

#### MikeML

Joined Oct 2, 2009
5,444
Since the forward voltage drop of Si diodes is >0.65V, it is tough to make a passive rectifier that is "linear" (VoutDC proportional to VinAC) .

You will need an active circuit utilizing an opamp called a "precision rectifier" Google it...

#### sn369

Joined Nov 19, 2015
3
Right I'm with you so from what I can gather it's essentially a full wave rectifier coupled an op amp to boost the Vin? Even if it's boosted wouldn't I still have the problem calculating what diodes and capacity I need?

#### dl324

Joined Mar 30, 2015
17,155
How are you going to use the rectified, filtered AC? Is it ground referenced (symmetrical about ground), or does it have a DC offset?
Right I'm with you so from what I can gather it's essentially a full wave rectifier coupled an op amp to boost the Vin?
No. A precision rectifier would rectify without incurring a diode voltage drop. This is important when you're trying to rectify a voltage where a diode drop is a significant portion of the voltage.
Even if it's boosted wouldn't I still have the problem calculating what diodes and capacity I need?
It depends on how the filtered DC is going to be used. If you're rectifying and filtering, are you really just looking for positive peak voltage?

#### wayneh

Joined Sep 9, 2010
17,505
I'm trying to convert an AC signal to a DC.
By using the word "signal", can we assume you are not talking about power conversion? For power, the specifications you need are well defined by the maximum load you want to serve, and the acceptable ripple.

For information processing, you need far more detail about what you are you doing and the specifications, in order for anyone to help you.

#### sn369

Joined Nov 19, 2015
3
Hi everyone. Firstly thanks for the messages. dl324 you were right I did need a precision rectifier. I think I got confused when I originally asked the question. I needed to convert an AC to DC of an equivalent voltage which I've done. Thanks for the help everyone and sorry for the confusion.

#### dl324

Joined Mar 30, 2015
17,155
I needed to convert an AC to DC of an equivalent voltage which I've done.
Are you just looking for the peak voltage? Or do you really need to full wave rectify and filter?

A peak detector is a simpler circuit.