# Full Wave Rectifier with Variable Load

Discussion in 'General Electronics Chat' started by sn369, Nov 19, 2015.

1. ### sn369 Thread Starter New Member

Nov 19, 2015
3
0
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

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
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...

3. ### sn369 Thread Starter New Member

Nov 19, 2015
3
0
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?

4. ### dl324 Distinguished Member

Mar 30, 2015
3,235
619
How are you going to use the rectified, filtered AC? Is it ground referenced (symmetrical about ground), or does it have a DC offset?
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.
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?

5. ### wayneh Expert

Sep 9, 2010
12,090
3,027
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.

6. ### sn369 Thread Starter New Member

Nov 19, 2015
3
0
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.

7. ### dl324 Distinguished Member

Mar 30, 2015
3,235
619
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.