If I have an AC voltage source that periodically falls to 0volts, is there a method to full-wave rectify only the AC portions of the wave?

For example, if the input wave looks something like this:

 /\    /\    /\             /\    /\    /\             /\    /\    /\  
/  \  /  \  /  \-----------/  \  /  \  /  \-----------/  \  /  \  /  \-----------
    \/    \/                   \/    \/                   \/    \/

Is there a simple method to achieve this:

  ____________               ____________               ____________
 /            \             /            \             /            \   
/              \-----------/              \-----------/              \-----------

Instead of this, which full-wave rectification gives:

  ____________               ____________               ____________
 /            `--__         /            `--__         /            `--__ 
/                  ``------/                  ``------/                  ``------

I suppose I'm thinking of a capacitor that just maintains the voltage from one peak of the AC to the next, but then dies off almost instantly, so that it doesn't continue into the 0volt range.

I just had a thought; I might be able to use logic gate(s) to give the capacitor a low-resistance path to ground, once the input drops below a certain voltage.

That's a bit more complex than I had in mind though, so I'd be interested to hear if there is a simple way to achieve this result.

 

OK...........from square 1 :

Is the first graph representing your o' scope trace for an AC source to be fed to a rectifier ??

You should be showing an unbroken sinewave in either 60 or 50 Hz based on what part of the world you are in

We'll go from there.

 

I don't think your third trace shows full wave rectification of the first.

I think this is what happens if you add a capacitor to a full wave rectifier.

Full wave rectifiers do not incorporate capacitors by themselves.

 

Do you have a load resistor across the cap?

 

Generally this is called demodulation of a amplitude modulated wave. This should help you find more info. You are on the right track for a simple demodulator.

 

What if you don't use a cap at all, then overamplify with an op-amp to sky the voltage as soon as you have any rectified signal at all? The op-amp will just max out at its positive input voltage. This assumes you don't need a high power output....

 

It looks like you just want a diode, unless you want to maintain the voltage, in which case a cap + suitably chosen resistor will work well.

 

Thanks for the replies, and sorry for my confused terminology. "Demodulation of a amplitude modulated wave" is exactly what I was after.

 

http://www.allaboutcircuits.com/vol_3/chpt_9/6.html

 

What is the time scale on your plots?

 

What is the time scale on your plots?

I had no particular time scale or frequency in mind. Well, I was working with a frequency of 100Hz, but thinking of the general case of a digital-like signal, where the "high" sections are a sine wave, and I want to make it more digital-like. "Demodulation" put me on the right track.