# AC to DC converter (using envelope detector)

Discussion in 'General Electronics Discussion' started by Abidethedude, Aug 31, 2012.

1. ### Abidethedude

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0
Aug 31, 2012
Hey guys, just a quick question.

I need to create a circuit that can convert AC signal to DC using an envelope detector (or envelope follower). Simply put, I want it to behave like this:

http://en.wikipedia.org/wiki/File:Analytic.svg

Basically I have an input source of 1VRMS and the range of frequency is up to 3k hz.

I found some information about envelope follower, but it deals with AM signals.

If anyone can point me out the direction where to find such information, that would be great!

saludos

2. ### duke37

5,364
772
Jan 9, 2011
Your example is an AM signal.
Do you just want to measure the amplitude of a constant sine wave?
It all depends on the accuracy you want.
A full wave precision rectifier followed by a low pass filter would be about as good as you can do.

3. ### Abidethedude

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Aug 31, 2012
Yes, just the amplitude of the constant sine wave.
Ill look into the full wave rectifier. Thanks!

4. ### Abidethedude

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Aug 31, 2012
I've looked at the full wave rectifier with low pass filter. Although it worked for a constant AC signal (with no varying in frequency), now I am looking for a way for an incoming AC signal, which has varying frequency, to output a DC signal.

5. ### Electrobrains

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6
Jan 2, 2012
It's a matter of your LP filter setup!

What is the minimum frequency of the carrier signal (you wrote the maximum is 3kHz)?

And what is the frequency range of the modulating signal (the envelope)?

If the two signals are close with respect to their frequencies, you will get problems. It might not be possible to detect the envelope, or you would need a high order LP filter.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Essentially you design your rectifier and filter with the lowest frequency in mind. Higher frequencies (within reason) will work with it too.

7. ### Abidethedude

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Aug 31, 2012
Thanks for responding!

Allright, so I guess here's what I want to achieve (in full detail)

Currently, I am in the process of building a theremin and the volume output will have around 1 VRMS (or 0.8 VRMS to be exact). The pitch frequency will have roughly around from 100hz to 3k hz. So you can think of having a function generator that will create sinusoidal wave of 1VRMS will the range frequency from 100hz to 3k hz.

My overall goal is to have the current AC amplitude convert to DC,

I am not familiar with frequency carrier (I've looked what it means, but still not quite clear)

Any thoughts?

8. ### duke37

5,364
772
Jan 9, 2011
It does not look as if you need anything special. With a voltage input of a volt or so, a simple diode detector will not be adequate but an "amplified diode" will do the job. This will need to feed a capacitor which should have a resistor across it to discharge it when the signal stops. The time constant of the resistor and capacitor (R * C) should be about 0.1sec

What will the output be used for? Does it need to be high voltage or current?

Have you contacted Dr Who?

9. ### Abidethedude

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Aug 31, 2012
The output will feed into an scaling amplifier which then will feed into an Arduino UNO board (the Ardino takes an analog voltage from 0-5 volts). The scaling amplifier is complete

When you say amplified diode, does that involve biasing the amplitude to compensate the loss from the voltage drop of the diode?

10. ### duke37

5,364
772
Jan 9, 2011
This is a circuit of a simple diode detector and an amplified version.
This could be made into a scaling amplifier by tapping down the resistor to provide the feedback.

I am not into the latest amplifiers but you will need one which is fairly agile and, if you only have a single polarity supply, you will want one which will work off this.

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11. ### foTONICS

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Sep 30, 2011
bandpass filter?

12. ### Abidethedude

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Aug 31, 2012
Sorry for the late response, had to deal with some paper work.

Anyway good news! I was able to get the results. For those who are wondering, here's what I did:

Basically I ended up using LM324 as my op amp. Since it works well with operating in small voltage, it was good to go.

Like I've said, the AC source is producing 0.8 VRMS (frequency does not matter at this point, since we are only considered converting AC to DC).

Here is my setup:

Source--> Summing Amplifier --> Envelope Follower --> Differentiate Amplifier --> Buffer with low pass capacitor.

In this case, I've used a 2V as a reference to boost up the signal in the summing amplifier and the differentiate. The reason why I did that is because I needed the signal to be in range where the op-amp is active. After the envelope follower, I needed to scale down back to it's original amplitude, hence a differentiate amplifier was used. The buffer is simply used to polish the signal.

Message me if you have any questions!

saludos