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# Add dc voltage to ac signal

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#### katsar

##### Newbie level 5
Hello
I am using a current transducer for measure current from a pc. The pc's current is a about 0,5 A. So I use a current to voltage converter with op amp to turn current into voltage. But my controller needs only positive signals so I have to add a dc offset to my signal. I have the circuit which uses a cap and two equal resistors but the output of that circuit is different from the original signal. The frequency is from 50 hz to 200 hz. So the solution is to use a summer with op amp. But I don't know how to build it. Can you help me? Can you show me how to do it? I had many failures trying to make it work but nothing. Thank you

If your op amp has a single polarity supply, then you might get by with this method:

The op amp needs to be the type that can operate from a single supply. Hence a 741 type is excluded.

It may not fit your setup. It's just to illustrate.

From what I see may this isn't suitable for my project. I need negative values too. I am using the TL071CP op amp which has +-15V.

From what I see may this isn't suitable for my project. I need negative values too. I am using the TL071CP op amp which has +-15V.

If you need the entire AC signal...

This schematic will lift it all into the positive region.

When no current is flowing in the leftmost wire, the output will be 2.5V.

The resistor values will need to be fine tuned as desired.

The sense resistor value is 200m?

The sense resistor value is 200m?

Not knowing how you arrange the current transducer, I used a simple low-ohm resistor to create a small voltage across it.

A low value is recommended in order not to subtract from the load any more than necessary.

Your current is stated to be 0.5 A.
Hence 0.2 ohm is sufficient to generate a voltage which can be detected by the op amp.
This is amplified 47x to fit the typical 0-5V range of a microcontroller.

Thank you very much. I will try it.

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