How can I convert AC current to DC voltage?

[mzypt]

I'm using data logger with DC voltage input and I need to log AC current with it. How can I convert AC current to DC voltage? The best way to do this is AD536 or similar RMS converter chip but it's quite expensive. Is there any other way to this, like opamp, resistor and capacitor circuitry?

 

[Old Nick]

Re: AC current converter

transimpedance amplifier to convert into voltage.

remove DC if necessary, then rectify and smooth. This should give you a DC level that corresponds to the Peak Voltage level (-some diode drops)

 

[lladnar23]

AC current converter

Depending upon the particulars, you might be able to make an envelope detector: Just run the AC through a series diode, then into a cap.

 

[mzypt]

AC current converter

Yep, this circuit works pretty well. I added one more opamp to compensate zero output 0.88V which I couldn't get rid of. . So I can raise the virtual ground level with 22k potresistor so that output reading is zero. But my datalogger has USB interface and I would like to use USB 5V as power source for run my design. The problem is that DC input ground and USB ground are the same so zero output compensation doesn't work. When input AC current is zero - logger measures 0.88volts. This is annoying since I can't get rid of that voltage. I tried another differential amp to compensate errorvoltage but it does'n seem to work that way, I think Vin1-Vin2=Vout works only if you have double powersource (+-5V).



Added after 7 minutes:

Oh and yet another question. Why this design needs transistor to amplify the AC input (10-30mV). I tried to drive AC directly to opamp, but it didn't work. With transistor it works well. Can it be that since transistor is drived by current and opamp by voltage - this voltage, 10-30mV is way too small for opamp. Well anyway I couldn't get any decent readings with AC input coupled with capacitor connected to opamp input.

 

[mzypt]

AC current converter

I tried to use ICL7660 to provide -5V. But somehow I got only -2.8Volts with 5V Vcc. Why only -2.8V? And when connecting to LM324 it dropped to 0.52V? I had two chips to try and both acts similar way. It has only few external components, two 10uF caps. Why I can't drive opamp with 7660? This would correct my initial problem with 0.88V "zero" output.
Please help me on this.

 

[mzypt]

AC current converter

No worries, I found the problem - there were a bad connection and 7660 didn't get 5V at all. Now everything works perfectly

 

[kareba]

Re: AC current converter

Hi mzypt

What is your rated current value?

 

[FvM]

AC current converter

The rectifier circuit design is like re-inventing the square wheel. A simple peak detector with a temperature dependant offset.
OP have been invented to get rid of these things. But you don't manage to use their properties right.

 

[kareba]

Re: AC current converter

can u specify the op