Precision rectifier opamp help..

[avicreats]

Hello everyone,
I have a precision rectifier for the front end of my circuit.The input to the rectifier is from 1 ohm resistor which is shorted to the current transformer. The rectifier i use converts ac voltage to pulsatting dc. So to make it into pure dc,i used a 1000micro f capacitor. Then my circuit was not linear as the o/p from rectifier was not proportional to the input..I have attached the circuit without the capacitor whwer it will be connected to pin 7 and ground...Help will be much appreciated

.

 

[ericgibbs]

hi avi,

Are you requiring Peak DC voltages from the 'smoothed' fullwave rectified output.?

E,

 

[crutschow]

The circuit you posted is a half-wave rectifier although you show a full-wave rectified signal at the output stage. If you want a full-wave rectified signal then you need a different circuit.

You can't connect a capacitor directly to the output of an op amp and expect to get a linear DC output. The poor op amp is madly attempting to charge and discharge the large capacitor as it tries to follow the rectified signal level which means it's likely operating in an output current limit mode.

If you want the average value of the rectified waveform then add a resistor in series with the output of the op amp to the capacitor filter. The resistor must be large enough to keep the peak current below the output current rating of the op amp.

 

[avicreats]

hi avi,

Are you requiring Peak DC voltages from the 'smoothed' fullwave rectified output.?

E,

If you want the average value of the rectified waveform then add a resistor in series with the output of the op amp to the capacitor filter. The resistor must be large enough to keep the peak current below the output current rating of the op amp.

An average dc voltage is needed as output..and As crustchow told, Il add a HIgh range resistor to the o/p to the capacitor and check it out...Thank you...

 

[ericgibbs]

hi avi,
This is a circuit I have used in the past to give a Peak value.

The LTSpice plots show a circuit with a constant current discharge version and a fixed resistor version.

To convert to RMS or Average just use a resistive divider on the output.

E.

 

[avicreats]

hi avi,
This is a circuit I have used in the past to give a Peak value.

The LTSpice plots show a circuit with a constant current discharge version and a fixed resistor version.

To convert to RMS or Average just use a resistive divider on the output.

E.

Thanks for the circuit.I think your circuit works on dual supply opamps..
But i need the circuit to work with a single supply rail..Thankyou..

 

[ericgibbs]

Thanks for the circuit.I think your circuit works on dual supply opamps..
But i need the circuit to work with a single supply rail..Thankyou..

hi,
The resistive discharge version is only a single supply.

E.

 

[avicreats]

hi,
The resistive discharge version is only a single supply.

E.

Yes i got that...Thank you..

 

[ericgibbs]

Yes i got that...Thank you..

hi,
This is the method I would use for setting Average or RMS Vout, use a 10K pot as this image.

The final OPA stage will also buffer the circuit when driving an external load.

What is the Vout driving.??

E

 

[crutschow]

This is the method I would use for setting Average or RMS Vout, use a 10K pot as this image.
..................................

That circuit will only measure the average and not true RMS of course, although you can adjust the resistor values to give the RMS value (similar to what an average responding DVM does).

 

[ericgibbs]

That circuit will only measure the average and not true RMS of course, although you can adjust the resistor values to give the RMS value (similar to what an average responding DVM does).

morning Carl,

That is what I said.

This is the method I would use for setting Average or RMS Vout, use a 10K pot as this image.

Eric

 

[crutschow]

morning Carl,

That is what I said.


Eric

OK. Just wanted to emphasize that the circuit generates a representative voltage of the RMS value based upon the average, not the true RMS. Perhaps I was overstating the obvious.

 

[Roff]

The circuit you posted is a half-wave rectifier although you show a full-wave rectified signal at the output stage. If you want a full-wave rectified signal then you need a different circuit.

It is a full wave rectifier. The gain is 0.5.

 

[crutschow]

It is a full wave rectifier. The gain is 0.5.

Yes, I see that now.

 

[avicreats]

Thanks everyone for ur help..

Actually I got my problem solved..I added a high value resistor in series to line of parallel connected capacitor were crust how to.ld..
And I'm working on Eric Gibbs schematics and will check out the results..
Thanks everyone..much appreciated..

 

[audioguru]

The circuit is a FULL-WAVE rectifier when it uses the LM358 opamp.
When the input is positive then the inverting opamp has its output diode disconnect it from the output so the output is simply half the input voltage due to the resistor values.
When the input is negative then the output of the opamp goes positive at half the voltage of the input.
The lousy old LM358 is low power and old so the circuit works only at low frequencies.

 

[avicreats]

The circuit is a FULL-WAVE rectifier when it uses the LM358 opamp.
When the input is positive then the inverting opamp has its output diode disconnect it from the output so the output is simply half the input voltage due to the resistor values.
When the input is negative then the output of the opamp goes positive at half the voltage of the input.
The lousy old LM358 is low power and old so the circuit works only at low frequencies.

Yes,Audio guru,the circuit works well for low frequency due to the slow operation of lm358. In fact the circuit is used to detect the line current of commercial building using a ct(current transformer) with the frequency of 50hertz..so it works a lot better for me...

 

[moisture]

hi avi,
This is a circuit I have used in the past to give a Peak value.

The LTSpice plots show a circuit with a constant current discharge version and a fixed resistor version.

To convert to RMS or Average just use a resistive divider on the output.

E.

Are you using 741 opamp for the single supply or resistive discharge version? I have tried it but not getting the required output, although simulation on Multisim works fine, what's the reason for this, opamp selection or something else? Please also mention opamp model #

 

[ericgibbs]

hi,
I used a CA3240 OPA.

The 741 OPA is not suitable for this application.

E

 

[audioguru]

It is time for the lousy old 741 opamp to be buried.
There are many much better opamps available today.