# Precision Full-Wave Rectifier Question

#### Granosx

Joined Apr 30, 2012
4
Hey all, I have a question regarding simulation of a precision full-wave rectifier circuit ( using op-amps ).

I have done a lot of research and still cannot get the issue resolved. I am using multisim 11 if that helps.

My professor ( he never taught a thing about it by the way ) asked me to create a precision full-wave rectifier with an input 110(√ 2) sin(2∏60t). In other words he wants about 155 volts if I am correct at 60 Hz, to give a full wave output using op amps. In my attempts, I have simulated a precision full-wave rectifier that works perfectly, but only when I use 1 volt input, anything above 5 and the output is ridiculous. I have attached my simulations thus far. The first image the output isn't working as it should, while the second one, I am getting a half-wave, I have even reversed a diode to opposite polarity and it does not help.

I am not asking for anyone to do my project or anything, but would like to be pointed in the right direction, as this professor just came out of a coma, and for whatever reason they allowed him to "teach" electronics 2.

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#### #12

Joined Nov 30, 2010
18,216
It looks to me like you have copied the classic full wave rectifier perfectly, but you only gave the op amps +/- 10 volts to work with.

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#### Granosx

Joined Apr 30, 2012
4
It looks to me like you have copied the classic full wave rectifier perfectly, but you only gave the op amps +/- 10 volts to work with.

I am using virtual op-amps in multisim, with +/- 5v as VCC/VEE respectively. Changing the voltages doesn't seem to change the output at all. Were you suggesting that I didn't give enough dc voltages to the op-amps?

thanks!

#### #12

Joined Nov 30, 2010
18,216
Sorry, I don't sim. I just know that in the real world an op amp can't output volts it doesn't have.
Seeing that you have made a perfect copy of a classic circuit, but it doesn't work, the problem is either the parameters you provided or the sim program isn't working right, and I am not qualified to decide that.

Next helper?

#### Granosx

Joined Apr 30, 2012
4
Sorry, I don't sim. I just know that in the real world an op amp can't output volts it doesn't have.

Thanks a ton! I will see what I can figure out and post back.

#### Audioguru

Joined Dec 20, 2007
11,249
It is easy to adjust the values of only two resistors in the first circuit so it can handle the very high input voltage.

The second circuit gives a half-wave output because it is supposed to be a precision half-wave rectifier. But it is missing an important input resistor. The value of the input resistor determines the input signal voltage it can handle, just like in the first circuit.

#### Granosx

Joined Apr 30, 2012
4
It is easy to adjust the values of only two resistors in the first circuit so it can handle the very high input voltage.

The second circuit gives a half-wave output because it is supposed to be a precision half-wave rectifier. But it is missing an important input resistor. The value of the input resistor determines the input signal voltage it can handle, just like in the first circuit.

Thanks so much! I was able to get the full-wave . I went with the classical model of precision full-wave, and just adjusted resistor values / op-amp settings, and was able to get full wave rectifier.

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#### #12

Joined Nov 30, 2010
18,216
It is easy to adjust the values of only two resistors in the first circuit so it can handle the very high input voltage.

The value of the input resistor determines the input signal voltage it can handle, just like in the first circuit.
I don't get it. Scaling the resistors did not change the output voltage magnitude. If these imaginary opamps can deliver nearly a hundred volts, and they are doing it in both circuits, what changed? The necessary curent changed from nearly 10 ma to less than 2 ma, but if an imaginary opamp can deliver nearly 100 volts, why can't it deliver 10 ma?

#### Audioguru

Joined Dec 20, 2007
11,249
I don't know why you are playing with imaginary circuits. I thought you are designing a circuit that works and uses normal opamps with a plus and minus 10V power supply.
Then the input resistor values must be very high so that they attenuate the 110V down to about 6V.

#### Ron H

Joined Apr 14, 2005
7,014
Thanks so much! I was able to get the full-wave . I went with the classical model of precision full-wave, and just adjusted resistor values / op-amp settings, and was able to get full wave rectifier.
The right hand end of R4 needs to be moved to the anode of D2.