[Moved] Precision rectifier shows lower values than what i have input

[GeekDOn]

Hi All

i have a problem with my full wave precision rectifier using LM358 , i observe lower voltages than what i have as input,
firstly i drew the circuit on a simulator and fed in dc as input with a variable resistor connected, the output measured is the same as the DC input ,

when i built the hardware i put an 8V battery as input , but on the output of the LM358 my multimeter reads voltages of around 3,5 Volts, i resorted to building a more simpler half wave rectifier which does the same, the question is , where did i drop that much volts?

When i put power on pin 8 of the LM358, with nothing on the Input (inverting input) i get some voltage of ~89mV , i cannot be generating power from nothing , where is the voltage coming from? since i do not have any input ?

:sad:

 

[albbg]

Re: Precision rectifier shows lower values than what i have input

Please post your circuit.

 

[GeekDOn]

Re: Precision rectifier shows lower values than what i have input

not sure if you want the circuit diagram of the precision rectifier or the one i have built .. but i have attached the physical circuit , you can find the diagram here : https://en.wikipedia.org/wiki/Precision_rectifier
i used LM358 as explained before, if you can try to use my scenario and tell me your observation?

 

[albbg]

Your circuit is an inverting half wave precision rectifier. This means that a positive voltage at the input will be transalted to zero at the output, while a negative voltage will be translated in a positive voltage, with the same absolute amplitude (you set the gain to unity) at the output.
But to do this the op-amp needs a dual supply voltage (pin 8 - GND of the circuit and GND of the circuit - pin 4), while I see you used a single supply source.

 

[GeekDOn]

Hi

Thanks for the Reply, im not really clear on it , from the diagram below , Pin 8 is my Vcc , while Pin 4 is my ground ,
when you say dual supply , do you mean i should modify my circuit as the one below:

Secondly :
from what you have explained its clear that
If Vin< 0, Vout = -Vin , and when
Vin > 0 , Vout = 0 the problem here is that i measure some ~82mV

but the Logic makes sense, if you can just confirm for me if the attached rectifier should work?

 

[albbg]

Yes, I think your circuit can work. As I said, you need a dual supply like this:

 

[crutschow]

There are a number of full-wave precision rectifiers that only require a single power supply, see this.

Edit: This one seems fairly simple requiring only two op amps or one dual (single-supply types), three resistors and a diode.

Below is a simulation of the circuit.