Help with OP-AMP circuit - Page 1
Forum section: electronics / projects

author: ajoseph88
author_rank: Contributor
timestamp: August 03, 2015, 05:22:37 pm
content: So this is the current feedback circuit, the resistance Rb is used to convert the current from the current transformer (0 to 10mA) to voltage, and that is passed through diode bridge and then passed through low pass filter, and if i measure the voltage across the capacitor with an oscilloscope the, the voltage seems to be good, it varies from 0 to approximately 4 V... but if i put an OP-AMP, and use it as a buffer amp, the output shows 6V and constantly increases... I know that there is something wrong with my circuit, and i tried connecting a resistor of 10K across the capacitor (in between capacitor and op-amp, so that the capacitor can discharge) but it did not work...Any help, much appreciated...

author: dom0
author_rank: Super Contributor
timestamp: August 03, 2015, 05:43:29 pm
content: What op? Supply voltage?Driving 1 µF with an op amp will be troublesome unless specific (infinite capacitive load drive) ops are used or the circuit is adjusted accordingly. Specifically the 1 µF seem unnecessary to me here.Current direction arrows are usually more drawn like these:http://www.talkingelectronics.com/projects/Diode%20-%20How%20A%20Diode%20Works/images/CurrentFlow-2.gifto avoid the diode confusion you note in your schematic.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2015, 06:01:56 pm
content: The current circuit is not very linear, especilly at small currents.The shunt resistor should be better behind the rectifier. To compensate for the extra voltage drop, the shunt should be smaller and the OP have some (e.g. 5 fold) amplification. This gives better linearitiy and lower maximum voltage at the sensor, with just 3 extra resistors.

author: dom0
author_rank: Super Contributor
timestamp: August 03, 2015, 06:25:24 pm
content: Then put a normal RC low pass, not just C, at the op output.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2015, 07:57:36 pm
content: The extra 1 µF at the output of the OP is bad. Something like a 1-10 K resistor to the µC is a good idea. It limits the current in case of a malfunction (the µC will likely survive) and filters noise from the µC. Depending on the ADC and sampling rate a small cap (e.g. 10 nF) directly auf the µC is good.If the µC has enough calculation power left, the ADC could also sample the AC-signal and do rectification (e.g. RMS calculation) in software. So no OP needed in this case.

author: dom0
author_rank: Super Contributor
timestamp: August 03, 2015, 09:01:27 pm
content: Quote from: dom0 on August 03, 2015, 05:43:29 pm

author: Circlotron
author_rank: Super Contributor
timestamp: August 04, 2015, 03:39:30 am
content: Your circuit has two problems.As others have mentioned, put the 510 ohm burden resistor =after= the bridge rectifier, i.e. where the 10K connects to it and ground. This will fix the problem of indeterminate forward voltage drop of the diodes because the current tranny is a *current* source so now diode voltage drop won't matter.Putting this resistor after the bridge will fix the second problem. You have discovered opamp Input Bias CurrentHaven't watched the video, found it just then on a Google search.This current is charging the 0.47uF cap and so making the output voltage drift upwards. With a bipolar opamp we are only talking about a microamp or so, with a fet opamp it's perhaps a thousand times smaller.Anyway, the resistor placed after the bridge will also stop the cap charging up and causing problems. Actually it will do something else too - it will prevent the cap just charging up to the peaks because as the bridge voltage waveform goes below cap voltage it will pull the RC downward and give a true averaging effect.Move one resistor and fix three problems!P.S. Is your opamp a "single supply" opamp like for example LM324? If not then it won't like having it's input taken right down to ground.

author: ajoseph88
author_rank: Contributor
timestamp: August 10, 2015, 03:27:10 pm
content: Quote from: Circlotron on August 04, 2015, 03:39:30 amYes, its a single supply opamp... it is LM324n.

author: zapta
author_rank: Super Contributor
timestamp: August 10, 2015, 06:00:56 pm
content: Quote from: DiligentMinds.com on August 03, 2015, 09:20:22 pmThis will also allow the 0.47uf to discharge as the input current drops. The original circuit is peak detector with very flow decay.