What circuit would you recommend to measure AC current - Page 1
Forum section: electronics / projects

author: kuon
author_rank: Regular Contributor
timestamp: February 13, 2023, 01:24:04 am
content: I have an AC current coming from current sense transformer, this onehttps://www.digikey.ch/en/products/detail/tdk-corporation/CCT261631-30-06-00/5126209I was going to do a simple circuit with a resistor, a rectifier and a capacitor and feed that to an MCU analog input, but thinking about it I had other ideas, like using voltage divider without rectifying and sampling at higher frequency...How would you do it?

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2023, 02:08:47 am
content: CT's with a load resistor have low output voltages, from the datasheet to get 100A without saturation you need a 100R load resistor which gives about 12mv/A output. I would use a precision rectifier using opamps(google it) then filter that output.

author: pqass
author_rank: Super Contributor
timestamp: February 13, 2023, 02:26:27 am
content: I would use a precision rectifier.This one seems well documented:https://resources.altium.com/p/current-transformers-how-projectIf you are going to calculate power factor, see:https://mousa-simple-projects.blogspot.com/2017/12/power-factor-measurment-using-arduino_18.html.Using comparators, the voltage and current sine waves are turned into square waves (at zero-crossing). XORing the square waves produces a pulse whose width determines worsening power facter.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 02:40:14 am
content: What is your waveform and the accuracy needs? Any of them can push you over a limit when you want to use a true RMS converter. Or, an equivalent algorithm to calculate RMS from properly sampled AC signal.

author: Psi
author_rank: Super Contributor
timestamp: February 13, 2023, 02:42:05 am
content: You can feed it into the MCU as an AC waveform if you want to.Put two series resistors (something like 10k) from the ADC pin to GND. So the adc pin will rest at mid scale.Have the correct load resistor across the current transformer and then put that between GND and through a series capacitor connected to the center of the 10k resistors.Then you will get an AC current waveform on the ADC.

author: kuon
author_rank: Regular Contributor
timestamp: February 13, 2023, 03:07:20 am
content: Waveform is main, so it is quite sinusoidal and I need 1A accuracy with the sensor I linked.Thanks for your suggestions. I'll experiment with both opamp and offseting the AC signal.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 04:12:58 am
content: Are you going to be able to sample the AC signal with ~5KHz sampling rate? And have enough math power for calculations?If any of them is pushing the capabilities a precision rectifier can give you a simpler solution. So that the analog pin will give you the analog current readout and no need for signal processing on the MCU.

author: BrianHG
author_rank: Super Contributor
timestamp: February 13, 2023, 04:26:15 am
content: Quote from: kuon on February 13, 2023, 03:07:20 amWhat's wrong with hall-effect current sensing ICs?EG:https://www.digikey.com/en/products/detail/allegro-microsystems/ACS758KCB-150B-PFF-T/2042747Smaller ones exist in the +/-31amp and below range. They are 1/10th the price and far more accurate for analyzing sub 1amp signals.EG:https://www.digikey.com/en/products/detail/allegro-microsystems/ACS711KEXLT-31AB-T/3868195I would sample at >=1Khz and software scan for the peaks.Also, the Allegro hall sensor also will show you DC currents.

author: bdunham7
author_rank: Super Contributor
timestamp: February 13, 2023, 05:24:58 am
content: Quote from: kuon on February 13, 2023, 03:07:20 amBeing mains means the voltage will be somewhat sinusoidal, but there's no guarantee at all that the current will be.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 13, 2023, 10:46:18 am
content: Use the 10 ohms burden resistor and lift one side to Vcc/2, then feed it to your ADC. Range will be +-1.4V around that Vcc/2.Simple (digital) high pass filter to remove the Vcc/2 offset and you're set.Do the same for voltage and the power calculation is a simple multiplication and averaging.

author: kuon
author_rank: Regular Contributor
timestamp: February 13, 2023, 11:27:55 am
content: Quote from: Jeroen3 on February 13, 2023, 10:46:18 amYeah I think I'll go this route, with a small transformer for voltage. I'll feed the vcc/2 to another adc channel and use it as ref.