Current sensing/measuring circuit

  • Thread starter Pagedown
  • Start date
  • #1
92
0
Hi everyone, I've been trying very hard to contruct an analog front end to measure current with an MCU, using a C.T(preferbly) or a sense resistor. Translating them into positive 0-5V.But find no luck as the I need a wide range and good resolution of 5mA.

Requirements :-

Line current : 0 - 10A 50/60hz AC
Resolution :5 mA

I need to translate them into 0-5V to be measure by my MCU to graph the instananeous current waveform, and of coz the rms values, peaks and so on.

The biggest problem is the 5mA requirement.Any help is greatly appreaciated.
 

Answers and Replies

  • #2
1,681
3
Use a small resistor .1 ohms or less and connect the differential inputs of an op-amp across it. Adjust the feedback around the opamp to get the scaling you need. You'll need a differential power supply like +/- 5 or 15 volts so tge opamp can sense the below-ground voltage on the "left side" of the current sense resistor.
 
  • #3
vk6kro
Science Advisor
4,081
40
You will also need to connect the opamp as a precision rectifier and feed it to a high resolution A to D converter.

That is about 2000 steps (10 A / 0.005 A) so you would need at least 11 bit (2048 step) resolution.
 
  • #4
1,681
3
You will also need to connect the opamp as a precision rectifier and feed it to a high resolution A to D converter.

That is about 2000 steps (10 A / 0.005 A) so you would need at least 11 bit (2048 step) resolution.


I assumed he was already behind the rectifier.

Pagedown -do you need to measure direction of the current or just amplitude?
 
  • #5
1,763
59
What is the resolution of your A/D converter? Most uPs are only 10 bits.

Do you need 5 mA resolution even at 10 amps? If not you may consider using a log amp. Instead of using an opamp between the sense resistor and the A/D, you could use a log amp. You might also use an opamp to optimally scale the output of the log amp to the A/D converter.

Here is one I have used. http://focus.ti.com/lit/ds/symlink/tl441.pdf

Note: If you don't need fast conversion times, you can get 11 bit resolution from a 10 bit A/D by averaging several readings.
 
  • #6
92
0
Ya, I will need the direction and also amplitude, thus the rectifier will cut-me off my direction. I will be using a 12-bit external ADC if needed.

Log amp will have this solved?
 
  • #7
1,763
59
Yes, if you need the direction as well as amplitude you will need a 12 bit A/D. Unfortunately, log amps, like logarithms, do not handle negative values elegantly.
 

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