Pulsed DC current measurement using arduino and shunt resistor

Question

I am currently working on a project ( to develop a constant current pulsed DC supply) in which current has to be measured using Arduino's analog input (Upto 20A) passing through a variable resistance RV1(shown now as 145 ohm).

Current through this resistor is measured using a shunt R1 of .0035 ohm. Capacity 20A-AC/70 mV max.

For that, an IC4559 Difference-Amplifier is used as shown below. I have set the gain to 70. as 70*.070 Volts gives 4.9 Volts, reaching to the Arduino analog input port's capacity.

Question: This configuration works, but in my original circuit there is no grounding near the Diodes (highlighted). When I remove the ground port near the Diodes, the value read by Arduino become absurd, is there any way to measure the load current without grounding the diodes by using the similar configuration?

Arduino Code:

void loop() { lcd.clear(); volts = analogRead(A0); lcd.print(volts); Serial.println(volts); delay(500); }

schematic

• JimmyB: Instead of that shunt+amplifier circuitry consider just using a Hall-effect sensor (ACS714). Available in different current ranges (up to 30A), 1.2mOhm internal resistance and galvanic isolation of 2kV. A single chip, no hassles.

Answer 1

When I remove the ground port near the Diodes, the value read by Arduino become absurd, is there any way to measure the load current without grounding the diodes by using the similar configuration?

No, not using anything similar. Think about what voltages are produced on the output of the three phase rectifier. The cathode's of D1, D2 and D3 will produce a voltage equal to the peak positive voltage of your AC power waveform. The anodes of D4, D5 and D6 will produce a voltage equal to the peak negative voltage of your AC power waveform. Then you go and ground that negative voltage thus shorting out the three-phase AC supply.

You need an isolation system or an isolation amplifier or a floating ADC isolated by digital signal path isolators. You just cannot sensibly connect the anodes of D4-6 to local ground without creating a big problem.

• Aksh Chordia: Can you suggest some IC(s) /circuit designs for the purpose? Thanks in advance.
• Andy aka: I'd consider using Analog devices' range of digital isolators like this one. It has three control channels from grounded circuits to floating circuits and one channel back from the floating circuit to the grounded circuit. The data sheets shows how you can connect an ADC to the floating side. It's ideal for SPI control of a floating ADC and also provides power to the ADC.

Answer 2

You need to have a reference potential somewhere, anyway. If the source is grounded, than you already have it. If it's floating, then you may attach a ground like you did.

In case a source already has a connection to ground, then you have to pay attention on max. common mode voltage of difference amlpifier. The schematisc is not very clear, a clearer view of the difference amp. would help.