AC Current measurement using shunt resistor and amplifer

[Venkatvel]

Hi , I wanted to build a current sense circuit for AC loads of maximum current 10A. But it has to be non isolated, so I am using shunt resistor 1mohm and current sense amplifier to measure the high side current . The problem is the common mode voltage of the amplifiers are upto 70V, but my AC input voltage is 230V . How to select the amplifier ? Please suggest


REPLY

 

[KlausST]

Hi,

There's nothing new. Therevare millions of solutions in the internet for 10A 230V AC measurement (or similar).

Why non isolated?
Why high side?

Draw a schematic with your actual situation... To show your needs.

Mind: We don't need to see your solution, we need to see and understand your requirements.

Klaus

 

[Venkatvel]

Hi Thanks for your reply. I couldn't find a suitable one . Cost is also critical for my application

All the circuits are non isolated , it is the requirement , Thats why I didnt go for using ACS730 or any current transformers or isolated circuits

Below is the Schematics. I am using a shunt resistor 1mohm . The current has to be measured on the AC line 230VAC, high side current monitoring is used . The accuracy required is 1% or 2% . The maximum current through the path will be 20A. I used the current sense amplifier INA180 but it has common mode voltage very less. Please let me know if this circuit works.

Also suggest any suitable amplifiers for this application that can sense the current from 200mA to 20A . Since this is high voltage , do we need add any transient Protection. the Requirement is to use only non isolated current measurement method. I am running out of time , your quick reply is much appreciated

 

[KlausST]

Hi,

Please answer the question why you don't want an isolated solution.
"It's a requirement" doesn't tell why.

INA180 and other similar current sense amplifiers are not made for AC voltages! Forget about them.

Again: show your current situation and explain the requirements. No solutions.
Otherwise I can't help.

Klaus

 

[schmitt trigger]

The only reason I can think of a high side current measurement is because you have a three phase delta connected load.
Back in the late 1980s we had such a requirement.

The solution we employed back then was to perform an AD conversion at the high side, and then with an optocoupler send it to the ground referred data acquisition.

 

[Venkatvel]

Hi I understand your question. But I do not have the exact answer for this. Sorry for that . The end application and use case is not known

the design is similar to the https://www.st.com/resource/en/user...f-acs-triac-with-stm32-stmicroelectronics.pdf used to power AC loads in my design. Isolation is important for the safety but this board appears to be non -isolated for some reason. The board design is completed by someone else but Now i wanted to add current measurement circuit to monitor the current. I couldnt find one of non-isolated type

I have attached the circuit for reference . Suggest some current sense amplifiers non -isolated to measure the AC current . Please help

--- Updated Aug 9, 2020 ---

The only reason I can think of a high side current measurement is because you have a three phase delta connected load.
Back in the late 1980s we had such a requirement.

The solution we employed back then was to perform an AD conversion at the high side, and then with an optocoupler send it to the ground referred data acquisition.

thanks for the response . The circuit is supposed to be non -isolated , I wanted to use an amplifier and interface to adc of MCU. Any suggestions on the amplifier part ?

 

[FvM]

As already stated by others, it makes no sense to use non-isolated sense amplifiers if shunt common mode voltage is exceeding a few ten volts. The ultimate reason is that offset and common mode errors will prevent meaningful accuracy. You didn't tell what the actual common mode voltage in your application is. Some single phase energy meter circuits manage to connect the shunt on the low side with small common mode voltage. If this isn't possible in your application you really need analog or digital isolation for the current measurement.

 

[Venkatvel]

As already stated by others, it makes no sense to use non-isolated sense amplifiers if shunt common mode voltage is exceeding a few ten volts. The ultimate reason is that offset and common mode errors will prevent meaningful accuracy. You didn't tell what the actual common mode voltage in your application is. Some single phase energy meter circuits manage to connect the shunt on the low side with small common mode voltage. If this isn't possible in your application you really need analog or digital isolation for the current measurement.

The loads are Fans which is outside the board , so high side shunt is connected between the AC supply and load . The common mode voltage will be 220V. I could see only high voltage DC current sense amplifiers are available .

 

[FvM]

Still more questions than answers. You seem to assume that the analog ground is tied to mains neutral, but why? And if so, what's against placing the shunt in the load neutral line instead of phase?

The common mode voltage according to your assumptions is +/- 310V, without considering transient overvoltage. There's no suitable non-isolated sense amplifier available. You could design it, but the effort for truly isolated solutions is less.

--- Updated Aug 9, 2020 ---

Or the other way around, if your analog ground is mains connected, why no tie it to phase?

 

[KlausST]

Hi,

Any isolated measurement can be used in non isolated applications.
But not the other way round.

Especially when you don't know the use case, the isolated solution is recommended.

You may post the INA circuit even more often, it doesn't change the facts.
I don't know any current sense amplifier that is made for 230V AC voltage.

You overcomplicate things.

Klaus

 

[BigBoss]

These guys are expert to measure such high currents..

0 to 50 A Integrated Current Sensor ICs | Allegro MicroSystems

Our family of fully integrated, high isolation current sensor solutions provides accurate measurement with very low resistance—all without the need for external isolation components.

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[Easy peasy]

for AC - a current transformer - CT - would seem to be the obvious ... ?