4 to 20mA Current measuring Circuit Max 4072

[akhilfrancis]

Hai,

I am using a pressure sensor, with 4 to 20 mA current output. For measuring current output i am using max 4072 IC. Is it going to work?

 

[KlausST]

Hi,

It depends on your circuit.

--> show us your circuit.

Klaus

 

[akhilfrancis]

This is is my circuit. Here I am using the 2 wire pressure Sensor, is Nah_8253 From Trafag - Ag. Here i Also use a comparator.
Here i am aiming to sense the high side current.

Is this circuit is correct or not.

 

[KlausST]

Hi,

1)
DC_Volt seems to be your supply voltage.
This voltage is across R48.... What is it good for? It makes the circuit useless in my eyes.

2)
The sensor is not involved in any signal measurement.
But from your description it seems you want to measure sensor current.

3)
The comparator is connected to the sensitive feedback of the OPAMP. Expect a lot of error.
(input bias current of both OPAMP and comparator... and when comparator switches.)

Klaus

 

[memarian]

Hi,

Your sensor current flow is independent from your current sensing resistor , so it seems that it will not work.

You could use RCV420 from TI directly and convert your 4-20ma to 0-5V and do what ever you like with it or

you could check design examples like :

https://www.maximintegrated.com/en/datasheet/index.mvp/id/8025/part/MAXREFDES4

or
**broken link removed**

 

[akhilfrancis]

I am Sensing High Side current

Is this is the correct circuit?

 

[memarian]

I am Sensing High Side current

Is this is the correct circuit?
View attachment 136289

Yes, This Circuit seems fine.

BTW, You should choose your sensing resistor correctly , the drop on the sensing resistor shouldn't be high, standard sensing resistor is around 250ohms in 4-20ma systems , you could use higher resistors (for example 500 ohms) , but you must be sure about your sensor voltage ,both at minimum (4ma) and at maximum (20ma) currents with your power supply .

 

[KlausST]

Hi,

The MAX4072 is for bidirectional current measurement. But your current is unidirectional.
It will work, but you loose measurement range.

What is the output voltage divider good for?

What is DC_Volt?
What is REFIN?
Where does the OUT go to? and what´s it´s input voltage range?

Klaus

 

[akhilfrancis]

Hi,

I want to measure unidirectional only.
DC_Volt = 24V
Ref_in = 2.5V.
O/P is going to Micro Controller. I am using STM32F103RB.

Maximum I/P range is 3.3V.
So i used R45 is 1.5E.

 

[KlausST]

Hi,

As already recommended:
I'd use one of the 4..20mA measurement circuits.

But if you want to build your own:
I'd use low side sensing

But if you want higb side sensing:
I'd use an unidirectional sensing chip

Your configuration has some drawbacks. This makes it difficult to use your circuit.

*****
One drawback is the 2.5V offset.
Either you live with it (it's the easiest method).
Then just feed the MAX_output to the ADC and subtract the offset from the digital value.

If you want to subtract it on the analog side, then I recommend a difference amplifier. (Opamp circuit)
But it has also drawbacks:
To be able to measure close to zero Amperes you need a rail to rail output opamp.
To be able to measure true zero, then you need a negative supply voltage for the Opamp, (and protection circuit for the ADC)

Klaus