4-20ma PLC Input Only Reading Lower Half of Range

[Ozzie01]

Could you explain why the sensor's resolution has been halved when you added the resistance?

Just curious

 

[standeven]

When a ~400 ohm resistor is added in parallel the input reads between 2-10 mA (but over the entire range, so the sensor is usable).

 

[Ozzie01]

When a ~400 ohm resistor is added in parallel the input reads between 2-10 mA (but over the entire range, so the sensor is usable).

Thanks for the reply standeven, so just bear with me a second to make sure I understand this correctly.

So when you added a 400 Ohm resistor in parallel this brought the total circuit resistance down to 194 Ohms, Thus leaving your sensor operate.

I=V/R thus 24/194 = 124 mA just I am bit puzzled as to how the sensor is operating over a 2-10 mA range.

I have obviously missed something here?

 

[Mickey]

Thanks for the reply standeven, so just bear with me a second to make sure I understand this correctly.

So when you added a 400 Ohm resistor in parallel this brought the total circuit resistance down to 194 Ohms, Thus leaving your sensor operate.

I=V/R thus 24/194 = 124 mA just I am bit puzzled as to how the sensor is operating over a 2-10 mA range.

I have obviously missed something here?

Because the parallel paths will splits the 4 to 20ma current, one thru the 400 ohms and the other thru the analog card's internal resistor. He will lose resolution within the PLC's analog card.

He should install an isolator that has a load of less then 200 ohms and retransmits 4 to 20ma into the higher load of the PLC.

 

[standeven]

You are correct Ozzie, but the total current (4-20 mA) is equal to the sum of the currents in each parallel branch. So the sensor outputs 4-20 mA, but the current measured at each resistor is half of that (assuming equal resistance). So the PLC only sees a range of 2-10 mA.

Edit - Mickey beat me by seconds.

 

[Ozzie01]

Great Thanks for clearing that one up guys!