Hi Everyone,
I am currently completing a project that involves sensing current from a 4-20mA industrial sensor. I am trying to figure out the advantages of simply putting the output across a resistor versus using an IC (such as a current shunt monitor) to measure the output. Obviously the IC would burn less power, but I just wanted to see if there were any other suggestions on why to use one over the other. Thanks in advance everyone!

 

hi sp,
Welcome AAA,
What is driving the 20mA loop.?
E

 

hi sp,
Welcome AAA,
What is driving the 20mA loop.?
E

Hello E! I have a power supply of 13V powering the loop.
Thanks - sp

 

It is designed to work into some kind of compatible 4-20ma reading device or circuit.
What are you using to record the reading?
Max.

 

hi,
Ok,
The usual way is to have say a 250R resistor in the loop, measure 1V thru 5V across the 250R.
No extra power needed, the down side is that there is only 8V compliance voltage, normally would not effect the loop/driver.

E

 

Hi Everyone,
I am currently completing a project that involves sensing current from a 4-20mA industrial sensor. I am trying to figure out the advantages of simply putting the output across a resistor versus using an IC (such as a current shunt monitor) to measure the output. Obviously the IC would burn less power, but I just wanted to see if there were any other suggestions on why to use one over the other. Thanks in advance everyone!

Are you looking for a continuous output voltage analog to the current, or a switch on/off with some hysteresis around a specific current?

Integrated shunt monitors generally are looking for mV rather than volts, as otherwise the burden voltage becomes significant.

 

It is designed to work into some kind of compatible 4-20ma reading device or circuit.
What are you using to record the reading?
Max.

Hello! Apologies, I should probably provide more background:

It is designed to work into some kind of compatible 4-20ma reading device or circuit.
What are you using to record the reading?
Max.

Hi Max - I am using an ADC and MCU to record the output from the sensor.

 

Are you looking for a continuous output voltage analog to the current, or a switch on/off with some hysteresis around a specific current?

Integrated shunt monitors generally are looking for mV rather than volts, as otherwise the burden voltage becomes significant.

Hello! I am turning the sensor on/off from an MCU, and then have the sensor output being recorded by an ADC and an MCU. I'm trying to figure out the best way to measure the current when I turn on the sensor.

 

Well as you've already alluded to, to measure current with an ADC you must convert to a voltage through a resistor or some other device. The resistor has to be small enough to be within the dynamic range of your sensors supply voltage. Generally that's 250ohms giving 1 - 4v. What is the supply voltage to your sensor?

 

Doh! Read the thread, 13v. You simply don't have enough volts to do much else other than a resistor.

 

Using the 250 ohm resistor gives you a 1-5V output which you read with your ADC. The 5V output at 20 mA through the 250 ohm resistor means 5*.02 = 0.1W dissipation. Maybe that's not enough to worry about?

 

Hi Everyone,
I am currently completing a project that involves sensing current from a 4-20mA industrial sensor. I am trying to figure out the advantages of simply putting the output across a resistor versus using an IC (such as a current shunt monitor) to measure the output. Obviously the IC would burn less power, but I just wanted to see if there were any other suggestions on why to use one over the other. Thanks in advance everyone!

The sensor could be passive ( you supply power ) 3 wires (+power gnd signal) 4 wires ( two power two signal)
Using a resistor, as stated before, is normal practice however; common ground loops could destroy the results.
Most of the 4-20mA lines are galvanic separated to avoid this.
Sometimes in the sensor (4 wire system) but most of the cases at the receiver.

Picbuster

 

The alternative to a resistor & ADC is a current to voltage converter (& ADC).

This one uses a 100ohm resistor, but uses a standard LM358 op-amp so has accuracy/drift limitations;

A better solution uses a TI RCV420 chip which has an internal 75ohm resistor, available from Mouser. I've not found any off-the-shelf boards based on this chip (if anyone knows of one, let me know ASAP before I roll my own).

Conventional current sense chips for motor and system supply current sensing, eg INA219 and the like, though they can be pressed into service, generally don't have the CMR or dynamic range that an industrial current loop needs.

 

Most sensor manufacturers suggest just using a simple resistor. The reason is it's both simple and very inexpensive. Using a good quality resistor with a good temperature coefficient will provide years of service problem free. The downside of this would be if the resistor for any reason opens your ADC will likely have a toasted channel. The odds are likely pretty high of that happening. Weigh out the good and bad and choose a solution which works best for your specific application.

Ron