Expert’s answer : Converting 4-20 mA to 1-5 volts

The short answer: it depends on your control system. Usually, the system will have an analog input to read those milliamps (mA) from the field transmitter.

However, sometimes the control system can convert the signal to  1-5 volts. Some companies use 1-5 volts as a standard reference rather than 4-20 mA.

Here, you need to understand how the card in your control system works. For instance, some cards already have the 250-ohms resistors, so you won’t need that in the field. Others require a resistor, or convert the signal to voltage, or read 0-20 mA rather than 4-20 mA.

4-20 mA transmitter
Courtesy of Prelectronics

And keep in mind the correlation between 4-20 mA and 1-5 volts. When you add a resistor to read your HART data, you’ll get a voltage drop on the resistor. I’ve seen this application in different scenarios, such as converting a sensor from analog to voltage and connecting it to a wireless transmitter to send the data wirelessly.

Just make sure you know the devices and control system you have and the standard followed by the company. As I said, you can find companies using 4-20 mA, 0-20 mA, 1-5 volts, digital protocols, and so on.

Once, I visited a company in Brazil that had its wires crossed. It used the black wire as positive and the white as negative. Usually in Brazil it goes the other way, but nobody wrote that in stone, so what can I say? Pay attention to the details!

These articles will tell you more about analog communication:

Analog and digital communication

If you want to learn more about the difference between HART and FOUNDATION Fieldbus as analog and digital communication, check this article out !

HART theory: Sending a digital communication through an analog signal

How it’s possible to send a digital communication through an analog signal? Find the answer in the article !


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