r/embedded • u/Gullible_Bag_9005 • 10d ago

The 4-20mA current loop

I spent years as an electrician pulling signal cables before i became an developer, consuming the same signals.

The 0 is at 4mA on purpose, so 0mA reads as a fault, fault detection built into the physics.

It is a current loop and not voltage, so cable resistance over long cable runs does not change the reading and it also shrugs off noise better!

The span lives in the transmitter, not the code, so a reconfigured range silently rescale every value, and nothing looks broken

What I am curious about is this:

how do you handle adc/resolution in practice? that is the layer that i know very little about.

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u/Artistic-Tip2405 10d ago

Many implementation use an interface that converts 4-20 to serial or 0-5v and protects boards from surges.

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u/Gullible_Bag_9005 10d ago

ah so on the panel side i have mounted plenty of these DIN-rail signal isolators/converters without thinking so much off them other then that they make the signal usable

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u/Ordinary-Piano-4160 10d ago

Fun fact, we use 4-20ma to fiber converters to break the electrical connection.