I always believe in simplicity in design when I’m working on PCB prototypes. But I saw both ends of the spectrum when I had my first project dealing with industrial 4-20mA current loop signals. If you’ve been dealing with digital electronics all these while, chances are you have never heard of 4-20mA loop signals.
Why 4-20mA Loop Signals
When you step into industrial electronics often dominated by PLCs, you’ll notice various sensors and transducer that uses a current loop to relay the measured value instead of voltages. The reason is simple. Electric current does not suffer any losses theoretically while voltage drops over a length of a cable. Electric current does not change when it travels from point A to point B. Hence, the preference of current loop system.
Here’s a typical connection of 4-20mA temperature sensor.
In industrial electronics, you’ll have sensors that measure parameters like temperature, humidity, or humidity and converts them into representative signals from 4mA to 20mA. The advantage with 4mA to 20mA current loop is that you’re able to detect a cable break or sensors malfunctioned if you’re getting a reading a less than 4mA reading. This is particularly important when you’re dealing with critical systems that demand immediate respond to faulty sensors or human tampering.
4-20mA Input Design
Designing a 4-20mA input was pretty straightforward. It’s about converting the current into voltage signals. If you’re still figuring out how your microcontroller could accurately measure current input, the answer is simple, you add a single resistor that connects the sensor input to Ground, to convert the current into a voltage signal, like this.
Now, you notice I’m using a 165R resistor and why is that so? That’s because I’m using a 3.3V microcontroller and I need to convert the 4-20mA into a full range 3.3V analog value. Just note that you need to select resistors with smaller tolerance for better accuracy.
4-20mA Output Design
While you can easily convert 4-20mA to input with a single resistor, designing a 4-20mA output is a totally different situation. I remember searching for a few solutions and chips for my PCB prototype. I wanted something simple to design, well, and this is what I actually implemented in the eventual production version of an industrial air dryer controller.
I’m using a DAC converter MCP4725 for a digital to analog conversion. You can always use an internal DAC if your MCU has one. But the real deal is the XTR111 chip. The values of R104, R105, and R106 form a divider for the 4-20mA range. The XTR111 itself came in the form of MSOP or VSON package, which can be tough to work on if you’re populating your PCB prototypes manually..
How far can you drive 4-20mA signal?
Theoretically, you can drive a current loop signal on an infinite length of a conductor as current does not drop over resistance. But that’s not the truth in reality. When it comes to 4-20mA sensors, it’s all about the load resistance that is supported by the sensor’s output. If you drive the current on a load that exceeds the supported resistance, the reading from the sensor will not be accurate.