The 4-20 mA current loop is the most widely used analog method for communicating and transmitting measurement signals in the world of industrial instrumentation and process control. It’s simple, reliable, and effective for transmitting sensor data over long distances and is a key component of troubleshooting and calibrating process systems.

But what makes it so effective? Let’s explore the fundamentals that underpin the 4-20 mA current loop, including Ohm’s Law, key components of the loop, the pros and cons of using this system, and more.

Understanding the Basics

Broadly speaking, a 4-20 mA current loop works by varying the electrical current in a circuit to represent a physical measurement — such as temperature, pressure, or flow rate. The loop typically includes a power supply, a sensor or transmitter, a receiver or controller, and wiring that connects them all in series.?

However, to better understand how a 4-20 mA current loop works, it’s important to know how Ohm’s Law relates to how the loop functions.

Ohm’s Law In Action

Ohm’s Law is a fundamental equation when dealing with electrical systems. The equation is as follows:

V = I × R

• V is voltage
• I is current (in amperes)
• R is resistance (in Ohms)

The law is especially helpful for validating the static values of circuit components, current levels, voltage supplies, and voltage drops. (A voltage drop is a phenomenon wherein the electrical potential of a current decreases as it flows through a circuit.)

In a 4-20 mA loop, each element of Ohm’s Law comes into play. The loop’s transmitter adjusts the electrical current (I), which it draws from a direct current (DC) power supply and which remains constant throughout the loop, based on the measured variable. However, the voltage (V) across the loop changes depending on the total resistance (R), which includes the wiring and any load devices.

Engineers use Ohm’s Law, at least in part, to avoid voltage drops in an 4-20 mA loop and ensure it has enough voltage to drive the required current through all components.

All this said, if you're new to electrical concepts like voltage drops and loops, comparing current flow to water flow can be helpful. So let’s review a helpful analogy.

Visualizing Current With a Water Analogy

Imagine a pipe system: You know that pipes can come in different circumferences, water can flow at different rates, and that pressure on the water can impact flow rates. Now, let’s translate those components to an electrical circuit:

• Current (I) is like the flow rate of water.
• Voltage (V) is the pressure pushing the water.
• Resistance (R) is the narrowness and physical constraints of the pipe.

Just as water flows from high to low pressure, electric current flows from high to low voltage. Resistance restricts the flow of the current, and voltage must be sufficient to overcome that resistance — just like a water pump must provide enough pressure to move water through a narrow pipe.

Now that you have a better sense of how a 4-20 mA current loop works, let’s review its anatomy and the pros and cons of using one.?

Components of a 4-20 mA Current Loop

A current loop is made up of several key components, each playing a specific role in signal transmission:

• Power supply: Usually 24V DC, it provides the energy to drive the loop.
• Transmitter: Converts a physical measurement into a proportional current (4-20 mA).
• Receiver: Often a Power Line Communication (PLC), Digital Code Squelch (DCS), or display that interprets the current signal.
• Wiring: Connects all components in series.
• Load resistor (optional): Converts current to voltage for analog input devices.

Each component must be carefully selected to ensure the loop functions correctly and reliably.

Benefits of 4-20 mA Current Loops

There are several reasons why 4-20 mA loops remain a standard in industrial settings:

• Noise resistance: Current signals are less affected by electrical noise than voltage signals.
• Long-distance capability: Maintains signal integrity (i.e., experiences little to no voltage drops) over hundreds of meters.
• Live zero (4 mA): Allows detection of faults like broken wires (which would read 0 mA).
• Simplicity: Easy to install, troubleshoot, and maintain.
• Compatibility: Supported by a wide range of industrial devices.

Drawbacks of 4-20 mA Current Loops

Despite their advantages, 4-20 mA loops do have some limitations:

• Single signal per loop: Each loop carries only one measurement.
• Power limitations: Devices must operate within the loop’s power budget.
• Wiring overhead: Large systems may require extensive wiring for multiple loops.
• Limited data: Analog signals don’t carry device diagnostics or metadata unless you pair them with a digital protocol like the Highway Addressable Remote Transducer (HART) Protocol.

How To Verify a 4-20 mA Current Loop

As is the case with any sort of system, it’s critical to stay on top of ensuring your 4-20 mA current loop is operating correctly — especially when it comes to troubleshooting and calibrating process systems. The first step is to verify the loop by testing components such as the system’s wiring, transmitter, inputs, and more.

Read our guide to learn more about verifying your 4-20 mA current loop.

Frequently Asked Questions (FAQs)

Q: Why does the loop start at 4 mA instead of 0 mA?
A: The 4 mA "live zero" allows the system to distinguish between a valid zero reading and a fault condition like a broken wire or power loss.?

Q: Can I use a 4-20 mA loop for digital communication?
A: Not directly. However, protocols like HART can overlay digital data on top of the analog signal.

Q: What happens if the loop resistance is too high?

A: The transmitter may not be able to maintain the correct current, leading to inaccurate readings or system failure.

Q: How do I measure a 4-20 mA signal?
A: Use a loop calibrator or multimeter in series with the loop to measure the current directly.

Q: Can multiple devices share a single loop?
A: Typically, no. Each loop is dedicated to one signal to ensure accuracy and avoid interference.