4–20 mA Explained

What Is a 4–20 mA Signal?

A 4–20 mA signal is an analog electrical signal commonly used to transmit a measurement from a field instrument to a control system.

A transmitter measures something physical—such as pressure, temperature, flow, or level—and represents that measurement as a current between 4 mA and 20 mA.

For example, a pressure transmitter configured for 0–100 PSI might produce:

Why Does the Signal Start at 4 mA?

One of the defining characteristics of the standard 4–20 mA signal is that zero percent of the measurement range is represented by 4 mA rather than 0 mA.

This creates what is often called a live zero.

A live zero makes it possible to distinguish a normal measurement at the bottom of the range from certain loop faults.

For example, in a 0–100 PSI application:

• 4 mA can represent a normal reading of 0 PSI.
• A signal significantly below 4 mA can indicate an abnormal condition.
• 12 mA represents the midpoint of the measurement range.
• 20 mA represents the top of the configured range.

LRV, URV, Zero, and Span

Two terms you'll encounter constantly when working with transmitters are LRV and URV.

• LRV — Lower Range Value: the process value represented by 4 mA.
• URV — Upper Range Value: the process value represented by 20 mA.

The difference between the URV and LRV is the configured measurement span.

The LRV does not have to be zero.

For example, a transmitter could be configured for 10–50 PSI:

This distinction is extremely important when troubleshooting a transmitter or PLC. Always use the transmitter's configured range rather than assuming that 4 mA equals zero engineering units.

The 4–20 mA Scaling Formula

To convert a measured current into an engineering value, calculate how far the current has progressed through its 16 mA signal span.

Another way to think about the formula is:

Once the percentage of span is known, apply it to the engineering range.

Worked Example: 0–100 PSI

4–20 mA Reference Table

For a linear 4–20 mA signal, these values are useful reference points to memorize.

These percentages apply regardless of the engineering range. Whether the transmitter measures 0–100 PSI, 0–500°F, or 10–50 PSI, 12 mA is still 50% of span.

How the 4–20 mA Loop Works

A 4–20 mA signal is not simply a voltage coming from the transmitter. The transmitter regulates the current flowing through the entire loop.

A typical loop contains:

• DC power supply
• Transmitter
• Control-system analog input
• Wiring and terminal connections
• Potentially additional loop devices

The current flowing through the loop is controlled by the transmitter according to the measured process value.

Why Use Current Instead of Voltage?

A major advantage of a current signal is that the same current flows through the series components of the loop.

Cable resistance therefore primarily creates a voltage drop rather than directly changing the signal current.

This makes current transmission practical over relatively long cable runs and in electrically noisy industrial environments.

Loop Voltage and Voltage Burden

Although the signal is a current, the loop still requires enough voltage to operate all of its components.

Every component in the loop consumes some voltage. This is commonly described as its voltage burden.

A simplified loop might look like:

If the available supply voltage is too low, the transmitter may not be able to maintain the desired loop current.

This is one reason that loop troubleshooting should include checking both the current and the available voltage.

Why Is a 250 Ω Resistor So Common?

A 250 Ω resistor converts the 4–20 mA current into a convenient 1–5 V signal.

At 4 mA:

At 20 mA:

How to Measure a 4–20 mA Signal

One of the most useful skills for an instrumentation technician is being able to measure the actual loop current.

When measuring current with a conventional multimeter, the meter must become part of the current path.

A clamp-style process meter or a dedicated loop calibrator can provide other ways of measuring loop current without opening the loop, depending on the instrument and measurement method.

Troubleshooting a 4–20 mA Loop

When the controller is displaying an unexpected engineering value, avoid immediately changing the PLC scaling. First determine what the field signal is actually doing.

Step 1: Check the transmitter configuration

Verify the configured LRV and URV. A transmitter configured for 10–50 PSI will not behave like one configured for 0–100 PSI.

Step 2: Measure the actual loop current

Determine whether the field device is actually producing the current you expect.

Step 3: Calculate the expected engineering value

Use the measured current and the transmitter's actual range.

Step 4: Compare the calculated value with the PLC

If the current and transmitter configuration are correct but the PLC value is wrong, investigate the analog input configuration and scaling.

Step 5: Check the loop electrically

Check supply voltage, wiring, terminal connections, grounding, loop resistance, and voltage available at the transmitter.

Common 4–20 mA Problems

4–20 mA and PLC Scaling

In a PLC application, there are usually several different representations of the same measurement.

The transmitter converts the physical measurement into current. The analog input converts that electrical signal into a digital value. The PLC program then converts that digital value into engineering units.

This distinction is important because a problem can occur at any stage.

If the loop current is correct but the HMI value is wrong, the transmitter may not be the problem at all.

Common Mistakes

• Assuming 4 mA always represents zero engineering units.
• Using the transmitter's maximum range instead of its configured LRV and URV.
• Forgetting that the signal span is 16 mA, not 20 mA.
• Scaling the PLC differently from the transmitter.
• Measuring voltage when the real question is what the loop current is.
• Putting a current meter across a voltage source instead of placing it in series.
• Replacing a transmitter before checking power, wiring, and the actual loop current.

A Practical Field Workflow

When troubleshooting a 4–20 mA instrument, a useful mental sequence is:

1. What should the transmitter be measuring?
2. What are its configured LRV and URV?
3. What current is actually present in the loop?
4. What engineering value does that current represent?
5. What raw value is the PLC receiving?
6. What engineering value is the PLC calculating?
7. What value is the HMI displaying?

Following the signal through those stages helps isolate whether the problem is in the process, transmitter, wiring, analog input, PLC scaling, or HMI.

4–20 mA Calculators

Use the ControlsCalc tools to perform the calculations discussed in this guide.