A 4-20 milliamp (mA) signal is a standard for transmitting analog signals in many industrial environments, including temperature, pressure, flow, and level measurements. This standard uses current to transmit information because current remains consistent regardless of cable resistance or length, ensuring accurate data over long distances.

1. What is a 4-20 mA Signal?

The 4-20 mA signal is a widely adopted method for conveying process variables in industrial applications. The signal uses a range of current between 4 mA and 20 mA to represent a range of values. The reason 4 mA is chosen as the lower limit is to differentiate between a live zero and a dead zero (broken wire or no power), while 20 mA represents the upper limit of the measurement.

2. The Basic 4-20 mA Circuit

A basic 4-20 mA current loop typically consists of:

• Power supply (usually 24V DC)
• Transmitter (the device that sends the signal)
• Receiver (the device that receives the signal, such as a PLC or indicator)
• Wiring connecting all components in series.

In the loop, the transmitter outputs a current signal that ranges from 4 mA (0%) to 20 mA (100%), which is read by the receiver.

3. Example Transmitters Using 4-20 mA Signals

Here are some common examples of transmitters that use 4-20 mA signals:

• Pressure Transmitters: Convert pressure readings into a proportional current signal.
• Temperature Transmitters: Connected to sensors like RTDs or thermocouples to transmit temperature data.
• Level Transmitters: Monitor liquid levels in tanks and output corresponding signals.
• Flow Transmitters: Measure flow rates of liquids or gases in pipelines.

4. Troubleshooting Under and Over-Range Values

In a well-functioning 4-20 mA system, the signal should always stay between 4 and 20 mA. However, issues may occur if the signal falls outside this range:

• Less than 4 mA: Could indicate a wiring fault (e.g., broken wire), sensor failure, or transmitter malfunction.
• More than 20 mA: Might indicate an over-range condition, calibration error, or faulty transmitter.

5. The Role of the 250 Ohm Resistor

A 250 Ohm resistor is often placed in the loop to convert the current signal into a voltage signal for devices that require voltage readings (e.g., PLCs). Using Ohm's Law, the resistor converts:

• 4 mA to 1V (4 mA * 250 Ohms = 1V)
• 20 mA to 5V (20 mA * 250 Ohms = 5V)

This creates a 1-5V signal that can be interpreted by voltage-based receivers.

6. Advantages of Using 4-20 mA Over Other Signals

Advantages of using 4-20 mA include:

• Resistance to noise: Current signals remain consistent, unlike voltage signals that degrade over long distances.
• Live zero: The 4 mA base signal ensures detection of system faults, such as broken wires or power failure.
• Simplicity: 4-20 mA is a global standard and easily integrates with many industrial devices.

7. Conclusion and Best Practices

Technicians should regularly calibrate 4-20 mA transmitters, perform routine wiring checks, and understand the role of resistors in converting current to voltage signals. By mastering these concepts, electrical technicians can effectively troubleshoot and maintain 4-20 mA circuits in industrial settings.