Understanding 4-20mA Signals: A Maintenance Technician's Guide
The 4-20mA current loop is the backbone of industrial instrumentation. Pressure transmitters, level sensors, temperature transmitters, flow meters — they all talk to your PLC or VFD using this simple, reliable analog signal. If you work in maintenance, understanding 4-20mA is non-negotiable.
Why 4-20mA Instead of 0-20mA?
The "4" in 4-20mA is what makes this standard brilliant. At 0% of the measured range, the signal is 4mA — not 0mA. This means a broken wire (0mA) is distinguishable from a legitimate zero reading (4mA). If your PLC reads 0mA, you know the wire is broken. If it reads 4mA, the sensor is working and reporting the minimum value.
This is called a "live zero" — and it's the reason 4-20mA has survived for 60+ years despite digital alternatives like HART, Foundation Fieldbus, and IO-Link.
How to Calculate the Process Value
The math is linear. 4mA = 0% of range. 20mA = 100% of range. For a pressure transmitter ranged 0-100 PSI:
- 4mA = 0 PSI
- 8mA = 25 PSI
- 12mA = 50 PSI
- 16mA = 75 PSI
- 20mA = 100 PSI
- Formula: PSI = (mA - 4) / 16 × 100
2-Wire vs 4-Wire Transmitters
A 2-wire (loop-powered) transmitter gets its power from the same two wires that carry the signal. The PLC or power supply provides 24VDC, and the transmitter modulates the current. This is the most common type and uses the fewest wires.
A 4-wire transmitter has separate power wires (24VDC+ and ground) and separate signal wires (4-20mA output and signal ground). These are used for transmitters that need more power than a 2-wire loop can provide, like radar level sensors.
How to Measure 4-20mA with a Multimeter
- Set your multimeter to DC milliamps (mA).
- For the easiest measurement, disconnect one wire at the PLC analog input terminal and put your meter in series.
- A healthy reading will be between 4.0mA and 20.0mA, proportional to the process value.
- Below 3.8mA: suspect a wiring fault or transmitter failure.
- Above 20.5mA: sensor may be over-ranged, or there's a wiring error.
- If you have a clamp-on mA meter, you can measure without breaking the circuit — these work on the wire without disconnecting anything.
Common 4-20mA Problems and Fixes
- 0mA (open circuit): Broken wire, loose terminal, or dead transmitter. Trace the wiring from the PLC back to the transmitter.
- Stuck at 4mA: Transmitter powered but sensor failed, or the process is genuinely at 0%. Check the transmitter local display.
- Erratic/noisy signal: Electromagnetic interference from VFDs or large motors. Route signal cables away from power cables. Use shielded cable with the shield grounded at the PLC end only.
- Reading 20.9mA+: The transmitter is saturated (process value exceeds the configured range). Re-range the transmitter or investigate the process.
- Slight offset (e.g., reads 4.2mA when process is at zero): Transmitter needs re-zeroing. Use the transmitter's zero/span adjustment or a HART communicator.
Always carry a mA loop calibrator in your tool bag. It lets you simulate a 4-20mA signal at the PLC end to verify your analog input card is reading correctly — isolating whether the problem is the transmitter/wiring or the PLC.
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