Understanding 4 to 20 Milliamperes Current Signals

Learning Objectives

• Basics and history of 4 to 20 milliamperes current signals.
• Industrial history and adoption.
• Advantages and disadvantages.

Industrial History

• Olden Days: Industries used mechanically operated devices (pneumatic sensors and controllers).
• Pneumatic Devices:
  • Used pressure for measurement and control.
  • Output signal: 3 to 15 PSI represents 0 to 100% of process variable.
  • Disadvantages: bulky, need compressed air, high maintenance costs, slow troubleshooting.
• 1950s Evolution: Introduction of analog transmitters, generating 4 to 20 milliamperes signals.
  • Advantages: simpler installation and configuration.
• 1970s Evolution: Introduction of programmable logic controllers (PLC).

Principles of 4 to 20 milliAmperes

• Pressure Transmitter Example:
  • Configured for 0 to 10 bar.
  • 4 mA = 0%, 20 mA = 100% process variable.
  • PLC converts received mA to pressure and displays it.
• Detailed Example:
  • 0 bar --> 4 mA --> PLC displays 0 bar.
  • 2.5 bar --> 8 mA --> PLC displays 2.5 bar.
  • 5 bar --> 12 mA --> PLC displays 5 bar.
  • 7.5 bar --> 16 mA --> PLC displays 7.5 bar.
  • 10 bar --> 20 mA --> PLC displays 10 bar.

Why Not 0 to 20 milliAmperes?

• Dead Zero Problem:
  • Faulty readings indistinguishable as 0 mA could be due to zero pressure or a fault.
• Solution - Live Zero
  • Start signal range at 4 mA, allowing identification of faults (0 mA indicates a fault).

Why 4 mA and 20 mA?

• Minimum Starting Value: Analog devices needed >3 mA for operation.
• 20% Bias Rule:
  • Followed from pneumatic standard (20% of 15 PSI = 3 PSI, similarly, 20% of 20 mA = 4 mA).
• Maximum Value: Must be less than the dangerous threshold of 30 mA for human safety.
  • 4 to 20 mA easier ratio (1:5) aligning with previous pneumatic standards (3-15 PSI).
• Previous Practices: Early attempts used 10 to 50 mA but later ceased due to exceeding safe current limits.

Advantages of 4 to 20 mA

• Safety and Simplicity:
  • Safe thresholds and simple calculations.
  • 4 mA allows for fault detection (live zero).
• Signal Conversions:
  • Easy to convert to 1-5 volts using a 250 ohm resistor.
  • Essential for PLC’s A-D-C which reads voltage.
• Noise Immunity: Current signals are more immune to electrical noise.
• Long Distance Travel: Can travel up to approximately 1 km with 24V DC power.
• Standardization: Widely accepted industrial standard, easy installation, and configuration.
• Troubleshooting: Simple equipment like a multimeter suffices.

Disadvantages of 4 to 20 mA

• Magnetic Interference:
  • Induced magnetic fields in straight wires (resolved by twisted pair cables).
• Limited Varied Data: Each pair carries only one process variable.

Conclusion

• Widely used due to safety, simplicity, effective conversion, noise immunity, and distance capability.
• Important factors: live zero, fault detection, minimal equipment need, and effective range.