Why I Need to Convert 4-20 mA to 1-5 VDC: A Practical Guide for Industrial Automation
In industrial control and instrumentation systems, signal conversion plays a crucial role in ensuring seamless communication between field devices and control equipment. One common requirement is converting a 4-20 mA current loop signal into a 1-5 VDC voltage signal. But why is this conversion necessary? What advantages does it bring? And how can it be implemented correctly? In this blog, we answer all these questions and provide a deep dive into the reasons, benefits, and methods for converting 4-20 mA to 1-5 VDC.
Understanding 4-20 mA and 1-5 VDC Signals
What is 4-20 mA?
The 4-20 mA current loop is the most widely used analog signal standard in industrial automation. It represents a physical quantity (like temperature, pressure, flow) as a proportional current signal.
- 4 mA = lowest measured value (e.g., 0% process value)
- 20 mA = highest measured value (e.g., 100% process value)
What is 1-5 VDC?
The 1-5 VDC signal is a voltage-based analog signal that represents the same range of physical quantities as the 4-20 mA signal.
- 1 V = 0% process value
- 5 V = 100% process value
Why Convert 4-20 mA to 1-5 VDC?
There are several practical reasons and scenarios where this conversion becomes necessary:
1. Compatibility with PLC/DCS Inputs
Some legacy or budget-friendly Programmable Logic Controllers (PLCs) or Distributed Control Systems (DCS) accept only voltage signals. Converting the 4-20 mA signal to 1-5 VDC enables the controller to interpret the analog input accurately.
2. Data Acquisition Systems
Many data acquisition (DAQ) modules and recorders are designed to read voltage rather than current. A 1-5 VDC signal is more suitable and easily integrated into these systems.
3. Reducing Wiring Costs in Short Runs
In compact or panel-mounted applications where signal runs are short and electrical noise is minimal, voltage signals like 1-5 VDC are acceptable and reduce the need for specialized current loop hardware.
4. Input Isolation and Signal Splitting
Voltage signals can be more easily buffered, scaled, or split using signal conditioners. For example, one 1-5 VDC signal can be sent to multiple receivers using a buffer amplifier, which is more challenging with current loops.
5. Voltage-Based Analog Input Cards
Many analog input cards, especially in low-cost automation systems, are configured for 0-10 V or 1-5 V input. Signal conversion allows you to use 4-20 mA sensors with these systems.
How to Convert 4-20 mA to 1-5 VDC
The conversion is straightforward using Ohm’s Law:
V = I × R
Where:
- V is voltage in volts
- I is current in amperes
- R is resistance in ohms
To convert 4-20 mA to 1-5 VDC:
- Use a 250-ohm precision resistor
- Place it in series with the current loop
| Current (mA) | Voltage (VDC) | Calculation |
|---|---|---|
| 4 mA | 1 V | 0.004 × 250 = 1 |
| 20 mA | 5 V | 0.020 × 250 = 5 |
Important Considerations:
- Use precision resistors (0.1% tolerance) to maintain accuracy
- Ensure the voltage drop does not exceed the input limitations of the receiving device
- Check that the transmitter can supply the required voltage to overcome the loop resistance
Pros and Cons of Conversion
| Advantages | Considerations |
| Enables compatibility with devices | Voltage signals are prone to noise |
| Simple implementation | Limited transmission distance (~10m) |
| Easy signal scaling and display | Requires precise resistor selection |
| Cost-effective for small systems | Not ideal for harsh environments |
Real-World Application Example
Scenario: A chemical plant uses a pressure transmitter with a 4-20 mA output to monitor reactor pressure. The available DCS only accepts 1-5 VDC analog inputs.
Solution: A 250-ohm precision resistor is installed across the input terminals. This converts the current loop into a 1-5 VDC signal compatible with the DCS without requiring expensive signal isolators.
Result: The plant achieves reliable data acquisition without hardware changes to the existing transmitter.
Alternative Solutions
If passive conversion using resistors isn’t viable, consider these options:
1. Signal Conditioners
These are dedicated modules that convert 4-20 mA to 1-5 VDC with added benefits like isolation, signal amplification, and linearization.
2. Analog Input Modules with Built-In Conversion
Some advanced PLCs/DCS offer modules that internally handle current-to-voltage signal conversion with configurable input types.
3. Loop-Powered Converters
Compact devices powered directly from the current loop that convert signals to a wide variety of formats including voltage.
Conclusion
Converting a 4-20 mA signal to 1-5 VDC is a practical and common requirement in automation, control, and instrumentation. Whether due to system limitations, signal compatibility, or integration needs, this conversion ensures that your sensors and controllers can communicate seamlessly.
By using precision resistors or dedicated signal conditioning hardware, engineers can achieve accurate, reliable, and cost-effective signal conversion. Understanding the principles behind this conversion not only solves immediate compatibility issues but also empowers better design decisions in future automation projects.