Why Convert Between 0-10V and 4-20mA?
In industrial automation and process control, two signal standards dominate: the 0-10V voltage signal and the 4-20mA current loop. While both carry analog measurement data, they behave very differently over distance and in noisy environments. Understanding when and how to convert between them is a fundamental skill for any controls engineer or instrumentation technician.
Key Differences at a Glance
| Property | 0-10V | 4-20mA |
|---|---|---|
| Signal Type | Voltage | Current |
| Noise Immunity | Lower | Higher |
| Max Cable Run | ~10–15 m (typical) | 300+ m |
| Live Zero | No (0V = zero) | Yes (4mA = zero) |
| Wire Resistance Effect | Significant | Minimal |
The live zero of the 4-20mA standard is a major practical advantage — if a wire breaks or a transmitter loses power, the signal drops to 0mA, which is distinguishable from the "zero" measurement value of 4mA. This allows control systems to detect faults automatically.
How the Conversion Works
The conversion between these two signal types is a linear mapping:
- 0V corresponds to 4mA
- 10V corresponds to 20mA
The relationship can be expressed as:
I (mA) = (V × 1.6) + 4
Where V is the input voltage (0–10V) and I is the output current in milliamps. Conversely, to go from 4-20mA back to 0-10V:
V = (I − 4) × 0.625
Types of Signal Converter Modules
1. Active Voltage-to-Current Converters
These modules accept a 0-10V input and output a proportional 4-20mA signal. They require a separate supply voltage (commonly 24VDC) and use an internal op-amp circuit to drive the current loop. They are the most common choice for PLC analog outputs that need to drive long cable runs.
2. Passive Signal Converters
Passive converters draw their operating power from the signal loop itself. They are compact and require no separate power supply, but they typically have tighter input voltage requirements and may introduce minor non-linearity.
3. DIN Rail Isolating Converters
For environments with significant ground loops or electrical noise (motor drives, welding equipment), isolating converters provide galvanic isolation between the input and output circuits. This prevents noise from coupling into the measurement signal and protects sensitive PLC inputs.
Wiring a Basic 0-10V to 4-20mA Converter
- Connect the 24VDC supply to the converter's PWR+ and PWR− terminals.
- Connect the 0-10V signal source (e.g., a PLC analog output) to the converter's IN+ and IN− terminals.
- Wire the converter's OUT+ terminal to the positive terminal of your receiving device (e.g., a valve positioner or flow meter input).
- Complete the loop by connecting the receiving device's negative terminal back to the converter's OUT− terminal.
- Verify polarity at every connection before powering up.
Troubleshooting Tips
- Output stuck at 4mA: Check that the 0-10V input signal is actually present and measured at the converter terminals — voltage drop on long signal cables can be an issue.
- Output stuck at 20mA: The input may be above 10V or there may be a short on the input terminals. Check the source device.
- Noisy/fluctuating output: Ensure signal and power cables are not routed alongside AC power cables. Consider a shielded cable with the shield grounded at one end only.
Summary
Converting between 0-10V and 4-20mA is straightforward once you understand the linear relationship and the practical constraints of each signal type. Choose an active, isolated converter for industrial environments where noise and cable distances are a concern, and always verify wiring polarity before commissioning.