The main reason most analog output devices use a 4 to 20 mA range instead of 0 to 20 mA is to ensure reliable operation and easier detection of faults.
Here’s a breakdown of why 4-20 mA is preferred:
- Zero Current Avoidance (Fault Detection):
A current of 0 mA could potentially mean a fault in the system, such as a broken wire or disconnected device. By starting at 4 mA instead of 0, it becomes easier to distinguish between normal operation and a fault. If the current drops below 4 mA, you can immediately recognize that something is wrong (like a wiring issue or device failure).
- Signal Stability:
Analog signals are usually transmitted over long distances, and at very low current levels, electrical noise or resistance in the wire can cause signal degradation. A 4 mA minimum ensures that the signal is strong enough to travel through the wires without too much interference.
- Standardization:
The 4-20 mA range has become a standard in industries like process control and instrumentation. This standardization allows for easier integration of different equipment and ensures compatibility between devices from different manufacturers.
- Linear Relationship:
In many systems, the current is used to represent a physical quantity (e.g., temperature, pressure, or flow rate). The 4-20 mA range provides a clear and predictable linear relationship, making it easier to interpret the readings.
In summary, the 4 to 20 mA range offers reliability, ease of fault detection, and stability in signal transmission, which is why it’s widely adopted in analog output devices.
