Four wires good, two wires bad
Original article date: September 1997
The relative merits of 4-wire and 2-wire signal/power transmission configurations in industrial sensors are the subject of some debate among controls and plant maintenance engineers.
In the 4-wire scheme, a pair of twisted wires is dedicated to the precise transmission of a 4-20mA current signal linearly proportional to the variable being sensed, while two additional wires are reserved for supplying power to the sensor.
The 2-wire method, on the other hand, uses the same pair of wires for transmitting the output signal and powering the sensor’s circuits.
To minimise signal noise (maximisation of S/N ratio), which is especially important in applications below 760°C, Calex has designed 4-way circuits into its infra-red thermometers. The resulting unlimited powering current allows for extremely low-noise circuit design and component selection.
Powering, noise and installation trades-off are listed. Clearly, a 4-wire sensor is the best application for all applications where process control accuracy and stability are early considerations.
In areas where a 2-wire device is being replaced by a 4-wire one, installation of the power supply at the sensor location will eliminate the need to rewire through conduits with existing 2-wire lines, and those wires may be used for signal transmission.
Table 1 Power/Noise Considerations
| 4-wire | 2-wire | |
| Power Supply | 12VAC | 16-40VDC (must be regulated) |
| Circuit Powering Current | Unlimited | 3.5mA maximum |
| Processing Electronics | Very low noise | Fair |
| Temperature Signal Accuracy | Not affected by power supply fluctuations | Affected by power supply fluctuations |
Calex Instrumentation
Tel: 01525 853800
Fax: 01525 851319
September 1997