Frequently Asked Question

A 4-20mA output sensor can be used for long distances. But is there a max. length? What should I take into account when designing the application?

A common rule of thumb is to limit the cable length to less than 30 meters for a current output sensor with a typical output of 4-20 mA. However, this can vary depending on the specific sensor and cable used.

To determine the maximum cable length for your sensor, you can use the following formula:

Maximum cable length = (supply voltage - (sensor output current * load resistance)) / (cable resistance per unit length * sensor output current)

Where:

• Supply voltage is the voltage provided to the sensor
• Sensor output current is the current generated by the sensor
• Load resistance is the resistance of the load connected to the sensor output
• Cable resistance per unit length is the resistance of the cable per unit length

By using this formula, you can calculate the maximum cable length that will provide accurate and reliable sensor output.

For example, the sensor outputs are represented with 4-20mA current, which means a maximum of 20mA is running through the circuit. If we simplify the whole loop as power, sensor, cable and the load as below:

Vsupply >= Vsensor + Vcable + Vload.

• Our sensor can be seen as a resistor of 250 Ohm, with 20mA current the Vsensor will be 5V.
• Vcable is depending on the cable type and the length.
  Take a length: 1000m  diameter: 0.34mm2 cable for instance, if you connect the sensor via that cable to your load. The resistance of the cable is about 57 Ohm x2 = 114 Ohm because the current runs from the cable to the load and then again back to the source.  Therefore the Vcable is about 114 x 20mA = 2.28V
• Vload is depending on the user application.
  Take a standard load of 250 Ohm for instance. The Vload = 5V.

If we add all the voltage up, we need at least 12.28V (in theory).