Two and three-wire connection

Sensors with digital output to function properly need to consume power for operation of the control circuit. Furthermore, the output circuit must complete a loop with the power supply and the load. This is done connecting to two wires o three wires.

The three-wire connection is the most common, two of the wires are sensor supply and the third wire is connected to the sensor output, internally one of the power cables should go to the output circuit to supply power to the load as it is shown in the following figure.

Three wire connection

In the two-wire connection, the wires are used to connect the output circuit to the load, the sensor can be seen as a switch, and, necessarily, the sensor is powered by the same two-wires.

Two wire connection

Some aspects to consider, in the two-wire connection, are:

1. The power consumed by the sensor is given by the product of the voltage at its ends multiplied by the current flowing through it (Ps=vs*is).
2. The load voltage is the difference between the voltage of the power supply and the sensor voltage.
3. When the sensor is active, the sensor output is connected to the load, under these conditions the voltage across the sensor drops to a very small value, enough so that the voltage reaching the load can activate it.
4. When the sensor output does not conduct, the current flowing through the loop (load, sensor, and source) is small enough so that the load is not active, but large enough to supply the needed power to the sensor operation.
5. The voltage (vs) and current (is) of the sensor cannot fall to zero because the sensor operating power would also be equal to zero and the sensor stops working.

In this sense not any load can be connected with a two-wire sensor, only those whose current required for activation is less than the operating current of the sensor (is) when its output is not driving. A clear advantage of the two-wire sensor to the three threads is the number of drivers needed for wiring.

Series and parallel connection

The following figure shows how the sensors two and three wires are connected in series and parallel.

Connection serie parallel