What is 2 wire sensor

    2-wire sensors are proximity sensing devices that can operate with ac/dc voltage according to their structure, have normally open or closed output types, and can be connected directly to the circuit in series and turn the load current on and off. The most important difference that distinguishes it from other sensors is that they always give leakage current whether they see the target or not. For this reason, receivers with large internal resistance cannot work properly.

Regardless of the supply voltage, a 2-wire sensor consists of the live end of 1 power supply and the other end of the cable coming directly from the non-polarized (most often relay, contactor or PLC optocoupler) source. Depending on the type of application, the load to be triggered can be connected to both + or - branches. At this point, the ohmic or resistive internal resistance of the receiver is of great importance.

 2-wire sensor wiring diagram

Two-wire sensors are commonly used in industrial control to simplify wiring. However, it is a mistake to see it as a competitor for many sensors. There are various technical difficulties that make it difficult to design a two-wire control circuit. Sensors operating with the two-wire method must operate over a wide range of power supplies. With the sensor off, the circuit can typically draw a minimum residual current of less than 1.5 milliamps. This leakage current can affect the triggering for mini relays such as PLC controllers or electronic time relays.

How does a 2-wire inductive sensor work ?

As can be clearly seen in the picture above, the triggering of the zener diode in the internal structure of the 2-wire sensors depends on the possible leakage current. Because trigger voltage is required for mosfet switching. The key word at this point is "receptor internal resistance". For example, in some plc systems, since the input optocoupler structure has lower resistance, triggering may occur even if the sensor does not see the target. For example, we cannot talk about the same risk for a contactor to be connected to the output.

What is the difference between 3-wire sensors and 2-wire sensors?

    As you know, the sensors can also be separated by cable configurations (such as npn-pnp or nc-no selection with the 4th cable) depending on the preference of the manufacturers. The most common are sensors with 2 wires and 3 wires.

     Two-wire devices are designed to wire in series with the load. In a 3-wire inductive configuration, two of the three terminals provide the supply, while the third is the output terminal and changes the load. Both types can be strategically connected in serial or parallel configurations to protect inputs or perform logic.

2-wire sensor connection

Two-wire sensors are the easiest to wire, but they can hinder overall system performance. Two-wire sensors pass additional current through the same line they pass through, which we mentioned above. Also, since each device powers the next devices, the response time is equal to the sum of the turn-on times of each device. Therefore, the answer to the question of whether 2-wire sensors can be connected in series depends on the value of the internal resistance level.

     In summary:
    A 2-wire sensor is basically a loop-operated device without requiring a separate supply voltage (source voltage is fed to the target device), while a 3-wire sensor is a self-powered device, meaning you supply the sensor with source voltage and you get results regardless of whether there is a load on the output.

What are the types of 2-wire sensors?

As you can imagine, there are many 2-wire sensor models, especially inductive and capacitive models.

According to their feed:

1- AC working between 380V-180 volts

2-Ac working between 240 -90 volts

3- AC working between 240v-14v

4 - Working between DC 265v- 90v

5- They are models that can operate between DC 33-12volt. (These models are only those on our website, manufacturers produce sensors at more feeding intervals.)

By output type:

NC: Normally closed

NO: Normally open

PNP (for DC models): With positive (+) turn

NPN (for DC models) : Has negative (-) turn

Attention please: The most confusion occurs between 2-wire proximity sensors and NAMUR sensors. You can learn about the NAMUR sensor, which is one of these types with two different working methods, here.

---------