2-Wire and 3-Wire Sensors

This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial

2-Wire Sensors

2-wire sensors are connected in series with the device that is acquiring the data.

Figure 1: 2-Wire Sensors

When the sensor is not activated, it must draw a minimum operating current normally referred to as off-state leakage current. In some cases this current is also referred to as residual current. This current is necessary in order to keep the sensor electronics working. The residual current is not a problem if the sensor is connected directly to another load other than the data acquisition device. For example, an industrial environment is not unusual to have these 2-wire sensors directly connected to motors and similar devices, which have a low impedance.

Residual current is a problem for the system if the sensor requires a residual current greater than what the digital I/O module has been designed to sustain. In this case the digital I/O module might incorrectly detect an "ON" state because the current being drawn is greater than it expects to provide. Most 2-wire sensors in the industry have an off-state leakage current or residual current no higher than 1.7 mA.

Similar to the "OFF" state, there is also a minimum current required to sustain the "ON" state of a sensor, called the minimum holding current. Typically this current ranges from 3 mA to 20 mA. The sensor will not operate correctly if the digital I/O module cannot sink or source this current.

3-Wire Sensors

3-wire sensors derive their power from a excitation terminal rather than through the digital output line. They are also called line-powered sensors by some manufacturers.


Figure 2: 3-Wire Sensors

The operating current that these devices pull from the digital I/O module is called burden current , which is typically in the neighborhood of 20 mA. It is important to notice that this current is provided by the excitation terminal.

Recommended NI Products that Connect to 2-Wire and 3-Wire Sensors

NI 6509
NI 6514
NI 6515
NI 6528

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