Sensor Wiring

Objectives

  • Explain the meaning of the terms sourcing and sinking.
  • Differentiate between active and passive sensors.
  • Analyze the wiring method required by various sensors.

Background

This lesson builds on knowledge about BJT NPN and PNP construction. If you'd like a refresher, check out our sessions on Diodes and Semiconductors.

Reality Check

For the sake of simplicity, we are showing sensors directly driving loads throughout this lesson.

This is not common.

In the real world sensors are often connected to an electronic control module (ECM) or a programmable logic controller (PLC) rather than a load, because sensors are often incapable of handling load currents.

Technical Characteristics

Power SupplyDirect Current
Working Voltage10 รท 30 V
Switching Distance (Sn)4 mm
Output LogicPNP
Output TypeNO
Hysteresis (% Sn)< 10 %
Max Switching Frequency1000 Hz
Repeatability (% Sn)<= 3
Max Output Current200 mA
Absorption< 15 mA @ 24Vdc
Voltage Drop< 1.8 V
Specs from a PNP NO proximity sensor

For example, this table lists the specifications for a common proximity sensor. Notice that the maximum output current is 200 mA (or 0.2A), while our example solenoid might draw as much as 0.5A. This strong current would cause the delicate sensor to burn out.

Instead, sensors are usually wired to the input terminals of an ECM or PLC, with the final load (e.g. lamp, or solenoid), connected to a specially designed output card. This arrangement isolates the sensor from the current draw by the load.

An example of how a proximity sensor might actually be wired into a circuit.
So enjoy the simplified circuits in this lesson, but never wire a sensor directly to a load without checking the sensor's specifications!
Nope, this is not what sinking means.

Sourcing Or Sinking?

Sourcing and sinking are descriptions of the way a sensor is wired into a circuit. They describe the current flow relationship between sensor and load.

Sourcing and sinking refer to conventional (positive to negative) current flow.

Sourcing

If a sensor requires sourcing circuitry, this means that when the sensor becomes active, current flows from the sensor output terminal (wire) to the load, and then from the load to ground (negative, or common).

Voltage Source
Sensor
Load
Ground

Sinking

If a sensor requires sinking circuitry, this means that when the sensor becomes active, current flows from the load into the sensor, and then from the sensor to the ground.

Voltage Source
Sensor
Load
Ground

Sourcing

Sinking

What will seem quite unique about sinking to the uninitiated is that the load will already have the positive voltage supply applied to one of the load terminals.

In DC circuits, some folks refer to sinking as ground-side switching.

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Atmospheric Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
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