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.
For the sake of simplicity, we are showing sensors directly driving loads throughout this lesson.
This is not common.
|Power Supply||Direct Current|
|Working Voltage||10 ÷ 30 V|
|Switching Distance (Sn)||4 mm|
|Hysteresis (% Sn)||< 10 %|
|Max Switching Frequency||1000 Hz|
|Repeatability (% Sn)||<= 3|
|Max Output Current||200 mA|
|Absorption||< 15 mA @ 24Vdc|
|Voltage Drop||< 1.8 V|
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.
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.
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).
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.
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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