Sinking Output?

To "source" something means the juice comes out of your device. To "sink" something means the the juice comes out of a source, through the load (if there is a separate load), and through your device to ground.

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Hi. A current-sourcing output will source current, a sinking output will sink current. Sourcing outputs are also called "PNP" and sinking outputs are also called "NPN". Here's the details.

+24V ..-------------------o---o "Sinking"| | NPN | | Sensor | .-. ..------o---. | | R | | | | | .--o-----. '-' | | | | | | | | | | OUT | |/ | '---------o---o | -| | | I (sink) | | | | | | '-------o--' | '----------------------o GND created by Andy´s ASCII-Circuit v1.24.140803 Beta

Here's a "sinking" NPN sensor, say, a proximity switch. Internally, at the output of the sensor is an NPN transistor. When an object comes in proximity to the sensor, it is made so that the NPN transistor at the output turns on, "sinking" current to GND. When the transistor is off, no current flows and you can read a voltage close to or equal to V+ at the output because of the + referenced load (R). Sensors of the "sinking" or NPN type are more common than sourcing.

These sensors work well with programmable controllers, which have optoisolators built in which have their commons tied to V+, like the resistor above. They can sometimes be used with logic level power supplies, and can sometimes be connected to logic inputs, but you have to be careful and read the sensor literature. One big problem with this type of output is that it is somewhat susceptible to electrical noise when the transistor is off (false ON). Also, note that usually these sensors have small signal transistors at their outputs, and can't switch more than 50 to 100 mA. Also, do not use these to switch relays or non-resistive loads -- inductive kick can crash the output transistor. Since it's built-in and non-repairable, that means you've got to throw the sensor out.

Possibly if you ask for an "NPN-type" sensor you might have more luck. Or, if you can be more specific as to your sensor requirements, someone can recommend something for you on this newsgroup.

Good luck Chris