On Sat, 15 Nov 2008 08:21:41 -0500, "Carl Boyd"
wrote:


"Karl Townsend" wrote in message
tanews.com...
I'm about to purchase a photo eye. http://tinyurl.com/6bjmeb

What's the difference between PNP and NPN? I just want an eye that's NC
powered by 12VDC that uses very little power.


Karl



Karl

It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load has
to be tied to the power supply, get the NPN.

To clarify a bit: an NPN sensor switches the negative wire; PNP
switches the positive. AN NPN sensor is a current sinking device; a
PNP is a current sourcing device.

Be careful when using the terms "sinking" and "sourcing" in the
context of industrial controls, especially PLCs. Some mfrs used (or
have in the past used) "sinking" to refer to a PLC input that's
compatible with a sinking sensor, even though the input itself is
sourcing current. Other mfrs use the convention that a sinking sensor
connects to a sourcing input. I'm not aware of any ambiguities in the
use of PNP/NPN. NPN devices are generally more flexible in systems
with mixed voltages.

(Fixed font)

PNP:
+V ---(sensor)-----------|
| |
|----(load)----| 0V

NPN:
+V |----(load)----|
| |
|-----------(sensor)--- 0V

--
Ned Simmons