Current Sourcing/Sinking

Can someone tell me the difference between current sourcing/sinking with devices such as 3 wire proximity sensors and such. I am unclear about the difference.
Thanks.

Sourcing/Sinking

Try this website:
http://www.patchn.com/npnpnp.htm

SOURCE/SINK

VERY SIMPLE:
CURRENT SOURCING OR PNP SENSORS SOURCE OR SUPPLY CURRENT WHICH MEANS THE OUTPUT BECOMES A POSITIVE WHEN ITS ON AND FLOAT WHEN THE SENSOR IS OFF SO YOU WOULD CONNECT YOUR LOAD BETWEEN THE OUTPUT AND A NEGATIVE.

CURRENT SINKING OR NPN SENSORS SINK CURRENT WHICH BASICALLY MEANS THE OUTPUT BECOMES A NEGATIVE WHEN THE SENSOR IS ON AND FLOAT WHEN THE SENSOR IS OFF SO YOU WOULD CONNECT YOUR LOAD BETWEEN THE OUTPUT AND A POSITIVE.

To expand on Netvisor's reply:

I hope I get ALL the facts right (and don't forget anything), so I don't get flamed.

An interesting thing you'll see, if you hook up a multimeter; PNP (Sourcing) works like you'd expect. If it's ON, you see (logic high)voltage; OFF - low (logic low) voltage.

NPN (sinking) can be trickier. The circuit blocks CURRENT until it is switched on, so you'll see voltage in the OFF state. In the ON state, the device acts as a current path and you'll see a low voltage.

AK



Ok guys: where did I screw up?

Just be careful in spec'ing modules or plcs from different manufacturers without looking at the field wiring specifications.

I like Terry's explanations, just look at what "thing" you are using.

-tommy23t

I was asked by our electricians couple times to explain the
difference. I even looked for other explanations like the
link posted by Mark, but I didn't like it myself. It seamed
this introduced more confusion than it was before. Then I
realized that non of them had problem wiring regular push
buttons or switches. See if this attachment helps.
Wiring is always the same (just stick to this for now):
brown: +
blue: -
black: output

panic mode said:

See if this attachment helps.

Click to expand...

That's pretty clever. I like it!

hi......
regarding pnp & npn sensor.........when i configure my PLC in sink mode i have to use a relay to hook up the pnp sensor to the plc(npn drives relay & plc gets contact from that relay).........
anyone of u know how can use bothpnp & npn sensors directly to the same plc?

lucifer said:

anyone of u know how can use bothpnp & npn sensors directly to the same plc?

Click to expand...

There are a couple ways to skin that cat, but no magic bullet. You'll need extra hardware in all cases. Some PLCs can handle either type of input, depending on how you wire them. But unless the inputs are isolated from each other, you're probably stuck with choosing which type you want anyway. Here are some ways around it:

1. A PNP/NPN converter. There are cheap little converters available, or you could build one yourself if you wanted to.

2. Relays. These won't be as fast as the transistor option, so putting them on a pulsing input/output may not be a good idea.

3. Pull-up/pull-down resistors. This has been covered on the message board elsewhere (LINK?). This is the cheapest way to handle the problem, but it could also end up LOOKING cheap. Also, the logic will be inverted. When the device output is ON, the PLC input will be OFF.

AK

anyone of u know how can use bothpnp & npn sensors directly to the same plc?

Click to expand...

You can have both sourcing and sinking input cards in your PLC. Just connect the sensors directly to the appropriate card. In the case of Modicon 800 series I/O their "true low" input card accepts the sinking (npn) signal directly and their "true high" input card accepts the sourcing (pnp) signal directly. As stated previously you can also use an isolated input card and wire each input as needed.