View Full Version : Sinking Input cards

Lead Maintenance
March 7th, 2006, 09:55 PM
Just so I have this 100% correct. PNP devices are used in conjunction with Sinking input cards. The online tutorial states that this type of setup is commonly used in Europe but I've noticed it is pretty common on our equipment. Thanks!:cool:

March 8th, 2006, 01:13 AM
Aparently there's a difference in terms between AB and other brands, according to a recent thread.
Look at how the input card gets wired.
If +24 turns the input point on, use a PNP Sensor.
If a ground turns the input point on, use an NPN Sensor.
Simply put, PNP switches the + side of the supply. NPN switches the negative side.
When someone calls an input card 'sinking', I assume you need to wire it to NPN (sinking) sensors. A 'sourcing' card takes PNP (sourcing) sensors.

For this discussion, we assume a 24DC supply, with the minus side grounded.
From a design standpoint, an NPN transistor is prefered over a PNP. That fact alone makes them the most common. Years ago, NPN was the norm - PNP was rare. The Collector (Output lead in PLC speak) of an NPN 'sinks' to ground when the transistor is turned on (makes a connection Collector to Emitter. Emitter is grounded). It supplies a low.

1) If an Input terminal is 'looking for a low input' to activate it (grounding the input turns it on), then use a sensor with an NPN output. The NPN output of the sensor 'supplies' a low or grounded output.

2) If an Input terminal is 'looking for a high input to activate it (connecting it to +24 turns it on) then use a sensor with a PNP output.
The PNP output of the sensor 'supplies' a high or +24DC.

Think of a sensor with an internal relay for it's output. If we wanted it to output +24DC when it's active, we wire the common of the relay to +24. The output would be +24 when the sensor is active, and floating or open circuit when it's off. To replace the relay with a transistor, we need to use a PNP transistor.

The opposite is true when we want to switch a ground (sink). We need an NPN transistor to switch the common or ground.

The same terminology applies to Output cards. If a solenoid has one end hard-wired to ground, then we need to 'supply' +24 to the other lead to turn it on. We need a 'Sourcing' (PNP) Output to send +24 to the solenoid.
If one end of the solenoid is hard-wired to +24, then we need a 'Sinking' Output (NPN) to supply a ground.

Think of how a light is wired in your car. All the lamps are connected to the frame ground. Minus side of the battery. Switches are use to connect the positive side of the battery to turn a light on. The 'switch' is 'sourcing' power to the lamp. If we wanted to replace the switch with a transistor, we need to use a PNP. In the early days, PNP transistors were more expensive than NPN, and didn't work very well as switches.
NPN's were much easier and cheaper to work with. The supporting electronics were also simpler. So when we went digital, we had to do everything backwards. Wire all the lamps, etc. to the Positive side of the battery, and switch the ground side. Instead of 'sourcing' loads by switching the plus side, we were now 'sinking' loads by supplying the ground. This is called Negative logic. Ground it to turn it on.

With today's semiconductors, PNP's work just as well as NPN's for the most part. Now the manufacturers can supply PNP outputs for the same price. With PNP switching, we are back to the norm of common grounding everything and switching the plus side.

Portions of this strange thinking from the digital world has found it's way into PLC speak. The manufacturers throw around terms like NPN, PNP, sourcing and sinking, and confuse everybody.
Remember the plus side of the supply is the 'source' of power, and you need a PNP transistor to switch it. The negative side of the supply is the 'sink', and you need an NPN transistor to switch it.
I hopes this clears the fog a little.

March 8th, 2006, 02:20 AM
Yes, I had a few words to say on the other thread.

At the end of the day, a PNP/sourcing device goes into an input card with a negative common. A NPN/sinking device goes into an input card with a positive common. The only thing that matters at the end of the day is voltage differential when the input device turns on. PNP turns on and delivers near 24VDC to the input card which has a negative common - near 24VDC voltage differential turns on the input. NPN in reverse of course.

Lead Maintenance
March 8th, 2006, 03:49 AM
Thanks all!!!!