NPN/PNP=What a Delima!

[rsdoran]

I was planning to show that this afternoon, even got a PM on the subject. There just are not enough hours in a day to get everything accomplished you would like too.

Here is another example showing a sinking plc output using a pull-up resistor with a sinking field device.

To calculate R you will need to know the current needed to activate the field device.

As I mentioned before the "logic" will be inverted, that means the field device will be energized when the PLC output is OFF.

Another thing to NOTE is that you have to be careful not to use this method for heavy loads which could be anything greater than 25ma...check your brand of plc to determine that part.

 

[keithkyll]

Which sends current to the load “Sinking” or Sourcing”.

How about a simple answer?
Sourcing supplies +24 DC to the load.
Sinking supplies ground (-24 or return) to the load.
Relay module example: Wire the module commons to +24 DC to make it sourcing. Wire -24 DC (return) to the module commons to make it sinking.

PNP transistors are used in Output sourcing modules.
NPN transistors are used in Output sinking modules.

If your trying to decide whether to design with sinking or sourcing output modules, use sinking. It's the most common. Internal to the module, it's easier and more efficient to use NPN transistors, so sinking modules provide the highest current at the lowest cost.

 

[manmeetvirdi]

NPN and PNP made clear

Hi Mr Doran
Fair enough now i understand but given a choich will u go for NPN or PNP?
regards
manmeet

 

[jprindle]

Personally, I think PNP is safer. A short (pinched wire) on a NPN will give you a false input. A short on a PNP will shut down the power supply, which in most cases will do no damage, but will let you know there is a problem. Just my opinion.

 

[rsdoran]

I am over my head as far as explaining.

Personally, I think PNP is safer. A short (pinched wire) on a NPN will give you a false input. A short on a PNP will shut down the power supply, which in most cases will do no damage, but will let you know there is a problem. Just my opinion.

A PNP can short and also provide a false signal AND may not shutdown the power supply...it depends and that was my whole point. The short would have to be to ground, another potential, etc before the supply voltage would be affected. For 2 wire devices the 2 wires from the device shorting or the device shorting could just be a full time connection to the plc input.

Another issue is that not all solid state DC devices are 2 wire, many are 3 or 4 wire it is not automatic that one will be safer then the other.

The issue is not understanding the terms NPN, PNP, sink, or source, what has to be understood is exactly what is/can be happening. Instead of trying to teach electronics someone came up with the terms sink and source;
SOURCE provides a path for DC + voltage
SINK provides a path to DC - (common)
In general that should be simple to understand...ie if the PLC input is Sinking (NPN) then the common (-DC) will be connected to the common terminal of the PLC input AND a device will need to provide a POWERED signal as an input.

Personally it actually confused me at first.

As far as designing a system goes you should not say, "I have this NPN card so must use PNP devices or vice versa", you should look at the application and understand the possibilities and use the appropriate hardware.

The main point of this was more of "technical tips" for those that have to be in a plant at 2:00 A.M. and the PNP (or NPN) field device is bad and all that is available is an NPN (or PNP) replacement device...it can still be used to make the system work if necessary, in many cases it can be because downtime can cost 1000's of dollars lost to the company.