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#1
Start by
Nagaraja ramaiah
10-21-2013 02:17 PM

Practical difference between Sink and Source in PLC

Can any body differentiate Practical difference between Sink and Source in PLC
10-21-2013 04:39 PM
Top #2
Vladimir Pak, P.Eng.
10-21-2013 04:39 PM
If PLC card supplies power to a field device, it has sourcing I/O. If it it doesn't and field device is powered externally then it has sinking I/O.
10-21-2013 06:47 PM
Top #3
Keith Foley
10-21-2013 06:47 PM
To expand on Vladimir's comment:
Assuming 24 vdc I/O cards.
On a transistor output card Sourceing is PNP or Active High. This means that you connect the output to the + side of the load device and connect the other side to 0vdc. If the output Card is Sinking it is NPN or active low meaning that you would tie the + side of the device to 24vdc and the common would connect to the output card.
On input cards if the input card is a sourceing card you would use NPN sensors. The cards common connection would be to a +24vdc source and the device (sensor) would give a path to 0vdc. If the input card was sinking you would use PNP sensors. The sensor would be connected to the + 24vdc source and the output from the device would connect to the PLC input card which would have a common connection to 0vdc.
10-21-2013 09:17 PM
Top #4
Vijayant Singh
10-21-2013 09:17 PM
thanks for the great comments .
10-22-2013 12:10 AM
Top #5
Andrew Sterne
10-22-2013 12:10 AM
The practical difference between sink and source is that for troubleshooting purposes, switching the positive line is much more intuitive and so is regularly used in conjunction with low voltage (24Vdc or ac) to make a robust system that is quite safe and also easy to maintain. Switching the negative line tends to be used in hazardous locations, but is less intuitive to troubleshoot.

To get an input to change state, one has a +24V (for instance) line going out to a switch and the switch connects or disconnects that voltage to the input. The other has a common line (24C, for instance) going out, and the switch connects or disconnects the common to the input. Usually the common is grounded and this is especially important for things like sensing whether the gasoline dispensing nozzle has been placed on the pump - you don't want sparks to fly when you're done pumping gas!

Further info:http://www.automationdirect.com/static/specs/sinksource.pdf
10-22-2013 02:40 AM
Top #6
Trayton Jay
10-22-2013 02:40 AM
Keep in mind the frame of reference of the product vendor, which can cause confusion if not understood up front. For instance, some (most) label their "device" (I/O module, sensor, etc.) according to the internal operation while others label according to the operation of what it connects with.
10-22-2013 04:44 AM
Top #7
Mark Lochhaas
10-22-2013 04:44 AM
I believe Automation Direct has a particularly good document on Sinking and Sourcing with respect to PLCs. It can be found at this URL.

http://www.automationdirect.com/static/specs/sinksource.pdf
10-22-2013 07:26 AM
Top #8
Vijayant Singh
10-22-2013 07:26 AM
For sinking type its NPN configuration of field devices are used in which current flows through the sensor/device and the sinking type input card of plc give the current to sensor/device, load is connected between +ve and sensor/device output. Sourcing type its PNP configuration of field device in which current flows from sensor/devices, load is connected between sensor/device output and common or-ve. for this a general input card is used or special cards are to available.
10-22-2013 09:59 AM
Top #9
Brian Hernandez
10-22-2013 09:59 AM
If you want to run two relays with different voltage ratings on the same module you'd need to wire the PLC as sinking. It will allow you to run several voltages ratings by switching their common path to ground which is negative (-). Source will limit your choice of using several voltages on one module.
10-22-2013 12:57 PM
Top #10
Mike Korkowski
10-22-2013 12:57 PM
Simply stated Current Sinking is connecting (switching) ground to the device (load) through as an example a sensor output, and Current Sourcing is connecting (switching) positive DC to the device (load). Banner Engineering has a very valuable "Handbook of Photoelectric Sensing. Take a look.
04-22-2016 01:45 AM
Top #11
Nasr
04-22-2016 01:45 AM
Can any one explain if i have to controllers working in parallel i want to shuttdown one controller (Say master) gives command to shuttdown the other controller what would be the sink and source relation in this scenario? should the commander be source or sink?  
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