Sinking And Sourcing

Question 1

Define sourcing
PLC and connections
Show path of current flow

Answer 1

Providing current
PLC provides v+ to load, device completes the circuit to the ground
If = PLC v+ → field device → ground

Question 2

Sinking
PLC and connections
Show current flow path

Answer 2

Receiving current
PLC provides a path to the ground and the device provides v+
Flow I= v+ → field device → PLC

Question 3

PNP transistor

Answer 3

This have a sourcing output so PLC must be sinking (most common)

Question 4

NPN transistor

Answer 4

It has sinking output so PLC has to source it

Question 5

More common transistor output type

Answer 5

It source current so the field device needs to be sinking type

Question 6

Transistor problem and solution

Answer 6

The transistor output tend to allow an amount of leakage current to flow even when the output is off, this can lead the output device to be partially on of “hum”
Solution:
Connect bleed resistors
Same value of input impedance will bleed half the leakage

Question 7

Sinking announcing important when
Failure will result in

Answer 7

When the input or output device is solid state

The device not working