What is PLC Sinking and Sourcing?

There are three popular types of output (Digital Output) cards.

1.Relay 2.Sourcing 3.Sinking.

Relay cards are very easy to interface but suffer from the mechanical limitation of low-speed operation and limited lifespan.

DC Sinking and Sourcing output card can switch output quickly and dont suffer from mechanical wear but exhibit some interface limitations.

Sourcing Output Card

Sourcing output cards are called P-N-P because the switching element was traditionally a P-N-P transistor.

PLC sourcing output module - Working

The PLC card connects V+ to the load.In sourcing output module during switch closure V+ is connected to the load and current flows as shown.

PLC sourcing output-Current flow

Advantages of Sourcing Output Card are - 1.Easier interface since only one power supply is required.

Disadvantage of PLC sourcing output card. 1.Voltage to the load is limited to the card supply voltage.

Sinking Output Card

Sinking output cards are called NPN because the switching element was traditionally a NPN transistor.

PLC sinking output module - working

These PLC cards connect ground to the load.

In sinking output module during the switch closure the load is connected to ground to the switch.The switch is hooked to ground and load is connected to the V+. Sinking PLC output module current flow

Advantages of PLC sinking output card. 1.Ability to switch voltages higher than card supply voltage.

Disadvantages of PLC sinking output module 1.Two power supplies are required for proper interfacing.