Difference Between Sinking Current and Sourcing Current: 7 Powerful Facts

Difference Between Sinking Current and Sourcing Current

The difference between sinking current and sourcing current is a fundamental concept in digital electronics and control system design. Understanding this difference is helps engineers correctly design microcontroller I/O, PLC outputs, sensor interfaces, and motor control circuits.

Incorrect use of sinking or sourcing outputs can lead to unstable logic levels, damaged I/O pins, or unreliable system operation. This article explains the difference between sinking current and sourcing current from current flow direction, logic state behavior, output drive capability, and real-world application scenarios.

What Is Sinking Current?

Difference Between Sinking Current and Sourcing Current in Low-Level Output

Sinking current occurs when a digital output is driven to a low logic level, and current flows from the external load into the output pin of the device. In this mode, the output port acts as a current sink.

A typical example is an MCU driving an LED:

The LED is connected to VCC through a current-limiting resistor
The MCU I/O pin outputs LOW
Current flows from VCC → LED → I/O pin → ground

In this configuration, the MCU is sinking current.

Key Characteristics of Sinking Current

Output logic state: LOW
Current direction: Load → Output pin
Higher current capability compared to sourcing
Widely used in PLCs and industrial I/O modules

Because many chips can sink more current than they can source, sinking current outputs are often preferred for driving LEDs, relays, optocouplers, and motor control inputs.

What Is Sourcing Current?

Difference Between Sinking Current and Sourcing Current in High-Level Output

Sourcing current occurs when a digital output is driven to a high logic level, and current flows from the output pin to the external load. In this case, the output port acts as a current source.

Example scenario:

The I/O pin outputs HIGH
The load is connected between the pin and ground
Current flows from the I/O pin → load → ground

Here, the device is sourcing current.

difference-between-sinking-current-and-sourcing-current.jpg

Schematic Diagram of Current Direction for Sourcing and Sinking Current

Key Characteristics of Sourcing Current

Output logic state: HIGH
Current direction: Output pin → Load
Lower maximum output current capability
More sensitive to voltage drop under load

Difference Between Sinking Current and Sourcing Current in Output Drive Capability

Sinking Current and Low-Level Voltage Rise

As sinking current increases, the internal transistor inside the chip experiences a higher voltage drop. This causes the output low-level voltage (VOL) to rise.

Designers must ensure that VOL remains below the logic low threshold. For example, TTL logic typically requires VOL ≤ 0.5V to avoid logic errors.

Sourcing Current and High-Level Voltage Drop

As sourcing current increases, internal resistance causes the output high-level voltage (VOH) to decrease.

VOH must remain above the minimum logic high threshold. TTL systems typically require VOH ≥ 2.4V for reliable operation.

Typical Application Scenarios

Difference Between Sinking Current and Sourcing Current in Real Circuits

Typical Applications of Sinking Current

LED indicator circuits
PLC digital outputs
Relay and solenoid drivers
Motor control input signals
Industrial automation equipment

Sinking outputs are widely used in industrial environments due to their stronger load-driving capability and higher tolerance to electrical noise.

Typical Applications of Sourcing Current

Logic signal transmission
High-impedance sensor interfaces
Reference voltage outputs

Because sourcing current capability is often limited (typically less than 5 mA), it is not recommended for driving high-current loads directly.

Difference Between Sinking Current and Sourcing Current in Datasheets

Datasheet values for sinking and sourcing current represent the maximum allowable output current under defined conditions. These values are absolute limits, not recommended continuous operating currents.

Designers should always apply safety margins and consider the total current across all I/O pins.

For additional technical reference, see this application note from Texas Instruments: Understanding Digital Output Drive Capability

Practical Design Recommendations

Prefer sinking outputs for LEDs, relays, and motor driver inputs
Verify VOL and VOH thresholds against logic requirements
Never exceed datasheet current ratings
Use external driver circuits for higher current loads

Conclusion

The difference between sinking current and sourcing current lies in current direction, logic output state, and drive capability. Sinking current flows into the output pin at low level, while sourcing current flows out of the pin at high level.

Proper understanding and correct application of these concepts help engineers design reliable, safe, and long-lasting digital and motor control systems.