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Current Sourcing and Sinking

Created on: 6 February 2013

Current sourcing and sinking is often mentioned in relation to electronics, digital systems and microcontrollers, but what is current sourcing and what is current sinking?

Current sourcing and sinking refers to the way that an external load is connected to a circuit, system, microcontroller or other electronic device.

Circuit Diagram

The circuit diagram below shows the difference between current sourcing and current sinking.

Current sourcing and current sinking

Current Sourcing (left) and Current Sinking (right)

The load in the circuit diagram is shown as a 1k resistor, but can be any load that draws a current such as an LED and series resistor, the coil of a relay, a light bulb, etc.

The device in the circuit diagram can be any electronic circuit or device such as a microcontroller, FPGA, CPLD, logic circuit, etc.

Current Sourcing

When a load is connected to a device so that the device supplies current to the load (sources current) then the configuration is said to be current sourcing.

An example is a series resistor and LED connected between a microcontroller pin and GND. When the microcontroller pin is switched high (logic 1) then the microcontroller will source current to the load. In this configuration a logic 1 will supply power to the load (switch the LED on) and a logic 0 will switch power to the load off (switch the LED off).

Current Sinking

When a load is connected to a device so that current flows from the power supply through the load and into the device, then the configuration is said to be current sinking. When current flows into the device, it is said to be sinking current.

An example of current sinking is when a series resistor and LED is connected between power (e.g. +5V) and a microcontroller pin. When the microcontroller pin is switched high (logic 1) then the current to the load is switched off. When the microcontroller switches the pin low (logic 0), current flows through the load.

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