The terms current source and sink, imply the direction of current flow.

For a source circuit, there is a "switch" connected between the positive terminal and load, where it sources the current through the load from a voltage supply.

For a sink circuit, there is a "switch" connected between the load and negative terminal, where the current sinks to ground through the load.

Is my understanding correct or not ?

What is the importance of understanding the source and sink concept in schematic design ?


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What you are describing is covered in "A' and "B" above. Your description for a sink is different. "C" shows perhaps a more common and intuitive description.

This a good question as a good understanding is needed. A current sink/source is not the same thing as a ground which can both sink and source current. This is important as not all components can both sink and source and misusing them can lead to confusion.

  • \$\begingroup\$ Hi, can you give one example of application of source and sink current circuit respectively ? \$\endgroup\$
    – nee
    Oct 17 '12 at 7:00
  • \$\begingroup\$ +'ve 78XX series voltage regulator \$\endgroup\$ Oct 17 '12 at 15:08
  • \$\begingroup\$ -'ve 79XX series voltage regulator \$\endgroup\$ Oct 17 '12 at 15:08
  • \$\begingroup\$ To my understanding, a voltage regulator is a voltage source, not a current source. The current on the load will be anything its impedance determines for the voltage supplied, thus not keeping it constant. I'd say an LED driver would be a better candidate for a current source practical example. \$\endgroup\$
    – fceconel
    Oct 17 '12 at 17:49
  • \$\begingroup\$ Ok, let me clarify: 78XX is a positive regulator and ONLY sources current -> it's not actually a voltage source in the ideal circuit way of thinking of things. The vice versa applies to the 79XX (-'ve voltage & sinks). PNP transistor is a current source and NPN transistor is a current sink. \$\endgroup\$ Oct 17 '12 at 17:58

In simple terms, a current source is a power source which (ideally) keeps the current through the load constant; therefore, it reduces the supplied voltage as much as the load impedance drops, and increases it when the load impedance rises.

A current sink can be considered a load with a special behavior, in which it'll increase its own impedance when the voltage across it increases, and decrease it as much as the voltage drops, thus keeping the current through it also constant.


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