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A Current source is a device that provides electrical energy regulated in such a way that the electrical current stays at the specified value.
While current sources have many applications within electronical devices like radio receivers, television sets, etc. they have little use in everyday applications. If one leaves the terminals of a current unconnected then the device will raise the voltage to try and maintain the correct current.
Comparison between a current source and a voltage source
Most of the sources of electrical energy (the mains, a battery,...) are voltage sources. Such sources provide constant voltage which means that a long as the amount of current asked to the source is within the source specifications the source output voltage stays constant.
A theoretical perfect voltage source would provide no energy when it is loaded by an infinite impedance (i.e. nothing is connected to the device terminals) and an infinite energy (and an infinite current) when it is short circuited. Such theoretical device would have a zero Ohms output impedance. A real life voltage source has a very low output impedance: typically less than 1 Ohms
Contrarily a current source provides a constant current which means that as long as the load connected to the source terminals has an impedance low enough the current stays constant.
A theoretical perfect current source would provide no energy to a short circuit and an infinite energy (and an infinite voltage) to an open circuit. A theoretical current source has an infinite output impedance
Example of Implementation
Transistor output configured in such a way that the load is wired from the output of the sensor to the common side of the power source so that when the transistor turns on voltage is sourced to the load. One form of current source is a current mirror.
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