In circuit analysis, an ideal current source is an active circuit element which maintains a constant current through its terminals. Its behavior is independent of any other element in the circuit. An ideal current source is generally represented as:
/-----\
| ^ | arrow indicates direction
| | | of current flow
\-----/
A current source is often used to produce signals for transmission over some distance. Since real-world wires have some resistance, by Ohm's Law there is a voltage drop across the length of the wire. For small voltages and long wires, this can affect the operation of a signal produced as a voltage source. A current source, while more complex and expensive than a voltage source, is not affected by voltage drop along wires because the current is constant along all points in any given mesh of a circuit. A mesh is a closed loop in a circuit which does not contain any other loops.
One way a current source can be used in signal transmission is to send the current through a resistor with a precisely known resistance value. The voltage drop along this resistor is then measured by the device receiving the signal. Current sources used in such a configuration are often designed to produce a current between 4mA and 20mA. This allows the current to pass through a 250Ω resistor to result in a voltage drop between 1 volt and 5 volts across the resistor. 4mA is used as the starting point so that if the device receiving the signal ever detects a voltage drop of zero, then it knows that communication has been lost and no signal is being received at all. Many devices are designed to report an error in this case.
wire resistance
+-----/\/\/----------+
| |
| +-----o positive
| | terminal
/-----\ >
| ^ | >
| | | 250Ω >
\-----/ >
| >
| |
| +-----o negative
| | terminal
+-----/\/\/----------+
wire resistance
In the above example, regardless of the wire resistances shown, the same current is flowing through the entire loop, as generated by the current source. The receiving device detects the voltage drop across the 250Ω resistor.
A real current source is more complex than an ideal current source. A real device would have a maximum amount of power and energy it can feed into the circuit, and if the load on the circuit is too high, it will not be able to generate the current which is expected of it. A well designed circuit will usually have a powerful enough current source that these issues can be ignored.