What is a Current Source?

Question

In EveryCircuit, they have a component called a "Current Source". For example, here is a circuit I've created with a 2A current source:

Is there a such thing as a 'current source' in the real world, or is this an abstract component that helps one regulate current (for example, without having to do a voltage source + resistor).

Ideal current source as a discrete component is not physical (because it simply cannot work if not in a circuit). But there are implementations of a practical current sources, which you can easily find with Google. — Eugene Sh., Jan 21 '20 at 21:30
@EugeneSh. thanks -- does that mean it's really a 'short-hand' in the circuit simulator to emulate a fixed current? — David542, Jan 21 '20 at 21:32
Think of a voltage source that changes its voltage output so that it is always outputting a constant current. You can have constant-voltage or constant-current. In a constant-voltage output, the current changes to maintain a voltage. In a constant-current, the voltage changes to maintain the current. — Ron Beyer, Jan 21 '20 at 21:33
most bench supplies have a constant current mode. many LED drivers are CC. the venerable LM317 can be used as a CC supply. of course, laws of physics apply; it's hard to push 10 amps into a billion ohms... — dandavis, Jan 21 '20 at 21:37
Does this answer your question? [What exactly is a current source?](https://electronics.stackexchange.com/questions/28546/what-exactly-is-a-current-source) — , Jan 21 '20 at 22:24

score 2 · Accepted Answer · answered Jan 21 '20 at 21:55

There is no such thing as a perfect current source (or a perfect voltage source, for that matter), but things like batteries are closer to a voltage source than a current source.

There are a few devices that are more like a current source over some range of operation, such as a PV cell when loaded or some types of nuclear battery.

Usually in real circuits, voltage sources are some energy source like a battery or AC voltage connected to some parts that result in a fairly constant voltage over some range of current draw.

Similarly, current sources are usually some energy source connected to a circuit that produces a fairly constant current over some range of output voltage.

Here is a simple example of a (far from ideal) current source:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

As you can see from the simulation below, the approximately 1mA current is fairly constant for output voltages from 0V to 8V, above which it decreases quite rapidly. In actuality it will work fine for output voltages well below 0V as well.

thanks for the answer and illustration here. What does the diode do in the above circuit, why is that required? — David542, Jan 21 '20 at 22:32
The LED acts as a voltage reference- its forward voltage minus about 0.7V (Vbe of the transistor) ends up across R1, which establishes the collector current over a wide range of collector voltages. — Spehro Pefhany, Jan 22 '20 at 02:20

score 1 · Answer 2 · answered Jan 21 '20 at 21:34

Yes, there are numerous practical circuits that approximate the behavior of an ideal current source, just as there are numerous practical circuits that approximate the behavior of an ideal voltage source. Like real voltage sources, real current sources have limitations. The output current may change depending on the output voltage, a certain circuit topology may only be able to produce positive output current values while another might only be able to produce negative values, etc.

And, in circuit theory, we have the Thevenin and Norton theorems that tell us that any non-ideal linear voltage source (i.e. an ideal voltage source with a series output resistor) has an equivalent current source (an ideal current source with a shunting conductor).

What is a Current Source?

2 Answers2