I came across this page on building your own current source.
What are current sources used for in real-world applications? The only places I've seen them used are textbooks.
Do you have any suggestions for some fun weekend projects involving a current source?
The page you point to uses a current source to drive an LED which is a very popular real-world application and is also a good weekend project ;) The color temperature of the LEDs changes with current so keeping the current constant is useful for some applications.
Current sources can also be used in battery charging applications where you need to maintain a constant current to properly charge a particular chemistry of battery.
I have used current sinks to test power supplies. A typically power supply test will be to run the power supply at it's rated current to verify proper voltage regulation.
Your multimeter may use a current source to measure resistance. Pass a current through an unknown resistance and measure a voltage.
Within circuits a current source (and sink) can be used to bias transistors (like a differential amplifier circuit).
Stepper motor drivers are constant current sources made by switching the motor windings on and off at high-frequency. They monitor the current on sense resistors and adjust the duty cycle appropriately.
If you want some examples of the loosy (not switch-mode) kind then Ethernet and CAN use simple resistor current sources and current mirror circuits to limit the current spikes and reduce EMI when transmitting.
Another example are laser diode power supplies. The diodes are extremely sensitive to over-currents and have a sharp U(I) characteristic at the operating point. Even small fluctuations in voltage can cause large currents and destroy the diode.
One more example is the diode testing mode of a multimeter. It will source a little more than 1mA to allow you to check diode and transistor polarity and forward voltages.
PS. All these examples are really constant current/constant voltage since their maximum output voltage is limited by the power supply voltage. In fact both kinds of real sources have a limit: current sources won't work if the load resistance is too high and voltage sources won't work if the load resistance is too low.
Solar cells behave like current sources-- their voltage remains relatively constant across different light levels, while the current changes roughly linearly. I've seen current sources used in solar call testing before.
LED brightness is proportional to current, so if you made an adjustable current source, you could make a dimmable LED flashlight-- that would be pretty cool.
Current sources are used to linearize transistor amplifiers, and as I understand it are used all over the place inside of ICs.
Many sensors and transducers out there are easy to instrument with constant current sources. RTDs are the first thing that come to mind, but really any resistive based transducer can be driven with a constant current source and then all you need to do is monitor the voltage drop across the element with something like an instrumentation amplifier to measure its output.
Like endolith mentioned, current sources are very important in many IC's, especially analog IC's including, amplifiers, op-amps, digital-to-analog converters. If I am remembering my basic amplifier design correctly constant current sources are common in the input stages of many designs to ensure proper biasing in the input FETs or BJTs.
Op-amps often (always?) use a current-source driven differential amplifier as the input stage, and it is this that confers the high-impedance to the input of the op-amp.
A current source provides a fixed current, regardless of the voltage across it. So the impedance of a current source is the ratio of change in voltage divided by the resulting change in current. Since for a current source, current doesn't change, the ideal current source therefore has infinite impedance. Real world current sources do pretty well, 10's of megohms being easily achieved.
