I'm building a bench power supply as my introductory project to electronics.
When it comes to adjustable voltage it's easy to see the usefulness of such feature but I can't figure out what is the usefulness of having adjustable contant current limiting on a power supply ?
Isn't ideal to have a power supply providing as much current as possible/needed for a circuit ?
Limiting the current and the voltage effectively limits power, since power \$P\$ is the product of voltage \$E\$ and current \$I\$:
\$ P = IE \$
Since bench supplies are commonly used for prototyping, where faults are commonly introduced mistakenly, this is a big win. Many faults that would normally destroy a device will instead not destroy the device if the total power is low. This is because many faults destroy components by excessive power generating heat faster than it can be carried away, causing (often microscopic) materials to melt or vaporize. If the power supply can't supply enough power to vaporize your components, then this just can't happen.
Also in some cases it can be handy to have a current source instead of a voltage source. Driving an LED, for example. A power supply with adjustable voltage and current limits can be either a current-limited voltage source, or a voltage-limited current source.
One thing that I dont see anyone mentioning is that a current-limiting supply is excellent at locating short circuits. Set it up to limit current, set your voltmeter to its millivolt scale and start probing around the power and ground net. Move only one lead at a time, looking for a lower and lower voltage between the two points. the lower the voltage drop, the closer you are to the short circuit. This makes short work (heh) of finding the tiny solder ball or bridge on a board filled with components.
I usually use constant current mode when powering a newly assembled board.
I set the supply to a minimum current and power up the project, then I slowly increase current limit while looking at the current consumption closely - if it does not increase above some calculated estimation, I'm certain I have no shorts or other heavy failures on the board.
Desk top lab power supplies are designed with current limit control so that, should you choose, you can control how much current gets to the load. This can be useful in several ways.
1) You can use this to test a circuit / component to a certain current level without having to use a resistor box in series from a fixed voltage supply.
2) The current limit can be used as a safety feature for either a new circuit or a faulty circuit under test. The controlled current limit will keep the circuit from blowing itself to bits or severely overheating if there were shorts, backwards components or bad semiconductors.
3) The variable current and voltage controls of the lab supply make it easy to step through a whole series of data points for a circuit or component to collect data so that you can plot a current versus voltage plot.
A current-limiting bench supply is an essential tool when prototyping. If one has enough space, and is operating at low enough power levels, that having the supply draw the maximum current from its source transform and dissipate all the power as heat would not pose a problem, a transformer, a regulator, some caps, a handful of resistors and two LM317 regulators would suffice [without current limiting one would only need a single LM317]. Commercial bench supplies aren't exactly free, but if your labor is worth much a small unit may be worth the money.
If you don't want to buy a commercial supply, it may still be worthwhile to build a simple board with a current limiter that has a few settings (perhaps use a switch instead of a pot if you like), a few fixed-voltage outputs (e.g. 5.0 volts and 3.3 volts), and a variable output. One could probably build such a thing on perfboard with an hour or two of work.
Even a board which only has an approximate 20mA current-limit setting may be useful. In many cases, one can program a processor-controlled board in such a way that it should take less than 15mA [by not having the processor enable any functions that would take more]. If one powers up such a board with a non-current-limited supply and something isn't right, it's possible that a mis-wired part could fail in such a way as to cause widespread damage. If one powers up the board with a roughly-20mA current-limited supply, the 20mA current limit will generally be low enough to prevent immediate damage.
PS--It's entirely possible that an accidental short which causes 20mA to flow where it shouldn't will cause some parts to be operated outside their Absolute Maximum Ratings. Any time a part is operated outside its AMR, one should expect that it may have been damaged thereby, and be aware of the possibility that a such damage might ironically alter a part's behavior in a way which would cause it to work in one's application even though an undamaged part would not. Nonetheless, when one's trying to get a design working for the first time, it's often helpful to assume that things will have probably escaped damage unless one sees evidence otherwise [especially when a 20mA current-limited supply makes it likely that they will have done so].
I think it would be valuable to add, as most answers suggest that CC mode = protection, that when relying on a power supply's constant current mode to protect components one should keep in mind that the transition between constant voltage and constant current modes is not instant, and in that time frame the unit will supply more current than intended, likely it's rated maximum. This transition time varies from unit to unit and also usually depends on the difference between it's maximum current rating and the set current limit (the bigger the difference, the faster the transition will be).
So if protecting the components is important, make sure to check your power supply's current overshoot at your intended settings and anticipated load changes before relying on it as your only protection.
