Knowing Ohm's law, the current drawn is directly proportional to the resistance. In some power supplies, you can limit the current. Let's say the voltage is 12 volts, it supplies 1 amp, and the resistance is 8 ohms. The current going through should be 1.5 amps, but the supply can only deliver 1. How can this defy Ohm's law, and limit the current?
What you do with a current limited power supply is set to maximum properties. The power supply will regulate its output voltage in such a way that the lowest condition is met.
I drew a graph for how a current limited power supply will act with varying resistance connected. Consider a 12V power supply limited to 1A.
- The X-scale varies the load resistor from 1 to 30Ω;
- The left Y-axis and red line represents the ouput voltage of the power supply. Clearly when the resistor is too low, the power supply output voltage drops, to fulfill the 1A max output;
- The right Y-axis and green line represents the output current. Once the maximum output voltage is reached the output current will drop inversely proportional with the resistor value.
- You can clearly see voltage regulation taking over from current regulation at R=12Ω
The name current limited power supply is a little misleading, because such a supply is also voltage limited. It delivers as much power as possible, without exceeding either the voltage limit or current limit.
If it's connected to an open circuit, it will be limited by voltage, because it takes no current to develop a voltage over an open circuit.
If it's connected to a short, it will be limited by current, because it takes to voltage to develop current through a short.
If it's connected to something else (0 < \$R_L < \infty\$), then it could be limited by current or voltage, but it's much more common to design circuits to be powered by voltage sources than current sources.
You can't violate Ohm's law. Under that situation, the voltage at the output would drop to 8 volts. If you were to continue reducing the load impedance, at some point something will go pop, and it will fail. A specification like 12V output is only valid under certain operating conditions, like a load of less than 1 amp. Outside that range, the voltage will not (and cannot) stay constant.
Power supplies often include "foldback" current limiting, so that with a low-impedance load, the voltage will drop enough (in a controlled way) so that the output power is reduced enough so that nothing pops. When the load goes back within the specified range, the supply will operate normally again.