Electronic – How to current-limiting supplies be possible


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?

Best Answer

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

enter image description here