let's consider a DC power supply of 5V with for instance 1A of maximum output current (i.e. its maximum output power if 5W).

Let's connect a load R = 1 Ohm. Ideally a voltage source of 5V will give to it a current equal to 5V/1Ohm = 5A. But it is out of the maximum current of our power supply.

What does it happen at circuit level? I suppose that our power supply will provide a current of 1A, so the voltage drop on the load will be equal to 1V. Where are the others 4V?

  • 1
    \$\begingroup\$ Read here learnabout-electronics.org/PSU/psu22.php and here powersupply.blogs.keysight.com/2012/07/… \$\endgroup\$
    – G36
    Oct 6, 2019 at 16:53
  • 1
    \$\begingroup\$ Without entering electronics level: most PSs these days have CV and CC modes, which means Constant Voltage and Constant Current mode respectively. This means that even if you set CV mode with Vout = 5 V, when load's current exceed the maximum current (set by you or the PS itself), PS automatically turns to CC mode, so it lowers Vout in order to produce a current equal to CC. \$\endgroup\$
    – thece
    Oct 6, 2019 at 17:01
  • 3
    \$\begingroup\$ Possible duplicate of What kinds of things can cause a power supply to cut out? \$\endgroup\$
    – Huisman
    Oct 6, 2019 at 17:09
  • \$\begingroup\$ unregulated sources may be higher than 1V but overheat and regulated sources will cutoff to prevent damage. Load regulation tests may predict small changes. try not to use 100% and learn to derate specs for better performance \$\endgroup\$ Oct 6, 2019 at 19:47

2 Answers 2


There are a number of things that can happen.

  1. The supply blows a protection fuse.
  2. A designed in circuit breaker trips.
  3. The supply let's out smoke and dies.
  4. The supply output voltage is dragged down to some much lower output voltage and delivers less current correspondingly.
  5. The overload makes the supply temperature rise to a point where a thermal switch cuts off either to input or output.
  6. The supply detects the current overload and shuts down the output.

Depending on the type of your power supply, the effects will vary.

If it is a smart power supply with CC and CV modes, the current will be limited to 1A (in CC mode) and the voltage will be reduced.

In the case of a dumb power supply, either a safety mechanism will be triggered (a fuse or a breaker) or it will continue to work at a undesired state. The voltage at the resistor and the current through it will depend on the characteristics of the components inside the power supply.(diodes, transformer winding, etc.)


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