I’m talking about like bench power supplies. Like how do they know if the positive and negative is shorted and it switches to constant current?
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They are controlled through feedback.
They measure the current and voltage all the time and adjust their output to reach the set voltage unless the current would have to get too large in wich case it controls to the constant current. There are no “natural constant voltage sources”, you always have to manipulate them to get that behavior.
Internally they work by pulse-width-modulation, wich means they quickly open/close a switch to connect their internal storage to the output only for a limited time. (The flickering voltage is then averaged to a more constant voltage with a capacitor)
If the measured output is too low then they increase the open time and decrease the closed time (they adjust the “duty cycle”) and vice versa if either output current or voltage get too large.
So when you short the output ports the controller notices the output current gets too large so the switch gets closed much longer wich decreases the output voltage until the maximum current isn’t violated anymore.
they don’t really switch between the two, it is always doing constant voltage but there is circuitry that measures the current and can turn down the voltage
so when the current is over the set limit the voltage is turned down until the current is back at the limit, when the current get under the limit the voltage get turning back up but only up to the voltage you set
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