I am researching power supply over-current protection modes, and I have a question about constant current.

Wikipedia claims (https://en.wikipedia.org/wiki/Foldback_(power_supply_design)) that the current remains constant while the voltage goes to zero. Doesn't this violate Ohm's Law? R is not changing, I is not changing, but somehow the voltage can change willy nilly?

What gives?

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    \$\begingroup\$ If R and I don't change, V doesn't change too. \$\endgroup\$ – PlasmaHH Mar 20 '17 at 15:52
  • \$\begingroup\$ Even if it's a switching or linear regulator, there are two main regulation modes: constant voltage or constant current. And they are ORed in feedback circuit. So, if one (constant voltage) fails (by a short or overload) the other (constant current) will take control. Thus, in constant current mode, you shouldn't expect constant voltage. \$\endgroup\$ – Rohat Kılıç Mar 20 '17 at 15:57
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    \$\begingroup\$ R is changing, and Ohm's law tells you that for a constant current and decreasing R the voltage decreases with R. When R approaches 0, so does V. \$\endgroup\$ – JimmyB Mar 20 '17 at 16:16
  • \$\begingroup\$ I see, I guess I misunderstood the part where the graph defines the behavior as the load changes. That should have been obvious. Sorry! \$\endgroup\$ – user2913869 Mar 20 '17 at 18:31

If you set your power supply to constant current mode, the voltage will change depending on the load. Pure Ohms law V = IR. If I is constant and you change R, V must change.

Obviously within the limits of how much V is available.

So when you set your current limit on the supply and watch the meters while you increase the load, at first V will be constant and the current needle will ramp up to the limit. After that the current needle will stay put and the voltage needle will drop.


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