Learning Library · Circuit basics · 4

Constant voltage vs. constant current

A bench power supply has two knobs: a voltage setting and a current limit. It can only obey one of them at a time. The load decides which one, and the supply switches between them automatically.

The two modes

Constant voltage (CV): the output holds the voltage setting and the load draws whatever current Ohm's law gives it, I = V ÷ R. This is the normal mode for powering a circuit.

Constant current (CC): once the load tries to draw more than the limit, the supply lowers its output voltage until the current equals the limit exactly. The voltage setting becomes a ceiling instead of a target.

Above that resistance the supply runs in CV. Below it, the supply runs in CC. Most supplies show which mode they are in with an indicator or a CV and CC label on the display.

Try this

• Set 12 V and a 1 A limit. The crossover load is 12 Ω. Drag the load from high to low resistance and watch the operating point slide along the top edge, then turn the corner and drop down the right edge.
• Drop the load to the lowest value. That is close to a short circuit, and the supply still delivers exactly the current limit at a very low voltage.
• Raise the current limit until the mode returns to CV. The output voltage jumps back up to the setting.

Why the current limit protects your circuit

Set the current limit just above what a healthy circuit should draw before you switch on. If there is a solder bridge, a reversed part, or a short, the supply goes straight into CC and limits the damage instead of delivering everything it can. The same behaviour lets a bench supply charge a battery or drive an LED at a fixed current.

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