Quick answer: CV regulates voltage until the load reaches the selected current limit. CC regulates current by adjusting voltage within the selected ceiling. A CV-to-CC transition may be expected operation, not a malfunction.
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Constant voltage operation
In CV mode the supply controls voltage at the active sensing point. The load determines current as long as demand stays below the selected current ceiling and the instrument’s operating limits. Lead resistance can create a different voltage at the DUT if sensing is local.
Constant current operation
In CC mode the supply adjusts voltage to hold the selected current, up to the voltage ceiling. If the load changes, the voltage needed to sustain that current changes too. A source may be unable to establish the requested operating point when the load or power limit prevents it.
Calculate an expected transition
For an ideal resistor, I = V/R. With 10V selected and a 1A current limit, a 20Ω resistor draws 0.5A in CV mode. A 5Ω resistor would demand 2A, so the ideal CC operating point is 5V at 1A. Real startup or electronic loads may not behave as fixed resistors.
CC is different from protection shutdown
A CV-to-CC transition may be ordinary operation. OCP or other fault functions can instead disable output or latch an error. Read the instrument’s documented behaviour before treating every low-voltage indication as a fault.
Applications and limits
CV/CC control supports many device tests and suitable sourcing tasks. Batteries require a validated chemistry-specific protocol and external-energy compatibility. Discharge, regenerative operation or dedicated photovoltaic/battery simulation requires documented equipment capabilities beyond generic CV/CC operation.
Practical checks
| Check | What to verify |
|---|---|
| Resistive load | Check the expected current using I = V/R. |
| Current ceiling | Choose a limit appropriate for the DUT and startup behaviour. |
| Voltage collapse | Identify CC operation before assuming a defective supply. |
| OCP | Check whether OCP latches off or has a different reset behaviour. |
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