When a bench power supply is rated for 0-30V, 0-5A, does that mean that it can supply more than 5A at voltages below 30V while keeping the max power constant, or is there a hard cap on the current?
The supply can deliver up to 5 amps at up to 30 volts.
The voltage and current ratings are both maximums, and are independent of each other - you can't expect to get 10 amps at 15 volts.
7\$\begingroup\$ Depending on the supply there might even be an additional power limit and the maximum current is only available at lower voltages. In this case the datasheet will give a derating curve for maximum current vs. voltage. \$\endgroup\$– jusacaJul 31, 2020 at 8:20
Depends on the power supply, but generally a bench power supply is rated at max everything, so that would be 5 A at 30 VDC, aka 150 W.