In the linear power supplies, when the power supply is delivering maximum current, like 3 amps in the TekPower HY1803 variable power supply, the power supply internally is producing a fixed (maximum) amount of power, independent of the voltage output. Any power that is not delivered to the output load is going into the heatsink on the back panel. When the power supply is delivering 1 volt at 3 amps, it is dissipating about 51 watts more in the heatsink than when it is delivering 18 volts at the same current. The heatsink can get extremely hot in the 1 volt 3 amp case, and it should be kept away from any objects (especially flammable) that obstruct free air circulation. When the heatsink gets very hot, the current rating of the output transistor is reduced at the same time as the voltage drop across it is maximum, so operating the power supply at 1 volt and 3 amps output is more damaging to the transistor than operating at 18 volts and 3 amps.