The Extech Digital DC Power Supply (EXPS) can operate in either voltage-control or current-control modes, and so has both current and voltage control knobs.

Most instructors are familiar with power supplies which have voltage controls. That is, you set the voltage and the supply tries to produce that voltage (as long as it can supply enough current). This is called voltage-control mode. As long as the circuit demands less than the maximum current possible, the current is determined by the resistance of the circuit.

The Extech power supply will act just like a conventional power supply if you set the current control at the maximum and adjust the voltage control as desired.

In contrast, in current-control mode the user sets the desired current, and the voltage is adjusted by the power supply (within limits) to achieve that current.

To operate the Extech in this mode, set the current control knob to zero, the voltage to a high value, and then increase the current control to the desired current.

As an example of a current-control application, consider the magnetic field of a solenoid experiment. The solenoid is a low-resistance coil. The magnetic field is a function of the current through the coils. The potential is immaterial. To control this experiment, one would set the voltage limit knob to a high level, and then increase the current limit from zero. The user can then just dial up the desired current.

Another way to look at it is that the Extech power supply always limits the voltage and the current so that neither one can exceed the limits set by the two controls – the voltage limit and the current limit. This means that you, or your students should not ever be able to destroy the power supply. Even if you directly short across the output leads and turn both controls up all the way, the current will be limited so that the power supply will not destroy itself. Note, however, this does not guarantee that you will not destroy an external circuit.