A DC power supply typically has knobs to limit the voltage and the current. The supply limits the voltage and current it puts out to whichever setting is more stringent.
Let's consider an example. Suppose you connect a 10 W resistor to the supply, set the current limit to 0.2 A and the voltage limit to 1 V. How much current flows through the resisitor and what is the voltage across the resistor?
The key relationship you need for a resistor is V = IR, where V is the voltage across the resistor, I is the current, and R is the resistance. First let's consider the current limit. If 0.2 A of current flow through a 10 W resistor, the voltage across the resistor is (0.2 A)(10 W) = 2 V. Since this exceeds the voltage limit, the operation of the power supply will be controlled by the voltage limit and the voltage across the resistor will be pegged at 1 V. The current through the resistor will then be 0.1 A.
The rotation rate of a DC motor is controlled by the applied voltage; the current needed to produce the rotation rate depends on the torque that resists the rotational motion of the motor. It therefore tends to vary over the course of each revolution of the motor.
A convenient strategy for operating the motor is as follows. Turn the voltage control nob to zero, and raise the current control setting at least half a turn. Now increase the voltage knob until you achieve the desired rotation rate. If you need to interrupt the motion temporarily, turn the current control knob to zero but leave the voltage knob setting as it is. When you want to resume operation, just raise the current setting until the motor rotates.
Updated 10/8/99 by Peter N. Saeta .