Constant-Current Systems

Author: J.B. Hoag

Our commercial power supply system is built in such a fashion that the voltage remains constant regardless of the current load upon it. The lights in a house may dim a little if we draw too much power, but this is due to the voltage drop along the feed wires. The voltage of the generators at the powerhouse is constant at about 115 volts.

There are certain applications which require the current to remain constant despite changes in the resistance of the load. The voltage may go up or down, but the current must be constant. This is the case with the power supply of high-powered oscillators, and with certain applications of arc lamps.

Fig. 29 L. Circuits for changing from a constant-voltage supply system to one of constant output current under varying loads

Figure 29 L shows circuits whereby a constant-voltage system may be transformed into a constant-current system. The reactances of the inductance L and of the capacitance C are made equal to each other at the supply frequency, i.e., 2πfL = 1/2πfC. When the output resistance R is small, the voltage across it decreases, but the current through it remains constant, and vice versa. A short-circuited load is not dangerous but an open circuit will develop high voltage, and perhaps puncture C. The constancy of the current is vitiated somewhat by the fact that the coil has resistance as well as inductance and by the fact that its inductance changes slightly when different currents pass through it.